Aug 12, 1991 - pSMP-6, +46 to -90, which has elements D, C, and B deleted pSMP- .... from each dish for a 12-day period and analyzed for hGH levels by.
Vol. 267,No. 17, Issue of June 15,pp. 11995-12003, 1992 Printed in U S.A.
THE*JOURNAL OF BIOLOGICAL CHEMISTRY (c)1992 by The American Society for Biochemistry and Molecular Biology, Inc
Positive and Negative &-Acting Regulatory Elements Mediate Expression of the Mouse Vascular Smooth Musclea-Actin Gene* (Received for publication, August 12, 1991)
Douglas N. Foster$, Bonhong MinO, Linda K. Foster$, ElizabethS. Stofletll, Siquan Sunll, Michael J. GetzT, and ArthurR. Strauchll** From the Ohio State Biochemistry Program and the ((Department of Cell Biology, Neurobiology, and Anatomy, The Ohio State University, Columbus, Ohio 43210 and the VDepartment of Biochemistry and Molecular Biology, Mayo Clinic Foundation, Rochester, Minnesota 55905
Segments of the 5‘-flanking region of the mouse early transcriptional response to high cell density in vascular smooth muscle a-actin gene were assayed for temporal studies of promoter activity during BC3H1 promoter activity in transfected mouse BC3H1 my- myogenic cell differentiation. ogenic cells and AKR-2B embryonic fibroblasts. The region between -150 and -191 that functions as a positive transcriptional element inmyogenic and fibroblastic cells contains a mammalian-specific inverted Regulation of actin gene expression is controlled by both CC(A/T)6GG-typeconsensus sequence. Expressionwas tissue-specific and developmental stage-specific mechanisms restricted to fully differentiated myogenic cells when (1).The functionalsignificance of multiple actinpolypeptides an additional sequence spanning -191 to -224 was in included inreporter gene constructs. This 33-base pair vertebrate species is not known, although transcriptional (bp)negativeregulatoryelement is 70% conserved switching between different actin isoform genes during develbetween the mouse and human genes and contains a opment suggests that maturing muscle cells may require sev10-bp motif at its3’ end that only partially resembles eral types of actin monomers for performing specialized cya CC(A/T)&G element. Retention of a GGGA motif at toskeletal and/or contractile functions. Although molecular the 3‘ boundary of the 33-bp region is sufficient to mechanisms governing the regulation of genes encoding the maintain full transcriptional repression in fibroblasts cardiac (2, 3) and skeletal (4,5) muscle-specific forms of a and is partlyresponsible for repression in undifferen- actinhave beenproposed by several researchgroups, an tiated myoblasts. Complete muscle tissue-restrictive equallydeveloped description of regulatory elementsthat expression requires an additional 8 bp from theCC(A/ control vascular smooth muscle (VSM)’ a-actin gene expresT)6GG-like element immediately 5’ to the GGGA motif, sion during myogenesis has not yetemerged. This actin polysince replacement of this region with an unrelated10- peptide is the predominantisoform found in vascular smooth bp sequence completely eliminated restrictive tranexpressed in bothdeveloping muscle ( 6 )and also is transiently scriptionalbehaviorinundifferentiated myoblasts. skeletal and cardiac muscle tissue in avian and mammalian The distal portion of the 5”flanking region between species (7, 8). To better understand thegenetic basis of VSM -224 and -1074 contains six E-box motifs(CANNTG) a-actin gene inductionand repression duringmammalian and mediateshighlevel transcription only in post- myogenesis, we recently isolated the full-lengthmouse VSM confluent BC3H1myoblasts. Analysis of reporter gene a-actin gene and identified the promoter region by nucleotide constructs including either the proximal E-box a t -240 sequence comparisons with the chicken and human VSM a or all six E-boxes indicate that the five distalE-boxes are not required for high level transcription. A 724- actin genes and cell transfection assays (9). A 373-bp portion bp segment of the 5”flanking region consisting of the of the 5’-flanking region proximal to the transcription start proximal E-box flanked upstream by a mammalian- site of the mouse VSM a-actingene contains a muscle-specific promoter that mediates expression of reporter gene fusion specific 352-bpregionwassufficientformaximal plasmids in transfected myogenic cells. Fourputative cistranscriptionalactivationin postconfluentBC3H1 resembleCC(A/ myoblasts. Deletion of the 352-bp region restricts the acting regulatory elementsthatpartially T)6GG-type sequence motifs described in the 5’-flanking re* This work was supported by National Heart, Lung, and Blood gions of other muscle protein genes (10) are located within Institute Grant HL 43370 (to A. R. S.), National Cancer Institute thisportion of the mouseVSM a-actin gene. Thethree Grant CA 33643 to (M. J. G.), KoreaScience andEngineering proximal CC(A/TIGGG-type elements are100%conserved in Foundation Grant913-0401-020-2 (to B. M.), and by state andfederal funds appropriated to the Ohio Agricultural Research and Develop- mouse and human species, whereas the distal CC(A/T),GGbe unique to themouse. Six elements ment Center, The Ohio State University (to D. N. F. and L. K. F.). type element appears to The costs of publication of this article were defrayed in part by the that exhibit the consensus sequence CANNTG, previously payment of pagecharges. Thisarticlemustthereforebehereby referred to as the E-box motif, also are locatedin the 5’marked “aduertisement” in accordance with 18 U.S.C. Section 1734 flanking region of the VSM a-actin gene between -240 and solely to indicate thisfact. $ Present address: Dept. of Animal Science, University of Minne- -1074 (9). E-box motifshave been shown to bindbasic helixloop-helix trum-activation proteinsinvolved in myogenic cell sota, St. Paul, MN55108. differentiation (11).Although their individual roles in VSM Present address: Dept. of Biochemistry,Kon-KukUniversity, Choongju-City, Korea. ** To whom correspondence should be addressed Dept. of Cell Biology, Neurobiology, and Anatomy, College of Medicine, The Ohio State University,333 W. Tenth Ave., Columbus, OH 43210-1239. Tel.: 614-292-9602; Fax: 614-292-7659.
The abbreviationsused are: VSM, vascular smooth muscle; CAT, chloramphenicolacetyltransferase; bp, basepair(s);hGH,human growthhormone;PCR, polymerase chainreaction;SMP,smooth muscle promoter.
