Identification and Elimination of the Competing N ...

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Aug. 2010, p. 5175–5180 0099-2240/10/$12.00 doi:10.1128/AEM.00834-10 Copyright © 2010, American Society for Microbiology. All Rights Reserved.

Vol. 76, No. 15

Identification and Elimination of the Competing N-Acetyldiaminopentane Pathway for Improved Production of Diaminopentane by Corynebacterium glutamicum䌤 Stefanie Kind,1 Weol Kyu Jeong,2 Hartwig Schro ¨der,2 2 1 Oskar Zelder, and Christoph Wittmann * Institute of Biochemical Engineering, Technische Universita ¨t Braunschweig, Braunschweig, Germany,1 and BASF SE, Fine Chemicals and Biotechnology, Ludwigshafen, Germany 2 Received 6 April 2010/Accepted 5 June 2010

The present work describes the development of a superior strain of Corynebacterium glutamicum for diaminopentane (cadaverine) production aimed at the identification and deletion of the underlying unknown N-acetyldiaminopentane pathway. This acetylated product variant, recently discovered, is a highly undesired by-product with respect to carbon yield and product purity. Initial studies with C. glutamicum DAP-3c, a previously derived tailor-made diaminopentane producer, showed that up to 20% of the product occurs in the unfavorable acetylated form. The strain revealed enzymatic activity for diaminopentane acetylation, requiring acetyl-coenzyme A (CoA) as a donor. Comparative transcriptome analysis of DAP-3c and its parent strain did not reveal significant differences in the expression levels of 17 potential candidates annotated as N-acetyltransferases. Targeted single deletion of several of the candidate genes showed NCgl1469 to be the responsible enzyme. NCgl1469 was functionally assigned as diaminopentane acetyltransferase. The deletion strain, designated C. glutamicum DAP-4, exhibited a complete lack of N-acetyldiaminopentane accumulation in medium. Hereby, the yield for diaminopentane increased by 11%. The mutant strain allowed the production of diaminopentane as the sole product. The deletion did not cause any negative growth effects, since the specific growth rate and glucose uptake rate remained unchanged. The identification and elimination of the responsible acetyltransferase gene, as presented here, display key contributions of a superior C. glutamicum strain producing diaminopentane as a future building block for bio-based polyamides. prise the elimination of by-products, the optimization of precursor or cofactor supply, the optimization of product export, and the release from undesired regulatory phenomena. From a metabolic viewpoint, diaminopentane is formed directly from lysine by decarboxylation, meaning that Corynebacterium glutamicum, which produces more than 1,000,000 metric tons of L-lysine per year, is a promising production organism. In a first proof of principle, a C. glutamicum wild type was modified by replacing homoserine dehydrogenase with heterologous lysine decarboxylase (cadA) from Escherichia coli (19, 24). The yield of diaminopentane was, however, rather low, underlining that this modification can only be a first step toward a competitive industrial strain. A milestone toward efficient diaminopentane production was the recent development of a tailor-made production strain of C. glutamicum with genetic optimization of the lysine pathway and the supply of the major precursor oxaloacetate (13). Through systematic analysis of this mutant, N-acetyldiaminopentane was discovered as a thus far unknown by-product. N-Acetyldiaminopentane was secreted by all examined production strains, in which its concentration was up to 20% of that of the desired product diaminopentane. The formation of this acetylated by-product is highly undesirable with respect to a high diaminopentane yield, with minimized carbon loss into other metabolites, and also to product purity for subsequent polymerization, in which high-grade monomers are required. From a metabolic perspective, the elimination of the underlying pathway appears crucial. Unfortunately, the enzyme responsible for acetylation of diaminopentane is not known, re-

Polyamides are polymers containing monomers joined by peptide bonds, with examples being nylons, aramids, and polyaspartates. They are commonly used in textiles, automotives, carpet, and sportswear due to their extreme durability and strength. The most prominent products, polyamides PA 6 and PA 6.6, have an annual market volume of about 6 million tons. Currently, polyamides are derived via chemical routes from fossil raw materials. Due to the shortage of these resources and problems of escalating CO2 production and global warming linked to the underlying processes, bio-based production using renewable resources arises as a promising alternative (11, 23, 27). In this context, fermentative production of diaminopentane (cadaverine) as a monomer building block for polyamides has recently come into focus (19). Using diaminopentane derived from microbial biosynthesis, polymerization with appropriate bioblocks, such as succinate (9, 22), provides completely bio-based products. Moreover, polyamides based on diaminopentane reveal excellent material properties (7). The experience of the past clearly shows that a superior production strain with a high yield, level of productivity, and titer requires substantial modification at different key points of the metabolism, which have to be identified by careful investigation of the underlying metabolism (28). The major targets typically com-

* Corresponding author. Mailing address: Biochemical Engineering Institute, Technische Universita¨t Braunschweig, Gaussstrasse 17, 38106 Braunschweig, Germany. Phone: 49-(0)531-391-7651. Fax: 49(0)531-391-7652. E-mail: [email protected]. 䌤 Published ahead of print on 18 June 2010. 5175

