is quite sluggish in turnover of methyl mercuric chloride at 1 min, n-butylmercuric chloride at 15 min, and phenylmercuric acetate at 15 min'. cis-. Butenylmercuric.
Molecular
basis
of bacterial
organomercurial
and inorganic MARK
CHRISTOPHER
T
AND
L. VERDINE
GREGORY
Department 02115,
WALSH,
D. DISTEFANO,
Chemistry and Molecular
of Biological
Pharmacology,
Bacteria
mediate
organic nontoxic sible for operon, that also
mercuric salts by metabolic conversion to elemental mercury, Hg(0). The genes responmercury resistance are organized in the mer and such operons are often found in plasmids
resistance
to organomercurial
bear drug resistance determinants. three of these mer genes, merR,
and in-
We have merB, and
merA, and have studied their protein products via protein overproduction and purification, and structural and functional characterization. MerR is a metalloregulatory DNA-binding protein that acts as a repressor.of both its own and structural gene transcription in the absence of Hg(II); in addition it acts as a positive effector of structural gene transcription when Hg(II) is present. MerB, organomercury lyase, catalyzes the protonolytic fragmentation of organomercurials to the parent hydrocarbon and Hg(II) by an apparent SE2
mechanism.
MerA,
FAD-containing
enzyme
and
with homology
mercuric
ion
redox-active
reductase,
is an
disulfide-containing
to glutathione
reductase.
It has
evolved the unique catalytic capacity to reduce Hg(II) to Hg(0) and thereby complete the detoxification scheme. WALSH, C. T; DISTEFANO, M. D.; MooRE, M. SHEWCHUK, L. M.; VERDINE, G. L. Molecular basis of bacterial resistance to organomercurial and inorganic mercuric salts. FASEB]. 2: 124-130; 1988.
J.;
Key Words: DNA-binding tox/ication mercuric reductase
protein
.
heavy
metal
de-
mercury resistanceorgano-
mercury lyase
ORGANOMERCURIAL
COMPOUNDS
ARE TOXIC to Organisms
by virtue of their avid ligation to thiol groups in proteins, which, coupled with their lipophiicity, leads to their bioaccumulation in the food chain (1). The predominant natural form is methyl mercury, arising from biomethylation of mercuric ions via methyl B12 in sewage and sediments (2); aryl mercurials are industrial contaminants, e.g., from the paper industry. The most common inorganic mercuric salt is HgS, comprising up to 7% mercury by weight of the ore, red cinnabar. 124
mercuric
MELISSA Harvard
to
J.
Medical
MOORE,
salts
LISA M. SHEWCHUK,
School, Boston,
M#{128}rssachusetts
USA
ABSTRACT
subcloned
resistance
Some 1010 tons of rock (average 80 tg Hg/kg) are weathered globally each year, releasing 800 tons of Hg(II) into the environment; the oceanic reservoir of mercury is estimated at 200 million tons. The dissolved Hg(II) concentration in water typically ranges from 0.03 to 2.0 sg/liter (i.e., up to 10-8 M) and, with this as backdrop, it is not surprising that bacteria have developed resistance mechanisms to detoxify both organomercurial and inorganic mercuric salts. As shown in the biogeochemical cycle for mercury in Fig. 1, bacteria recycle organomercurials back to nontoxic, volatile, elemental Hg(0) by a two-enzyme strategy. The first enzyme, organomercury lyase (merB gene product), catalyzes a protonolytic fragmentation of a carbon-mercury bond to yield, for example, Hg(II) and methane from a methyl mercuric salt. The second enzyme, mercuric ion reductase (merA gene product), takes the Hg(II) ions thus produced and catalyzes a two-electron reduction to yield Hg(0), using NADPH as the electron donor. Both RHgX and HgX2 salts are toxic because of their coordination to protein sulfhydrylgroups, whereas Hg(0) is inert, nontoxic, and lipophilic, and simply volatilizes out of the bacterial cell. Because of the novel chemistry of these two enzymes we have sought to purify and characterize them both structurally and enzymologically to delineate their catalytic mechanisms. The genes that encode these two enzymes and additional mer resistance components are, in general, plasmid encoded, and in some cases are present on transposable elements (e.g., Tn501) within the plasmids. These genes are inducible with low levels of mercury, are widespread in bacterial populations, and often segregate with antibiotic resistance genes as in the Shigella R100 plasmid (3). The most common mer resistance systems, exemplified by Tn501 and R100, are the so-called narrow-spectrum resistance determinants that, lacking the merB gene, confer resistance to Hg(II) salts but not organomercurials. Broad-spectrum resistance to organomercurials in Escherichia coli and Staphylococcus aureas requires a functional merB gene in addition to the other mer genes. The mer genes of both Tn501 and R100 have been sequenced (4-6) as has the merB gene for Staphylococcus (7).These genes are clustered physically and functionally in an operon; a typical organization is shown in Fig. 2. Each operon contains a regulatory gene, merR, and four or five structural genes. merT
0892-6638/88/0002-0124/$01.50. © FASEB
polymerase-binding shown in Fig. 3B. vents genes
Hg#{176},, 2+ Hg i-
_________
reductase
Figure
1. Biogeochemical
CH5 Hg’
organomercury lyase
cycle of mercury.