11995
11996
cis-Regulatory Regions of the V S M a-Actin Gene
7, +46 to -59, which has elements D, C, B, and A deleted but the cu-actin gene expression currently is notknown,thefour of the polymerase chain reactions CC(A/T),;GG-type andsix E-box elements appear to operate TATA box left intact. The products were ligated topBLCAT3thathad beendigestedwith SalIand together as a muscle-specific promoter in cell transfection BarnHI and then transformed into Escherichia coli strain HBlOlcells studies (9). for propagation. To further characterize the contribution of the variouscisSite-directed mutant STU4CAT and deletion mutant A195CAT acting regulatory elements in promoting transcription of the were constructed using PCR amplification. Oligonucleotide primers designed with 5’ SalI and 3’ BamHI restriction sites were made on mouse VSM a-actin gene, we prepared plasmids containing different segments of the 5”flanking region linked to a func- an Applied Biosystems model 394 DNA/RNA synthesizer and deover a Sephadex G-25 NAP column (Pharmacia LKB Biotechtional chloramphenicol acetyltransferase (CAT) reportergene salted nology Inc.) indistilled water. Thefollowing primers were used (lower (12) and performed cell transfection studies using BC3H1 case letters denote a clamping sequence for the restriction enzymes, cells and mouse AKR-2B embryonic fibroblasts. The results whereasunderlinedletters indicate SalI,BamHI,or PstIsites. Italicized of cell transfection analysis indicate that the distal portions letters indicate a StuI site and boldface capitals denote the VSM aof the 5“flankingregion containing oneor more E-box motifs actin gene sequence): ( a ) STU4, agtcatGTCGACCCATAACGAGCfunction as positive cis-acting elements that become maxi- T G A G C T G C C T a a a g g c c t a a G A G C A G A A C A G A G G A VSMPBam, catgatGGATCCTGACAGCGACTGGCT; mally activated when BC3Hlmyoblastsenter a post- ATG;(b) and ( c ) A195: gatcatCTGCAGGGGAGCAGAACAGAGG. confluent, growth-arrested state. A33-bpsequencelocated Due to fortuitous cloning, the A195CAT clone has a PstI site in between -224 and -191 in theproximal 5’-flanking region of place of the SalIsite. For PCR, primersa + b and c + b were annealed to 100 ng of template DNA (pSMP3) and amplified 35 cycles in a the VSM a-actin gene appears to contain a negative regulatory element, since deletion of this segment augments tran- Perkin-Elmer Cetus Instruments thermalcyclerusing standard PCR kit reaction mixes and times (94FC X 1 min, 50FC X 2 min, 72FC X scription in subconfluent,preconfluent,andpostconfluent 3 min). PCR products were purified from 1%agarose gel slices using BC3H1 myoblasts, as well as embryonic mouse fibroblasts. In Costar Spin-X columns. Fragmentswere then digested with SalI and BC3H1 myogenic cells, but not embryonic fibroblasts, the BamHI and ligated into the promoterless pBLCAT3 reporter gene negative regulatory region encompasses theimperfect CC(A/ plasmid that was similarly digested and treated with calf intestinal T),;GG-typemotif, which is unique to the mouse VSM a-actin phosphatase. Transformationwas done into E.coli HBlOl cells using gene (9). Substitution of this motif with an unrelated 10-bp standard techniques. Stock cultures of mouse BC3H1 myogenic cells (15) were obtained sequence issufficient to completely alleviate a repressed tranfrom Dr. D. Schubert, the Salk Institute, La Jolla, CA, and mainscriptional state in undifferentiated BC3H1 myoblasts. Tem- tained in culture as described previously (16). The AKR-2B mouse poral analysis of promoter activity also was performed using embryonic cell line was provided by Dr. Harold Moses, Vanderbilt a human growth hormone (hGH) reporter gene assay that University, Nashville, TN andwas maintained using standard culture permits transcriptional activity to be monitored throughout conditions (9). Cesiumchloride gradient-purified fusion constructs the myoblast cytodifferentiation process using a single popu- containing the CAT gene(describedabove) were transfectedinto lation of transfected cells (13). The results of these experi- BC3H1 myoblasts and AKR-2B fibroblast cells during logarithmic phase growth (30-50% confluent in serum-supplemented medium) ments confirmed that the VSM a-actin promoter was maxi- using thecalcium phosphate precipitation method (17). Cultures were mally activated when acontinuously cultivatedgroup of trans- exposed 16-18 h to equimolar amounts of each plasmid construct fected myoblastsreached a density-arrested growth state. (10-12.5 pg of DNA), after which time the DNA precipitates were Moreover, promoter responsiveness to high cell density was removed and the cells washed and re-fed with serum-supplemented observed only if portions of the 5”flanking region between medium (9).AKR-2B cells were treated for 2 minwith medium -372 and -1074 were included inthehGH fusion gene containing 10% glycerol following exposure to DNA precipitates to enhance transfection efficiency (17). constructs. The results of these studiesimply that full expresThe following cell extracts were prepared from transfected BC3Hl sion of the VSM a-actin gene during BC3H1 myogenic cell cells at thefollowing densities, as detailed under “Results”: “subcondifferentiation may depend on the density-dependent induc- fluent myoblasts” (50-70% confluent), “preconfluentmyoblasts” (80tion of trans-activation factors that are targeted to upstream 90% confluent),and“postconfluent myoblasts” (100% confluent). cis-actingregulatorysequences comprising a putative cell AKR-2Bfibroblasts were harvested for extractpreparation when fully confluent 48 h after transfection. Assays for CAT activity were density response domain. performed essentially as described (17) using 14[C]chloramphenicol (Amersham Corp., 57 mCi/mmol) and equivalent amounts of cytoplasmic extract (typically 1 pg of protein/l50 p1 of reaction volume). Construction of Mouse VSM a-Actin Promotor:CAT Fusion Vectors Acetylated products were separated by thin-layer chromatography using Whatman Silica Gel 60A multichannel plates and exposed to and Transient Transfection into Myogenic and Fibroblastic Cell.Seven plasmid constructs were prepared using deleted portions of the Kodak XAR-5 film (Eastman Kodak, Rochester, NY). Video images of replicateTLCautoradiographs were analyzed usinga Tracor VSM (?-actin 5”flanking regulatory region to drive CAT reporter gene transcription in transiently transfected cell lines. Constructs Northern image analysis computer(model TN8502, Tracor Northern, were made that contained one,two, three, or four CC(A/ThGG-type Middleton, WI) to estimate the percentconversion of 14[C]chloramelements (denotedA through D in Fig. 1) linked to the reportergene. phenicol to its acetylated derivatives. Transient Transfection of Mouse VSM a-Actin Promotor:hGH FUTwo additional reporter gene constructs contained all four CC(A/ T),;GG-type elements plusE-box motifs from the less well conserved sion Constructs for Kinetic Assays in BC3HI Myogenic Cells-EquiVSM a-actin 5”flanking region between -224 and -1074. Deletions molar amounts of four plasmid constructs that utilized deleted porwere prepared using oligonucleotide primers and polymerase chain tions of the VSM a-actin 5“flanking regulatory region to drive hGH reactions (PCR) to amplify sequences that were ligated to the CAT reporter gene transcription were transfected into proliferating, undifreporter gene vector pBLCAT3 (14). Oligonucleotide primers that ferentiated BC3H1myoblasts. Three of the constructsused (pXGH5, included 5’ SalI and 3’ BarnHI linkers for subcloning were annealed pSB18a-3-hGH, and p372-hGH)have been described previously (9). The fourth construct, pl91-hGH, was prepared by digesting CAT t o 1 pg of template (pSB18a-3-hGH) (9) and amplified 30 cycles in a gel-purified insert (+46 DNA thermalcycler (Perkin-Elmer Cetus Instruments). The seven construct pSMP-4with Sal1 and BarnHI. The to -191, which has CC(A/T)6GG-type element D deleted but includes constructs were designated as follows (shown inFig. 