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TABLE 1. Deletion of selected genes potentially encoding diaminopentane acetylation in the rationally derived diaminopentane producer C. glutamicum DAP-3ca Mutant genotype

Deleted locus tag

Primer

⌬act1

NCgl0848

act1-1 act1-2 act1-3 act1-4

5⬘-CCCACTCGAGAACACGTAGAGATCATCG-3⬘ 5⬘-CCATTCATCGTTGGTGATGGGTCCACTGATGTGACAGTGG-3⬘ 5⬘-CCACTGTCACATCAGTGGACCCATCACCAACGATGAATGG-3⬘ 5⬘-GCATACTAGTGCAGATGATGTCACGTCAGC-3⬘

⌬act2

NCgl1208


5⬘-TGGTGTTCCTGGAAGTCCTC⫺3⬘ 5⬘-CAGAGTCAACGCAAACGGTCCATTTTGTGGCATTGCTGGC-3⬘ 5⬘-GCCAGCAATGCCACAAAATGGACCGTTTGCGTTGACTCTG-3⬘ 5⬘-CCAGGTTCTCAACGAGCTAG⫺3⬘

⌬act3

NCgl1469


5⬘-GCTCCTCGAGGCATTGTATACTGCGACCACT-3⬘ 5⬘-CGATTCCGTGATTAAGAAGCGCTTCAACCAGAACATCGAC-3⬘ 5⬘-GTCGATGTTCTGGTTGAAGCGCTTCTTAATCACGGAATCG-3⬘ 5⬘-CGGTACTAGTGTAGTGAGCCAAGACATGG-3⬘

⌬act4

NCgl1614


5⬘-TTGTCTCGAGCGTAGGCTTCATGGTCTTGG-3⬘ 5⬘-GCTCGTACAACGAAACATTGCCCAAGGATAGGAAACAAAGG-3⬘ 5⬘-CCTTTGTTTCCTATCCTTGGGCAATGTTTCGTTGTACGAGC-3⬘ 5⬘-TGGCACTAGTGAACATCGAGCTCGCTAGAAG-3⬘

⌬act5

NCgl2090


5⬘-ACCGACTAGTGACTGAACTATGCCTCTGAG-3⬘ 5⬘-GTGTTGGTGGATATCTACATCGTCACTGACTTGCTCAGGCAG-3⬘ 5⬘-CTGCCTGAGCAAGTCAGTGACGATGTAGATATCCACCAACAC-3⬘ 5⬘-CCAAAACAGTCTGGTTAACTAC-3⬘

⌬act6

NCgl2487


5⬘-ATCCCTCGAGACTTCCACGCTTTCTAC-3⬘ 5⬘-GAGTGTTGCCAGCTTCCACCCTCCTTCAAAAGCAATAGTGC-3⬘ 5⬘-GCACTATTGCTTTTGAAGGAGGGTGGAAGCTGGCAACACTC-3⬘ 3⬘-AGGATCTAGAATCAGACTCATTGGAGTCG-3⬘

Primer sequence

a Constructed mutants, corresponding locus tag identification, and site-specific primer sequences used for the construction (1–4) of each deletion vector and for the verification of each deletion (1, 4) by PCR.

quiring its identification prior to a rational genetic engineering strategy. This is, however, complicated by the fact that a large number of candidates have to be considered. In the genome of C. glutamicum, about 20 different genes are annotated as proteins with acetyltransferase activity. Among these, members of the class of GCN5-related N-acetyltransferases (GNAT) have the common feature that they each transfer an acetyl group from acetyl-coenzyme A (CoA) as a donor to a primary amino group of small molecules or proteins (8, 20, 21, 25, 29). In this regard, the present work describes a detailed study of a superior diaminopentane-producing strain aimed at deletion of the competing pathway N-acetyldiaminopentane. This included the characterization of the enzymatic reaction involved and the identification of the corresponding gene by systems-level profiling, followed by targeted deletion of the corresponding gene and investigation of the obtained mutant. MATERIALS AND METHODS Strains and plasmids. The strains used in the present work comprised the wild-type Corynebacterium glutamicum ATCC 13032 (ATCC, Manassas, VA), the lysine producer C. glutamicum 11424, exhibiting different modifications of the lysine biosynthetic pathway and anaplerotic carboxylation (2), and the diaminopentane producer C. glutamicum DAP-3c, rationally derived from C. glutamicum 11424 by codon-optimized expression of lysine decarboxylase (13). For performing genetic engineering work, Escherichia coli strains DH5␣ and NM522 and plasmids pTc and pClik int sacB were applied as described previously (13). Cultivation and growth conditions. Cultivation was performed as described previously (13). The first preculture was grown in complex medium. For the second preculture and main culture, a minimal medium was applied (2, 13). The