Adapted
[Sediment
from
with the RNA polymerase-binding loci. Thus, the dual repressor function of MerR in mediating both negative autoregulation and repression of structural gene expression can be rationalized by consideration of the fact that the overlapping promoter regions for both mer mRNA molecules are occupied by MerR (Fig. 3B). The molecular basis of mer gene transcriptional activation by MerR in the presence of coactivator Hg(II) is currently under examination, but preliminary results indicate that MerR binds to the same site regardless of the presence or absence of the coeffector. Thus it appears that MerR does not necessarily achieve repression by physical occlusion of the RNA polymerasebinding site; furthermore, the activation of RNA polymerase binding to the promotor by MerR appears to be mediated through the same operator site as that which is active in repression. The phenomenon of traninteraction
+
CH3Hg
mercuric
transcription and divergent
sites for the mer structural genes, as This protein-DNA interaction preof the downstream mer structural (upstream) merR gene by a direct
I
ref 3.
and merP encode Hg(II) and RHgX transport and periplasmic binding proteins, respectively (8), whereas merA encodes mercuric ion reductase and merB organomercury lyase. merD, on the other hand, has not yet been shown to encode a translated protein (8). Of these, we have focused on the merR, merB, and merA gene products for subcloning, protein overproduction and purification, and structural and mechanistic characterization. Below, we summarize results on each.
scriptional repression at the same operator
MerR,
and activation being modulated site, as observed in the case
of
is apparently
precedented in an analogous case involving regulation of the arabinose operon by the AraC protein (16). Further studies that are aimed at delineating the nature of the active transcription complex in mer structural gene expression are currently in progress.
MerR:
METALLOREGULATORY
DNA-BINDING
PROTEIN
The effector
MerR
protein is both for the mer structural
a positive and a negative genes and is of interest as
a prototypic sion systems. heat shock
system for metal-regulated gene expresOther examples may include eukaryotic (9) and metallothionein transcriptional regulation (10, 11), as well as iron uptake gene regulation in bacteria (12). As a positive effector, both the MerR protein and low levels of Hg(II) (ca. i0 M) are required to turn on mer structural gene transcription (see ref 13 and refs cited therein). In the absence of Hg(II) the MerR protein acts as a repressor to prevent structural gene transcription and also appears to negatively autoregulate its own synthesis (13, 14); the latter property necessarily dictates that the repressor be present in vanishingly small quantities and thus renders its isolation and purification virtually impossible from host cells bearing the wild-type mer operon. By cloning the merR gene, with its 5’ negative autoregulatory region deleted by Bal-31 exonuclease digestion, behind the tac promoter we recently obtained good overproduction of the MerR protein. We have purified it to homogeneity and verified that it is a 144-residue polypeptide (16 kDa), transcribed divergently from the mer structural genes, and is active as a 32-kDa dimer (15). Specific interaction with the mer promoter/operator DNA region was detected by DNase footprinting studies (Fig. 3A), which showed that the MerR dimer protects a palindromic 26-base-pair (bp) DNA fragment that straddles the -10/-35 RNA BACTERIAL
RESISTANCE TO MERCURIALS
Initial analysis of the primary sequence of the 144residue MerR polypeptide suggests two possible domains (15). The NH2-terminal half has two 20-residue stretches with a propensity for formation of a DNA-binding helix-turn-helix motif at residues 9-29 and 55-75. This supersecondary structural motif is the DNA recognition element in a variety of prokaryotic repressor proteins (see ref 17 for a compilation). The second domain of interest is in the COOH-terminal portion where residues 115-126 contain a C X C H X7 C sequence .
.
.
.
.
0
R
transport protein regulatory protein
Under
study
reductase
merR
-*
regulatory
protein
Controlling
mer
DNA activity -‘
enzyme detoxifying
to -+
Hg” by reduction
Hg#{176}
enzyme cleaving organomercurials to RH and Hg
Figure 2. Typical arrangement of genes in a mer operon conferring broad-spectrum resistance to both organomercurial and inorganic mercuric salts.