1):pSMP-1, +46 t o -1063, which includes the entire 5”flanking region in pSB18a-3- elements C, B, and A) was ligatedinto SalI-,BarnHI-digestedlalkaline hGH minus 11 bp (+1indicates the start of transcription); pSMP-2, phosphatase-treatedp+GH5 (Nichols Institute, San Juan Capistrano, CA) and then transformed intoE. coli DH5 cells for propagation. +46 to -724; pSMP-3, +46 to -224, which has the distal portion of Constructs purified on cesium chloride gradients were transfected the 5”flanking portion deleted but has CC(A/T)6GG-type elements D through A intact; pSMP-4, +46 to -191, which has the D element into replicate cultures of BC3H1 myoblasts plated in serum-suppledeleted; pSMP-5, +46 to -150, which has elementsD and C deleted mented medium a t 2.5 X IO5 cells/lOO-mrn dish using the methods pSMP-6, +46 to -90, which has elementsD, C, and B deleted pSMP- described above. Medium aliquots were removedevery 12 or 24 h EXPERIMENTALPROCEDURES
cis-Regulatory Regions
of the V S M a-Actin Gene
11997
Densitometry was used to estimate the percentage of radiolabeled substrate that was converted to monoacetylated and diacetylatedproducts (Fig. 3). Differences inthe level of substrateacetylation observedbetween extractsprepared from BC3H1 myoblasts transfected with various reporter gene constructs are interpreted to reflect the ability of myoblasts a t certain stages of development to utilize portions of the VSM a-actin 5'-flanking region as transcriptional regulatory elements. Differences in transcriptional activity between each fusion plasmid vary only as a function of how much VSM a actin 5'-flanking region is included in the construct, since all myoblast preparations were transfected using an equimolar amount of fusion plasmid DNA. Moreover, myoblast cultures representing eachof the three developmental stages examined RESULTS in the SMP:CAT reporter gene studies were processed as a Fusion plasmid constructswere prepared using oligonucle- group throughout the transfection and posttransfection peotide primers andpolymerase chain reactions tosegregate the riods. When harvested for extract preparation, parallel sets of myoblasts transfected with each of the seven SMP:CAT CC(A/T)&G-type motifs found in theVSM a-actin 5"flanking region and to further bisect the upstream portionof this reporter gene constructs were indistinguishable with respect region that was previously shown to mediate transcriptional to cell density and morphological appearance. The level of activity in BC3Hl myogenic cells and aortic smooth muscle reporter gene expression in subconfluent myoblasts (Figs. 2 cells (9). The seven reporter gene constructs are shown dia- and 3, SUB-MB) is quite low forall constructs examined except SMP-4,which includes CC(A/T)&G-type elements C grammatically inFig. l and are abbreviated as SMP-1 through SMP-7 in the text and legends. Transfections were performed through A but deletes elementD. Extracts preparedfrom cells using two groupsof BC3H1 myoblasts maintaineda t different transfected with this construct displayed a high level of sub90% of theCATsubstrate was density states to studyhow early, density-activated events in strateacetylation;nearly converted to the acetylated forms (Fig. 3). Similarresults the cytodifferentiation processaffected promoteractivity. were prepared from denser prepMyoblasts in logarithmic stage growth were seeded in serum- were obtained when extracts postconfluent supplemented medium at an initial plating densityof 5 X lo5 arations of preconfluent (PRE-MB) and cells/lOO-cm dish. After a brief recovery period, one group of (POST-MB) BC3Hl myoblasts following transfection with cells (Group A) was transfected overnight and harvested 2 SMP-4, aswell as in transfected AKR-2B fibroblasts(Figs. 2 dayslaterassubconfluentmyoblasts (40-50% confluent). and 3). Correspondingly high levels of CAT activity were not Group B was maintained for4 days after subculture, provided observed in extracts preparedfrom myogenic and fibroblastic with fresh serum-supplemented medium, and then transfected cells transfected with the shorter constructs SMP-5, through overnight. Two days later, extracts were prepared from half SMP-7 (Figs. 2 and 3). These results indicate thata positive cis-acting element present in SMP-4 may be essential for of the GroupB cells harvested as preconfluent myoblasts (8090% confluent). The other half of Group B was refed with basic promoter activity. This region spans between -191 and serum-supplemented medium and maintained another2 days -150 and includes the CC(A/T)&G-type element C, which to allow the cells to become fully confluent before harvesting is 100%conserved in the mouse and human VSM a-actin genes. However, this region of the 5"flanking region apparthemaspostconfluentmyoblasts(100%confluent).Postently was not sufficient to direct muscle-specific transcripconfluent BC3H1 myoblasts are arrayed in the culture dish tion, since SMP-4 wasactive in both myogenic and fibroin overlapping sheets two to three cell layers thick. Autoradiographs depicting the results of CAT assays per- blastic cell types. Analysis of extracts prepared from preconformed on extracts preparedfrom BC3H1 cells and confluent fluent and postconfluent BC3H1 myoblasts (Figs. 2 and 3) mouse AKR-2B embryonic fibroblasts are shown in Fig. 2. showed that aslightlymore upstreamportion of the 5'flanking region corresponding toa 33-bp region that includes D C B A T the mousegene-specific CC(A/T)&G-typeelement D was pSMP-I -1063CAT required for restricted expression in muscle cells. Although the SMP-3 construct that contains CC(A/T)&G-type elepSMP-2 -724 + ment D was not expressed insubconfluent myoblasts or pSMP-3 -224 fibroblastic cells, high level expression was observed in the developmentally more advancedpreconfluentandpostpSMP-4 -191 confluent myoblast preparations.Thelargestconstructs, pSMP-5 -1 50 SMP-1 and SMP-2,were comparable in activitywith SMP-3 in transfected AKR-2B fibroblasts and stimulated only very pSMP-6 -90 low amounts of transcriptional activity. On the other hand, pSMP-7 -59 the relative activities of SMP-1 through and SMP-3 were very different for each of the transfected BC3H1 myoblast FIG. 1. The organization of mouse VSM a-actin promoter:CAT fusion plasmids. Constructs were prepared using oli- preparations. None of these constructswere active in subcongonucleotide primersand polymerase chainreactionsto amplify fluent myoblasts(Figs. 2and 3). In contrast, extracts prepared mouse sequences for subsequent ligation to pBLCAT3. The parent frompostconfluentmyoblaststransfected with SMP convector that served as template (pSB18a-3-hGH) contains a 1074-bp structs 1,2, or 3 all showed highlevels of acetylation activity. portion of the VSM a-actin 5"flanking region. T, TATAA consensus However, the performance of the three constructs appeared sequence; A-D, CC(A/T)6GG-typeelements. The constructs,as shown from top to bottom, are abbreviated as SMP-I through SMP- to be graded in preconfluent myoblasts, with activity being inversely related to the length of the VSM a-actin 5'-flanking 7. The numbers on the left side refer to the 5' end point of each construct. region included in the reporter plasmid. SMP-3, which confrom each dish for a 12-day period and analyzed for hGH levels by radioimmunoassay usinga '2511-labeledhGH monoclonal antibody mixture (Allegro" hGH Immunoassay System, Nichols Institute). Cell cultures were refed on days4 and 8 with serum-supplementedmedium following removal of medium samples for radioimmunoassay. Cell growth was monitored during the 12-day observationperiod using a set of 35-mm culture dishes initially seeded at the same density as the cells used for transfection (3.7 X lo4cells/35 mm dish).Cells were pulse-labeled on each of 10 days following transfection for 6 h with 1 FCi of ["Hlthymidine (6.7 Ci/mmol, ICN,Irvine, CA) andthen harvested by scraping into a solution of phosphate-buffered saline plus 0.1% sodium dodecyl sulfate. Trichloroacetic acid-precipitable counts were collected on Whatman GF/C filters that were washed with ice-cold 5% trichloroacetic acid and 95% ethanol and analyzed by liquid scintillation spectrometry.
&-Regulatory Regions of the VSM cY-ActinGene
11998
SUB-MB
POST-ME
PRE-MB
FIG.2. The results of CAT assays on transfected myogenic and fibroblastic cells. Extracts were prepared lromtransfectedsubconfluentBC3Hl myoblasts (SUB-MB), preconfluent HC3H1 myoblasts (PRE-MB), postconfluentBC3Hl myoblasts (POSTMH),andAKR-2Bembryonic fibroblasts (AKR-PH). The numbers below " each autoradiograph refer to the seven 1 2 3 4 5 6 7 SMP:CAT constructs shown in Fig. 1. Replicate CAT assays are shownonly for AKR-2B the seven AKR-2B fibroblast transfections to illustrate the degree of experimental variability. The level of nonspecific substrate acetylation obtained when cell extract is omitted from the reaction mixtureis negligible, as shown in the control lane (C).
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FIG. 3. CAT activity in myogenic and fibroblastic cells transfected with VSM a-actin promoter:CAT fusion plasmids. Digitized autoradiograph images were analyzed using comto deterputer-assisteddensitometry mine percent substrate acetylation per pg of cytoplasmic extract as prepared from transiently transfected BC3Hl myoblasts and AKR-2B fibroblasts. The numbers below each chart refer to each of the seven CAT constructs shown in Fig. 1. Panels are identified using abbreviations for each cell type, as described in the legend for Fig. 2.
2
3
4
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PromotecCAT Fusion Plasmid
tains only the fourCC(A/T)6GG-type elements, was most active, whereas SMP-1, which contains six E-box motifs in addition to the four CC(A/T)6GG-type elements,was the least active in preconfluentmyoblasts. Reporter gene constructs containing deletion and substitution mutations were prepared and transfected into BC3Hl myoblasts and AKR-2B fibroblasts to further delineate the position of the putative negative regulatory element located between -192 and -224 in SMP-3. Both mutants were designed to alter the structure of the mouse gene-specific CC(A/ T)BGG-like motif D between -194 and -203. Construct A195CAT contained a truncated form of element D carrying only the two 3'-endGG residues from this motif. In the STU4CAT construct, the entire element D was replaced bya 6-bp StuI restriction site flanked on both by AA sides residues. This substitution maintains thesize and position of element
D but scrambles its sequence to produce a GGCC core (Fig. 4). Expression was compared with SMP-3,which is repressed insubconfluentmyoblastsandAKR-2B fibroblasts, and SMP-4, which is fully active in bothcell types (refer to Figs. 2 and 3). Transfected BC3H1 myoblasts were harvested 24 h after transfection when in the subconfluent growth state to ensure that thecells were undifferentiated and transcriptionally repressed with respect to VSM a-actin gene expression. As summarized in Fig. 4, the behavior of the STU4CAT and A195CAT constructs was different in myoblasts and fibroblasts. Under conditions where SMP-4 was highly expressed in both types of cells, STU4CAT was virtually inactive in fibroblasts butfully expressed in preconfluent myoblasts. The base-line activity observed in fibroblastic cells transfected with STU4CAT mimicked the activity produced by SMP-3 that contains a correct element D. Relative to SMP-4, con-
of the VSM a-Actin Gene
cis-Regulatory Regions
11999
p372-hGH also resembles the 224-bp insert present in SMP3 since both contain the four CC(A/ThGG-type motifs. We previously showed that the insert from p372-hGH behaves as ‘SMP-4 94 99 a muscle-specific transcriptional control element that can be expressed by both BC3H1 myogenic cells and rabbit aorta .. 4 SMP-3 c5 -224 smooth muscle cells but not AKR-2B fibroblasts (9).Finally, pl91-hGHcontainsthethree proximal CC(A/TIfiGG-type CCTGTTTCGG elements (A, B, and C; refer to Fig. 1) and represents the sru I 1 3 STU4 99 same portion of the VSM a-actin 5”flanking region found in -224 SMP-4. By utilizing these constructs, the temporal analysis AAAGGCCTAA specifically focuses on density-dependent activationof ( a ) an 0 unrestricted promoter (plSl-hGH), ( b ) a short, muscle-re,95 1 r 1 n n IDelta 6 62 195 strictive regulatory region containing asingle E-box plus GG additional mammalian gene-specific upstream sequences FIG. 4. The results of fibroblast and myoblast transfections (p372-hGH), and (c) a longer, muscle-restrictive regulatory with variant promoter:CATfusion plasmids containing defective negative regulatory elements. AKR-2B fibroblasts ( F E ) and region containing multipleE-box domains (pSB18a-hGH). Culture medium was changed on posttransfection days 4 sulxonfluent BC3H1 myoblasts ( M B ) were transfected with SMP-4 (containing a deleted CC(A/T)&G-like element D), SMP-3 (contain- and 8 after first removing aliquots for radioimmunoassays. ing a native element D), STU4CAT (containing a scrambled element Complete replacement of the medium enabled us to distin111, or A195CAT (containing a truncated element D). The sequences guish hGH accumulation from de nouo synthesis, since reso l modified regions are shown below each construct diagram. A-C, toration of secreted hGH levels following a medium change (‘C(A/T),;GG-type consensus elements; T, TATAA box. The underwas possible only if the reporter gene constructs continued to lined sequence denotes the StuI restriction enzyme recognition site. ‘l’ransfected cells were harvested while in logarithmic stage growth. be active in the transfected myoblasts. Analysis of medium The percent acetylation for each construct is presented for both cell