125
A 1 2 3 4 56 II
II
I
lism, with the carbon-cobalt bonds in methyland 5’-deoxyadenosyl-coenzyme B12 the best studied (18, 19). Organomercurial bonds, unlike many other carbonmetal bonds, are quite stable in water and are resistant to protonolytic decomposition; less than 1% protonolytic fragmentation of CH3HgC1 to OH4 occurs after treatment with concentrated HC1 for 100 mm. Thus the task of this enzyme is not a trivial one. To facilitate investigation we subcloned the merB gene, encoded on plasmid R831 in E. coli J53-1, behind the phage T7 promotor. Using a double-plasmid, T7 RNA polymerase system (20) we overproduced MerB to 3% of the cell protein. This permitted its ready purification to homogeneity in an active form (21), which is a monomer of 22 kDa without detectable metals or cofactors. Broad substrate specificity for primary, secondary, and tertiary alkyl mercuric halides as well as for allyl, vinyl, and aryl mercuric halides was observed. The enzyme is quite sluggish in turnover of methyl mercuric chloride at 1 min, n-butylmercuric chloride at 15 min, and phenylmercuric acetate at 15 min’. cis-
7 GC I
I
I
I
Butenylmercuric chloride is the fastest substrate yet detected at 240 min5 (22).Although these numbers are not impressive on an absolute scale, the methyl case is
B 26 bp Protected Region
I
I 5 3
mwmRNA
#{149} -CCATATCGCTTGACTCCGTACATGAGTACGGAAGTAAGGTTACGCTATCCAATTT-3’
-GGTATAGCGAACTGAGGCATGTACTCATGCCTTCATTCCAATGCGATAGGTTAAA-5’
m,A mANA
Figure
I protection assay for the interaction of MerR of the Tn501 mer operon. Lanes 1-7 correspond to 0, 2, 50, 500, 0.8, 20, and 200 pM MerR protein. B) Partial structure of the operator/promotor region for the Tn501 meT operon. The 26-bp protected region refers to DNA that is protected from DNase attack in the presence of MerA protein, as shown in A. with
3. A) DNase
the regulatory
region
that is a candidate for an Hg(II)-binding site. Both the NH2and 000H-terminal domains of MerR are being probed by engineering of mutant MerR proteins in which putatively critical residues have been specifically replaced. The mutants will be analyzed directly for DNA-binding properties, metal ion responsiveness, and in vivo resistance phenotype effects. The ultimate goal is to understand precisely how a metal-based molecular switch affects specific gene transcription. MerB:
ORGANOMERCURY
LYASE
io-
108-fold and the aryl mercurial 6 x fold over the nonenzymic rates (23). To probe how the enzyme might cleave carbonmercury bonds, we analyzed the stereochemical outcomes with cis-butenylmercuric chloride and endonorbornyl mercuric bromide (2). Both substrates were processed with retention of configuration in D2O, which argues against radical intermediates. A discrete carbonium ion seems unlikely because that should add water to produce an alcohol product, not the observed alkane. A third possibility, a discrete carbanion, seems too high in energy (consider CH3HgC1 proceeding via a naked CH3 species) to be rapidly produced, suggesting that a concerted SE2 mechanism of carbon-mercury cleavage and carbon-hydrogen formation, as proposed for nonenzymic protonolysis of vinyl and alkyl mercurials (24-26), is occurring in the enzyme’s active site. This would be the first case of an enzymic SE2 process. A mechanistic scheme is proposed in Fig. 4 where coordination by two enzyme cysteine SH groups is hypotheaccelerated
-
sRHgSR
CII
H
RH)
..ii
4 -
Organomercury
that it utilizes Carbon-metal 126
lyase,
MerB,
an organometallic bonds are rare
is an unusual enzyme in species as a substrate. participants in metabo-
A
RSH
>
S-H
Excess
Figure 4. A mechanistic proposal cury lyase by an SE2 mechanism.
+
RSHQSR
A-H for catalytic
action
of organomer-
WALSH
ET AL.
sized to activate the carbon-mercury bond for protonolysis (23) because such nucleophiic acceleration of electrophilic reactions at carbon-mercury bonds (e.g., with 12) are known (27). Cleavage via the four-center transition state shown would yield the product hydrocarbon, R-H, and enzyme bis-coordinated Hg(II). Only with exogenous thiols present in the buffer will Hg(II) exchange occur, leaving the enzyme ready for another catalytic cycle (22).