conditioned by myoblasts transfected with pXGH5 showed types in columnsonthe right. Valuesreflect themean of three that hGH secretion continued throughout the 12-day obserseparate transfections. vation period despite two complete medium changes (Fig. 5). These results indicated that reporter gene constructs were not struct A195CAT also was inactive in fibroblasts and only completely lost from the actively growing population of transpartially repressed in myoblasts. Theseobservations sug- fectedmyoblasts. Fig. 6, top, shows that the proliferative gested that the boundaries of the operativenegative regulatory activity of the myoblasts declined sharply after day 4, as element may be cell-type specific. In fibroblasts, the essential indicated by reduced [3H]thymidine uptake. On day 4, the restrictive domain includes only the GGGA motif between transfected myoblasts were 100% confluent, as assessed by -195 and -192. This region also confers significant transcrip- light microscopic examination, indicating that the subsequent tional repression in myoblasts although full muscle-restrictive reduction in thymidine uptake was due to density arrest of behavior comparable with that exhibitedby SMP-3 requires cell growth. Also shown in Fig. 6 are the valuesfor hGH an intact element D (Fig. 4). Substitution of the unrelated secretion by myoblasts transfected with each of the VSM aStuI motif for element D in SMP-3 effectively eliminates actin 5”flanking region:hGH constructs. Consistent with retranscriptional repression in undifferentiated myoblasts. sults obtained using myoblasts transfected with pXGH5, reFusion plasmids prepared using portions of the VSM aporter gene retention was indicated by the sustained output actin 5’-flanking region linked to a functional hGH reporter of hGH from myoblasts transfected with pSBlsa-hGH, p372gene also were used toexaminethetemporalaspects of hGH, and pl91-hGH, although the temporal behavior of each transcriptional activation as BC3H1 myoblasts approached construct was significantly different over the 12-day period. and entered a confluent, density-arrested growth state and Construct pl91-hGH initially was expressed a t high levels for continued on to mature into fully differentiated myocytes. the first4 days of culture after transfection and then declined Unlike the CAT assays described above, the secreted hGH 8 days exhibiting a trend expected by during the remaining reporter gene product permits transcriptional activity to be continuously monitored ina transfected cell population with- simple two-step dilutionof a fixed pool of secreted hGH(Fig. outterminating individual cultures. Asingle, low density 25 r preparation of myoblasts was divided into four groups and transfected while in logarithmic phasegrowth with constructs pXGH5, plSl-hGH, p372-hGH, and pSB18a-hGH. Transfected myoblastswere cultivated to postconfluent density over a 12-day period using serum-supplemented medium. pXGH5 contained the hGH structuralgene driven by the constitutive metallothionein promoter and served to monitor plasmid retention by the transiently transfected myoblasts during the 12-day observation period. pSB18a-hGH contains a 1074-bp 1 3 5 7 9 1 2 portion of the VSM a-actin 5”flanking region and is directly Days after Transfection comparable with SMP-1described above, since it contains six FIG. 5 . Temporal analysis of hGH secretion by BC3H1 myoE-box motifs and all four CC(A/T)fiGG-type elements.p372- blasts transfected with aconstitutive promoter:hGH reporter hGH is similar to SMP-2 in that it contains a single E-box gene construct. Myoblasts were transfected with construct pXGH5, motifplusallfour CC(A/T)fiGG-typeelements. However, and conditioned medium aliquots were analyzed for secreted hGH at unlike SMP-2, itlacks a 352-bp segment located between the various times during the 12-day posttransfectionobservation period. first andsecond E-boxes. The 108-bpregion located upstream Culture medium was replaced on days 4 and 8. Data shown between the day 9 and 12 time pointswas collected 10.5 days after transfection. from the first E-box in p372-hGH is 85% conserved between Thedatarepresentthemean S.E. for two separate myoblast mouse and humanVSM a-actin genes but only49% conserved transfections performed at the same time as transfections described between the mouse and chicken genes (9). The insert from in the legend for Fig. 6. % Acelylatlon ~~~~
C
B
A
T * l
FB
~
MB
”
~
~
*
12000
cis-Regulatory Regionsof the VSM a-Actin Gene 130
I
E
-1 was sufficientto inducemuscle cell-specific transcriptional activity (9). A larger segment of the 5’-flanking region, how0,ever, encompassing theregion between -1074 and -1 stimulated nearly 10-fold higher levels of transcription in BC3H1 ‘0 52 myogenic cells. VSM a-actin expression in BC3Hl cells is aE density-activated (16, 18),and theobserved differential activf 28 ity of the 1074-bp segment ininducible BC3Hl cells compared 0 with rabbit aortic smoothmuscle cells, whichare constitutive 1 3 5 7 9 1 1 for VSM a-actin expression (19, 20), impliedthat a hypothetDays after Transfection ical cell density response domain might be located between -1074 and -372. Inspection of the 5“flanking region sequence revealed five E-box motifs between -1074 and -372 plus a slightly more proximal E-box (Fig. 7, E - l ) , which is 100% conserved between the mouse, rat, and humanVSM a actin 5’-flanking regions but not present in the chicken gene (9, 28). E-box motifs bind muscle-specific basic helix-loophelix trans-activation factors, as well as nonmuscle cell transcriptional factors such E2A as and the putative neurogenesispromoting protein encoded by the achaete-scute gene in Dro1 3 5 7 8 1 1 sophila. The CC(A/T)6GG motif represents another class of Days afler Transfection cis-acting regulatory elements (21-25), which reside in the5’flanking regions of several contractile proteingenes including the gene encoding VSM a-actin (Fig. 7). Portions of the 5’-flankingregion from themouse VSM aactin gene containing segregated E-box and CC(A/T)&Gtype motifs were ligated tofunctionalreporter genes and transfected into BC3H1 myoblasts to evaluate their effectiveness as transcriptional control elements at various stages of the cytodifferentiationprocess. Reporter gene constructs con1 3 5 7 9 1 1 taining the TATAA box alone (SMP-7) or the TATAA box Days after Transfection plus CC(A/T)sGG-type elementA (SMP-6) or CC(A/ThGGFIG. 6. Temporal analysis of VSM a-actin promoter:hGH type elementsA and B (SMP-5) were not significantly active activity during BC3H1 myoblast cytodifferentiation. Prolif- inBC3H1myoblasts or mouse embryonicfibroblasts. In eratingBC3H1 myoblasts were transfected with pl91-hGH (top recently published studies, a 151-bp segment of the chicken panel), pSB18a-hGH (middle panel),and p373-hGH (bottom panel) pl91-hpH
104
(I)
and monitored for hGH secretion over a 12-day observation period. A parallel set of dishes containing myoblasts were incubated with radiolabeled thymidine to measure DNA synthesis during the posttransfection period (top panel, solid line; right y axis). The medium was replaced on days 4 and 8 after transfection. Datais expressed for k S.E. for threeseparate myoblast eachtimepointasthemean transfections.
6, top). The dilution effect was observed on days 5 and 9, one full day after complete medium replacements were performed. In contrast, the expression of pSB18a-hGH increasedsteadily overthe 12-day observation period, withslightdecreases evident afterday 4and day8 due to complete mediumchanges at these times (Fig. 6, middle). By day12, the transfected myoblasts were completely density-arrestedandhad fully differentiated into highly elongated myocytes, as judged by diminished thymidine uptake and light microscopic evaluation, respectively. Myoblasts transfected with pSB18a-hGH doubled the output of hGH between days 4 and 8 and continof the ued tosecrete the hormoneat high levels up to the end experiment at day 12. In contrast, p372-hGHwas not significantly up-regulated until very late in the time course study at day 9, after which time there was about a 4-fold increase in hGH secretion culminatingat day 12 (Fig. 6, bottom). The results suggest that, whereas the muscle-specific pSB18ahGH construct may respond to cellular events that precede complete cell growth arrest, activation of the shorter musclespecific transcriptional regulatory element present in constructp372-hGHis more closely linkedto full myoblast cytodifferentiation. DISCUSSION
Previous studies have shown that a segment of the mouse VSM a-actin 5”flanking region located between -372 and
-1 074
-724
-850
A Cell-type Restricted Mouse (-) Element
-330 t
-,,:1.ig7
Unreslricted Mouse VSM a-Actin Promoter
.
““”“I
B
.
. ”_,
L
. . . .
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-
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Chicken (-) Element
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FIG. 7. Diagram illustrating cis-acting regulatory elements in the mouse VSM a-actin 5‘-flanking region. Panel A , the relative positions of E-box motifs in the 1074-bp 5”flanking region contained in both the SMP-1 and pSB18a-hGH reporter gene constructs described inthisreport. A putative cell density response element (CDRE)is located between -372 and -724. Panel B , a more detailed view of the proximal 330-bp portion of the 5”flanking region that includes four CC(A/T)6GG-type motifs ( A - D ) , the first E-box ( E - I ) ,the boundaries of nonselective core promoters for both the mouse and chicken VSM a-actin genes, and boundaries of the negative transcriptional regulatory elements for both mouse and chicken genes. The sequence of the 33-bp negative regulatory region is enclosed in dashed line brackets. Open boxes on both sides of the solid line in panel B denote CC(A/T)6GG-typemotifs that are identical in both themouse and chicken genes. The cross-hatched boxes below the solid line refer to CC(A/T)&G-type elements C and D, which are not present in the chicken and are either unique to mammalian species (element C) or only found in the mouse gene (element D). The start of transcription is indicatedby +l.
cis-Regulatory Regions of the VSM a-Actin Gene
12001
VSM a-actin 5”flanking region, which corresponds in size region of the chicken gene. However the 12-bp negative elemouse CC(A/T)&G element D and position to the mouse promoter usedin SMP-5, was ment comprising both the shown to be relatively ineffective at mediating transcription and the adjacent GA pair (-203 to -192) is 75 and 83% homologous to human and chickengenes, respectively. Interintransfectedskeletalmyoblasts,fibroblasts,andcardiac estingly, this conserved 12-bp motif is contained within a myocytes from chicken embryos but surprisingly robust in promoting reporter gene expression in chicken aortic smooth large cis-acting element located between -257 and -151 in muscle cells (26, 28). Although we have not performed trans- the chicken VSM a-actin gene that was recently shown to fections with authentic mouse aortic smooth muscle cells, we exhibit both negative and positive modes of transcriptional control in a species-dependent and cell type-specific manner have found that SMP-5 is completely inactiveinprimary culture aortic smooth muscle cellsfrom therabbit,’ suggesting (28). The differential expression we have observed between that theoperative genomic elements involved in VSM a-actin fibroblastic and myogenic cells from the mouse suggests that gene expression may be different in birds and mammals. there may be acomplex array of trans-acting factors that Inclusion of CC(A/T),GG-type element C in reporter con- interact within thevicinity of element D to produce cell typestruct SMP-4 resulted in nearly 90% substrate acetylation inspecific patterns of transcriptional repression. undifferentiated, subconfluent myoblasts and 100% acetylaThe activity of the muscle-specific transcriptional control tionin developmentallymore advancedpreconfluentand element(s) contained in SMP-1 and SMP-2 was most propostconfluent myoblasts. However element C did not mediate nounced in postconfluentmyoblasts. SMP-1 and SMP-2 both a muscle-specific pattern of transcriptional activation since contain E-box motifs from 5’-flanking region sequences lomore that80% substrate acetylation alsowas noted in embry- cated between -1074 and -224; SMP-2 contains a single Eonic fibroblasts that do not express VSM a-actin mRNA (9). box, and SMP-1 contains all six E-boxes. trans-Activation These observations indicated that the region between -191 factors specific for E-box motifs in the VSM a-actin 5’and -150 encompassingelementC,constitutes an “unreflanking region have not yet been identified, although it is stricted” promoter (Fig. 7), which behaves similarly in both reasonable to suggest that such factors may accumulate in fibroblastsand myoblasts. Theunrestricted VSM a-actin postconfluent BC3H1 myoblasts and mediate transcription promoter in the mouse gene extends to -191, which is 70 bp from the SMP-1 and SMP-2 promoters. On the other hand, farther upstream than the functionallyanalogous, “core pro- undifferentiated preconfluent myoblasts are not yet contactmoter” located between -122 and -1 of the chicken VSM a - inhibited for cell growth and, therefore, are not in a densityactin gene (26,28) (Fig. 7). Thus, althougha segment of DNA activatedcondition, whichmaybenecessaryformaximal containing two CC(A/T)6GG-typemotifsissufficientfor expression of basic helix-loop-helix proteins and full activaVSM a-actin promoter activation in chicken, a portion of the tion of the SMP-1 and SMP-2 constructs. Additionally, the 5”flanking region occupied by three CC(A/T)6GG-type ele- early stages of myoblast cytodifferentiation may initially inments is required in the mouse. volve the inactivation