MerA:
MERCURIC
ION
the following
Men
product,
exhibits
Hg(II)
to Hg(0)
+
NADP
+
NADPH
+
H
-‘
Hg(0)
+
reductases
N jj
Tn21
p1258
:
sequenced
to
date,
whereas
AS
two
C
II
I
76
-
MerA
8
119
leu gly
-
27
-
82
lys pro gly glu val
-
met thr
ala ala
pro
-
37
-
met thr
asp ser
ala
-
14
134
-
thr
val asn val gly
555
-
gIn leu ser Icysicysi
ala gly
-
-
140 561
_IJ_l_
pairs
in the
proteins
of the
Tn501 mer operon.
pairs
of mercuric
reductase,
we have
initiated
a muta-
genesis/protein engineering approach (31, 32, 34). We have generated the single ala and ser and double ala mutants listed in Table 1, and have isolated the wildtype and each mutant enzyme in 100-mg quantities via a tac promoter-based overproduction scheme coupled with Orange A matrix affinity chromatography. The mutants were assessed for both in vivo Hg(II) resistance metric,
ties
phenotype and in vitro and catalytic properties.
physical,
include
spectrophoto-
The catalytic properNADPH oxidation,
Hg(II)-dependent oxidase activit)c NADPH-thioNADP transhydrogenation, and 5,5 ‘-dithiobis(2-nitrobenzoate) reduction as well as anaerobic 203Hg(II) volatilization [via reduction to Hg(0)]. Anaerobic reoxidation of reduced enzyme-bound fiavin by Hg(II), which probes a single NADPH-dependent
catalytic
event,
was
also
examined.
Among the active site mutants, ser135 cys140 and ala135 cys140 retain ca. 1-2% levels of the wild-type Hg(II) reductase activity whereas the cys135 ser140 and cys135 ala140 mutants are 5-10 times lower still. Thus both cys135 and cys14o are clearly of catalytic consequence in Hg(II) reduction with the ala135 ala140 double mutant having only 1/2300 the rate of Enz-FADH2 reoxidation by Hg(II) of that calculated for the wild type. On the other hand, all active site mutants do show high
ss aa
levels
471 aa
Figure 5. Primary sequence positioning of cysteines (small black bars) in various mercuric reductases and human glutathione reductase. Alignment of the active site cysteine pairs (AS) shows that Tn501, Tn21, and p1258 mercuric reductases have two additional conserved pairs of cysteine residues (N and C) not found in human glutathione reductase (see refs 4, 5, 32a).
BACTERIAL RESISTANCE TO MERCURIALS
Cysteine
561
547 aa
liii
6.
cysteine pair of MerA, to its active site for reduction. A variant of this “bucket brigade” scheme is displayed in Fig. 7. To assess the respective roles of the three cysteine
of transhydrogenase activity, some up to sixfold wild type, which suggests that at least the NADPH-FAD half-reaction is unimpaired. The ala135 cysj40 and ser135 cys140 enzymes colorfully signal their mutant constitution for they are green and red, respectively, rather than the yellow of wild-type enzyme. This optical perturbation is regiospecific because cys135 R140 enzyme species are consistently yellow. The interpretation, in agreement with the glutathione reductase X-ray structure (35), is of a cys14o thiolateFAD charge transfer complex as shown in Fig. 8. The enzyme color of the green and red mutant enzymes returns to that of unperturbed FAD yellow as the cys14o
that
ii
Human Glutathione Reductase
ala sen ala Icysicysi
-
2RSH
unpaired cysteines are not (Fig. 5). Collected in Fig. 6 are all the cysteine pairs found in the Tn501 MerR, P, T, and A gene products. Given the avidity of Hg(II) for bis-thiol ligation, Brown (33) has proposed a scheme of sequential “handing over” of Hg(II) from one thiol pair to the next, starting with MerP, to MerT, to the 10,13
Tn501
-
ala
stoichiometry:
Most enzymological studies have been conducted with the Tn501-encoded enzyme (28, 29) for which the DNA sequence is available (5). Initial studies revealed that this FAD-containing enzyme possesses a redox-active site disulfide spanning residues 135-140. In this region, mercuric reductase displays remarkable active site homology with the fiavin-containing disulfide oxidoreductases, glutathione reductase and lipoamide dehydrogenase (30). However, only mercuric reductase can generate Hg(0) catalytically with a turnover number of ca. 400 min. In recent work we have begun to investigate what structural features contribute to this unique activity (31, 32). In addition to the cysteine pair at positions 135 and 140, Tn501 mercuric reductase has two more pairs of cysteines at residues 10,13 and 558,559 (there being a total of 561 residues). These pairs are conserved in all mercuric
21
31
Figure
Hg(SR)i
Ieu val
MerP
ala
Ehis
-
113
REDUCTASE
Mercuric ion reductase, the merA gene the unique catalytic ability to reduce
with
MerR
of the
127
merR
merT
merP
merA
merD
wwwwwwwwwwwwwww’
I
I
I
I
I
functionl
I
unknown
r.gulator protein (XerR)
Hg#{176}
Cytoplasm
transport protein O(.rT)
Periplasm
0 scavenger protein (NerP)
Figure reductase
7. Proposed (MerA)
scheme
for sequestration
by a bucket
brigade
mechanism
of Hg(II) ions in the periplasm involving the paired cysteine
thiolate anion 5-6, reflecting
is titrated. This thiol exhibits a pKa of the 2-3 pKa unit stabilization of the thiolate anion in the charge transfer complex. Analysis of the NH2- and COOH-terminal cysteine pairs by protein engineering reveals quite different outcomes from mutagenesis of these residues. The cysteine at 10 and/or 13 can be changed to alanine without de-
and their sequential passage to the active site of mercuric residues of MerP, MerT, and MerA. Adapted from ref 33.
tectable effect on the in vivo mercury resistance phenotype. Thus this pair is dispensable, and indeed an 86-501 proteolytic fragment (29) of wild-type MerA is fully active in vitro. The 000H-terminal cysteines at 558,559, however, are required because the ala558 ala559 mutant is inactive for Hg(II) reduction (but fully active in transhydrogenation) in vitro and imparts an HgC12
10-
E U
/ C
9.2 8E
C 5,
0
U
#{149} 7.
0
UI.
6-
C 0
C 0
o
5-
U C
I...-
0
0
U
‘C
w
4-
3
C 0 U C
I.1 /
a,
E
.,-
4
5
I
I
6
7
8
9
10
pH 3-
w 2I0I 300
I 400
I 500
Wavelength Figure
8. Dissipation
of the cys140 thiolate
128
of the charge transfer pertubation, in the ala,,5 cys,40 mutant
anion
resulting mercuric
I 600
(nm)
in the change ion
700
of enzyme
color from red to yellow, by reversible
titration
reductase.
WALSH ET AL.
TABLE 1. Current known MerA mutations whose catalytic and structural properties have been examined
4.
P.; A.;
BARRINEAU,
SUMMERS,
mercury
NH,
terminus
Redox-active
COOH
Mutant
Native
Region
disulfide
cys,o cys,,
1) cys10 ala,, 2) ala10 ala,,
cys,,,
3) ala,,5 4) ser,,, 5) cys,,, 6) cys1,, 7) ala13,
cys140
8) ala,,8 ala,,g
cys,,8 cys,59
terminus
cys,40 cys,40 ala,40 ser140 ala140
supersensitive phenotype (31, 32) in vivo to cells carrying this merA gene mutation. Whether one or both of these COOH-terminal cysteine residues functions to shuttle Hg(II) into the active site, and/or provide tn- or tetradentate coordination to Hg(II) along with cys135 and cys140 at the moment of Hg(II) reduction, remains to be determined. CONCLUSIONS Bacteria
display
to many toxic heavy metal ions, including silver, gold, mercury, chromium, arsenic, nickel, and cadmium. Of these the molecular basis of mercury resistance is now the best understood and the toxicity of mercury to higher life forms the most publicized. Dissection of the
often
plasmid
encoded,
bacterial strategy for metabolism of organomercurial and inorganic Hg(II) salts has turned up three remarkable proteins from this corner of bioinorganic toxicol-
ogy. The MerR protein is a metalloregulatory DNAbinding protein that acts as a metal-specific negative and positive switch for transcription of the mer genes. The merB gene product, organomercury lyase, is a novel catalyst working to effect protonolytic fragmentation of a carbon-metal bond by an apparently concerted four-center SE2 transition state. It also shows protonolytic activity for a few organotin compounds (23). Mercuric ion reductase, the merA gene product, is a newly discovered member of the class of redox-active disulfidecontaining flavoproteins uniquely evolved to reduce Hg(II) to Hg(0). We acknowledge financial support from the National Institutes of Health, General Medical Institute. Also, we thank our coworkers Drs. Au, Begley, Fox, O’Halloran, Schultz, and WaIts for their past contributions, acknowledged in specific references, to the mer research in this group, which was conducted in the biology and chemistry departments at MIT
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