of constitutively expressed,negative Promoter activitywas restricted to myogenic cells when the regulatory factors that serve to sustain a repressed transcripregion spanning between -224 and -191 wasincluded in tional condition by binding to CC(A/T)6GG-type elementD. reporter gene construct SMP-3. The expression of SMP-3 The loss of negative regulatory factors that bind to element was attentuated in both undifferentiated subconfluent myo- D may provide one explanation for the relatively high level of blasts and fibroblasts, suggesting that the 33-bp region be- activity exhibited by SMP-3 in preconfluent myoblasts. Futween -224 and -191 contains anegative transcriptional ture studies will be directed toward evaluating thevalidity of element(s). This segment of the 5”flanking region includes these hypotheses. the CC(A/T),GG-like element Dpreviously shownto be To examine the temporal aspects of transcriptional actiunique to the mouse VSM a-actin gene (9). The size of the vation during BC3H1 myoblast cytodifferentiation, we preoperative negative element within the 33-bp region appears pared three fusion plasmids that contained segments of the to be cell-type dependent. Analysis of constructs containing VSM a-actin 5’-flanking region ligated to a functional hGH a deletion withinelement Drevealed thatretention of a gene. This strategy was used to maintain continuity between GGGA motif between -195 and -192 was sufficient to mainindividual time points in myoblast transfection experiments tain a fully repressed transcriptional state in AKR-2B fibroand thuspreserve evolvingcell density-dependent adaptations blasts, whereas it was only partially able to do so in subconthat occur during cytodifferentiation, e.g. changes in cell-cell fluent BC3H1 myoblasts. Full repression was still observed in fibroblasts transfected with STU4CAT in which the two and/or cell-matrix contact. We reasoned that subtle cellular GG residues at the 5’-end of the 4-bp motif were converted adaptations that affect VSM a-actin transcriptional activity t o AA. These observations suggest that the downstream GA might be obscured if different culture dishes were used for each time point in the developmental series. The pl91-hGH residues at -193 and -192 may play a significant rolein mediating the negative regulatory response in fibroblast cell reporter gene construct used in the analysis contained the types. This pair of residues is conserved in the mouse, rat, same insert as the SMP-4 CAT reporter gene construct and human, and chicken VSM a-actin genes (9, 26, 28). On the was expressed in actively proliferating myoblasts at very high other hand,a full-sizeelement D was required for muscle cell- levels up to 4 days after transfection. The activity of ~ 1 9 1 restricted behavior, since a substitution mutant containing a hGH rapidly declined as the myoblasts entered a period of size-matched but unrelated sequence in place of D was as density-arrested growth at day 5 . This observation suggested active in subconfluent myoblasts as SMP-4, which lacks D. that plSl-hGH, which lacks the 12-bp negativeregulatory element describedabove,became inactivatedasthepostThisfindingalso implies thatthe region upstreamfrom element D located between-224 and -204 that was left intact confluent myoblasts developed into fully differentiated myoin the STU4CAT variant probably does not contribute to the cytes. Virtually the same results were obtained in four inderestricted behavior of SMP-3 in subconfluent myoblasts. This pendent transfection experiments with pl91-hGH. Although 21-bp region is 67% homologous to the human VSM a-actin we cannot completely rule out the possible loss of pl91-hGH from transiently transfected myoblasts prior to the onset of gene sequence and not at all similar to the corresponding growth arrest, it is important to note that other constructs ”,J. C. Reeser and A. R.Strauch, unpuhlished data. containing portions of the VSM ru-actin 5”flanking region
12002
cis-Regulatory Regions
of the V S M a-Actin Gene
remained transcriptionally active in proliferating myoblasts. closely linked to absolute density arrest of cell growth and Inaddition, myoblasts transfectedwiththeconstitutively complete cytodifferentiation.The five distalE-box motifs expressedpXGH5reporter gene constructsecretedhGH present only in construct pSB18a-hGH may amplify tranthroughout the 12-day observation period, despite two com- scription during the early stages of the cytodifferentiation plete medium changes. process perhaps by serving as high affinity binding sites for Seemingly differentresults were obtainedwithSMP-4, basic helix-loop-helix proteins that areinduced as a response which contained the same portion of the 5'-flanking region to enhanced cell-cell or cell-matrix contacts. On the other as pl91-hGH and was fully active in preconfluent and post- hand, the five distal E-boxes do not appear to be absolutely confluent myoblasts. The apparent discrepancy may be ex- required for high level transcriptional activation of the VSM plained in termsof myogenic cell developmental stageeffects. a-actin gene,since CAT construct SMP-2, which contains Myoblasts were transfected with SMP-4when 50% confluent only the proximal E-box plus about 450 bp of the 5'-flanking and then harvested 2 or 4 days later as either preconfluent region located between the first and second E-box (refer to (80-90% confluent) or postconfluent (100% confluent) myo- Fig. 7), functioned nearly as well as SMP-1 in preconfluent blasts, respectively. In both cases, the cells transfected with and postconfluent myoblasts. Preliminaryresultsobtained SMP-4 were observed to be undifferentiated at the time of using BC3Hl myoblasts stably-transfected with fusion gene harvest based on light microscopic evaluation techniquesrou- constructs containing 5'-flanking region sequenceslocated between -724 and -1 linked to hGH indicate that this tinely employed in our laboratory (27). Myoblasts used for region pl91-hGH reporter gene assays were likewise transfected at alone exclusive of E-boxes 2 through 6 is sufficient to promote low densitybut were maintainedfor a full12 daysafter high level, density-dependent tran~cription.~ Finally, the low transfection. Unlike myoblasts containing SMP-4, the myo- activity of p372-hGH in preconfluent myoblasts (Fig. 6, day blasts transfected with pl91-hGH became completely density 3 ) was very different from that exhibitedby the larger, more arrested 6 days after transfection and continued on to fully active SMP-2 construct in myoblasts at a similar developdifferentiate into myocytes. We speculate that positive tran- mentalstage, even thoughbothconstructscontainedthe scriptional regulatory elements residing in pl91-hGH and proximal E-box. Taken together, these observations imply SMP-4, which allow full expression in undifferentiated myo- that a portion of the 5"flanking region of the VSM a-actin blasts, become silenced in mature myocytes. For example, gene devoid of E-boxes and locatedbetween -724 and -372 fully differentiated myocytes may accumulate regulatory fac- may bevery important for mediating the early transcriptional tors that directly or indirectly inactivate transcription from response to high cell density. Gel mobility shift assays and the exogenous pl91-hGH fusion plasmid. Althoughthe mech- nuclease protection experiments should resolve the nature of DNA-protein interactions within the 5"flanking region of the anism of thisinactivationisnot known,myocyte-specific VSM a-actin gene and indicate whether these interactions factors may block, bind, or covalently modify factors that process. Recently, normally mediate transcriptional activation at, or in the vi- are modified during the cytodifferentiation cinity of, one ormore of the three CC(A/T)6GG-type elementswe showed that substrate-associatedmacromolecules isolated present in pl91-hGH. Alternatively, newly synthesized, my- from mature BC3H1myocytes were capable of inducing myoocyte-specific trans-activation factors that bind to distalcis- blast cytodifferentiation in the absence of cell-cell contacts response to high cell regulatory sequences in the 5'-flankingregion of the gene, e.g. (27). In this context, the transcriptional E-box motifs or a hypothetical cell density response domain density may be initiated by the accumulation of threshold (see below), may complex with and/or relocate pre-existing amounts of substrate-associated macromolecules in the extracellular microenvironment. Of particular interest in future transcriptional factors that are targeted to CC(A/T)6GG-type elements A, B, and/or C (or their intervening sequences) in studies will be the identification of trans-activation factors inducedby substrate-associated macromolecules that may subconfluent myoblasts. Inthisscheme,theseproximal bind tospecific regions of the mouse VSM a-actin 5"flanking CC(A/T)6GG-type elements could function in subconfluent myoblasts simply as serum response elements inview of their region. homology to the c-fos gene (2, 21) and demonstratedpositive REFERENCES response to serum growth factor^.^ In the intact 13-kilobase VSMa-actin gene, however, developmental stage-specific 1. Kedes, L. H., and Stockdale, F. E.(eds) (1989) Cellular and Molecular Biology of Muscle Development, Alan R. Liss, Inc., transcription may be achievedbycoupling these putative New York, pp. 429-789 serum response elements to more distalcis-acting regulatory 2. Sartorelli, V., Webster, K. A., and Kedes, L. (1990) Genes & Deu. sequences that serve as binding sites for muscle-specific trans4,1811-1822 activation factors. 3. French, B. A,, Chow, K.-L., Olson, E. N., and Schwartz, R. J. Contrasting results were obtained from studies on myo(1991) Mol. Cell. Biol. 1 1 , 2439-2450 blaststransfectedwiththeother two hGHreporter gene 4. Muscat, G. E. O., and Kedes, L. (1987) Mol. Cell. Biol. 7 , 40894099 constructs. pSB18a-hGH resembles construct SMP-1 in that 5. Grichnik, J. M., Bergsma, D. J., and Schwartz, R. J (1986) Nucleic both share the same 1074-bp portion of 5'-flanking region. Acids Res. 1 4 , 1683-1701 Accordingly, the expression of pSB18a-hGH increased stead6. Vandekerckhove, J., and Weber, K. (1981) Eur. J. Biochem. 113, ily during the post-transfection observation period that cul595-603 minated a t day 12 when the densely packed, transfected cells 7. Ruzicka, D. L., and Schwartz, R. J. (1988) J. Cell Bid. 107, had completely differentiated intohighly elongated myocytes. 2575-2586 8. Sawtell, N. M., and Lessard, J. L. (1989) J. Cell Biol. 109, 2929However, the p372-hGH construct was not activated until 2937 after day 9 whenthe cells became completelygrowth-arrested. These datasuggest that although pSB18a-hGH responds pos- 9. Min. B.. Foster. D. N., and Strauch, A. R. (1990) J. Biol. Chem. 265,16667-16675 itively to cellular adaptations that accompany the gradual 10. Minty, A., and Kedes, L. (1986) Mol. Cell. Biol. 6, 2125-2136 increase in myoblast density, transcriptional activation pro- 11. Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., vided by the 5"flanking region insert in p372-hGH is more Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, " E . S. Stoflet, L. J. Schmidt, P. K. Elder, D. N. Foster, A. R. Strauch, and M. J. Getz, submitted for publication.
D. N., Foster, J. G., Cogan, L. K., Foster, and A. R., Strauch, work in progress.
cis-Regulatory Regions S., Zhuang, Y., and Lassar, A. (1991) Science 2 5 1 , 761-766 12. Gorman, C. M., Moffat, L. F., and Howard, B. H. (1982) Mol. Cell. Biol. 2, 1044-1051 13. Seldin, R. F., Burke-Howie,K., Rowe, M. E., Goodman, H M., and Moore, D. D. (1986) Mol. Cell. Biol. 6 , 3173-3179 14. Luckow, B., and Schutz, G. (1987) Nucleic Acids Res. 15, 5490 15. Schubert, D., Harris, A. J., Devine, C. E., and Heinemann, S. (1974) J . Cell Biol. 6 1 , 398-413 16. Strauch, A. R., and Rubenstein, P. A (1984) J. Biol. Chern. 259, 3152-3159 17. Ausubel, F. M., Brent, R., Kingston, R. E., Moore, D. D., Seidman, J. G., Smith, J . L., and Struhl, K. (1987) Current Protocols i n Molecular Biology, John Wiley & Sons, Inc., New York 18. Strauch, A. R., Offord, J . D., Chalkley, R., and Rubenstein, P. A (1986) J. Biol. Chem. 2 6 1 , 849-855 19. Strauch, A. R., and Reeser, J. C. (1989) J. Biol. Chem. 2 6 4 , 8345-8355
of the VSM a-Actin Gene
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20. Liau, G., Janat, M. F., and Wirth, P. J. (1990) J. Cell. Physiol. 142,236-246 21. Boxer, L. M., Prywes, R., Roeder, R. G., and Kedes, L. (1989) Mol. Cell. Biol. 9, 515-522 22. Prywes, R., and Roeder, R.G. (1987) Mol. Cell. Biol. 7,34823489 23. Schroter, H., Shaw, P. E., and Nordheim, A. (1987) Nucleic Acids Res. 1 5 , 10145-10158 24. Treisman, R. (1987) EMBO J. 6 , 2711-2717 25. Walsh, K. (1989) Mol. Cell. Biol. 9, 2191-2201 26. Carroll, S. L., Bergsma, D. J., and Schwartz, R. J (1988) Mol. Cell. Biol. 8 , 241-250 27. Strauch, A. R., Berman, M. D., and Miller, H. R. (1991) J. Cell. Physiol. 1 4 6 , 337-348 28. Blank, R. S., McQuinn, T. C., Yin, K. C., Thompson,M M., Takeyasu, K., Schwartz, R. J., and Owens, G. K (1992) J. Biol. Chern. 267,984-989