medium for the culture of the strains with the episomal replicating plasmid pClik 5a MCS ldcC additionally contained 25 ␮g ml⫺1 kanamycin. Chemicals. Tryptone, beef extract, yeast extract, and agar were obtained from Difco Laboratories (Detroit). All other chemicals were analytical grade and obtained from Sigma-Aldrich (Steinheim, Germany), Merck (Darmstadt, Germany), or Fluka (Buchs, Switzerland). Recombinant DNA techniques. Construction, purification, and analysis of plasmid DNA and transformation of E. coli and C. glutamicum were performed as described previously (13). Targeted gene deletion. The targeted deletion of genes was carried out as described previously (13), using the corresponding integrative plasmid which cannot replicate in C. glutamicum (1, 3, 4). Genes were deleted by replacement of the coding region with a shortened gene fragment. The primers used for verification of the genetic changes are listed in Table 1. Analysis of substrates and products. The concentration of glucose was quantified in 1:10-diluted cultivation supernatant by a glucose analyzer (2300 Stat Plus; Yellow Springs Instrument, OH). Determination of cell concentration was performed as described previously (12). Amino acid quantification was carried out by high-performance liquid chromatography (HPLC) (14). The same method was adapted by using a modified gradient for quantification of biological polyamines, including 1,5-diaminopentane, 1,4-diaminobutane, 1,3-diaminopropane, and N-acetyl-1,5-diaminopentane. RNA extraction. For total RNA extraction, exponentially growing cells (1.5 mg [dry weight] cells) were harvested by centrifugation (13,000 ⫻ g, 30 s, room temperature). The cell pellet was flash-frozen in liquid nitrogen and kept at ⫺80°C until further processing. Frozen cells were thawed on ice, resuspended in 1 ml lysis buffer (4 M guanidinium thiocyanate, 150 mM sodium acetate [pH 5.2], 18.5 mM N-lauroylsarcosinate), mixed with 600 mg soda-lime glass (0.045- to 0.038-mm precision glass beads; Worf Glaskugeln, Mainz, Germany), and then mechanically disrupted (4°C, 2 times for 60 s each time, 6.5 m/s) using FastPrep-24 (MP Biomedicals, Solon, OH). After removal of cell debris by centrifugation (13,000 ⫻ g, 1 min, 4°C), the supernatant was mixed vigorously with 1 ml acid phenol solution (aquaphenol-chloroform-isoamyl alcohol, 50:48:2) for 30 s.

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After centrifugation (13,000 ⫻ g, 5 min, room temperature), the upper phase was transferred into 1 ml chloroform-isoamyl alcohol (48:2), followed by mixing and centrifugation (5 min, 13,000 ⫻ g, room temperature). The upper phase (700 ␮l) was mixed with 70 ␮l 3 M sodium acetate (pH 5.2) and 1 ml 100% isopropanol and incubated for 1 h at ⫺80°C. After centrifugation (13,000 ⫻ g, 15 min, room temperature), the pellet was resolved in 180 ␮l RNA storage buffer (20 mM sodium phosphate, 1 mM EDTA). For DNA digestion, 20 ␮l of 10⫻ DNase buffer (200 mM sodium acetate, 100 mM MgCl2, 100 mM NaCl) and DNase (30 U; Invitrogen, Karlsruhe, Germany) was added. Subsequently, RNA was purified (innuPrep RNA kit; Analytik Jena, Jena, Germany). Quality control and quantification of RNA were performed using the RNA 6000 Nano LabChip kit and the 2100 Bioanalyzer (Agilent, Santa Clara, CA). As a quality measure, all samples had to have a 23S/16S ratio higher than 1.5 and an RNA integrity number (RIN) between 9.4 and 10. Gene expression analysis. The comparative analysis of gene expression was carried out using DNA microarrays after extraction of total RNA from growing cultures of C. glutamicum, followed by fluorescence labeling and fragmentation. First, a DNA microarray for gene expression analysis was designed from the C. glutamicum genome (GenBank accession no. NC_006958) as a custom array using the online software Agilent eArray. The set of oligonucleotide probes for the genes of interest was designed using the following parameters: (i) 60-bp probe length, (ii) 5 probes per gene, (iii) antisense probe orientation, and (iv) 80°C for the preferred probe temperature of melting. Fluorescence-labeled RNA of C. glutamicum was prepared by direct chemical labeling of 1 ␮g native mRNA using the ULS fluorescent labeling kit (Kreatech Diagnostics, Amsterdam, Netherlands), according to the manufacturer’s protocol. After removal of free ULS labeling using KREApure columns (Kreatech Diagnostics, Amsterdam, Netherlands), the degree of labeling was measured with a NanoDrop-1000 UV/Vis spectral photometer (Peqlab, Erlangen, Germany). Afterwards, 300 ng of RNA labeled with Cy3 and Cy5, respectively, was pooled and fragmented in blocking agent and fragmentation buffer (Agilent, Santa Clara, CA) for 30 min at 60°C (Kreatech Diagnostics, Amsterdam, Netherlands). For competitive hybridization, equivalent amounts of RNA from the two samples to be compared were diluted in 25 ␮l hybridization buffer (Agilent, Santa Clara, CA). Out of the total volume of 50 ␮l, 40 ␮l of each solution was added onto one array and then hybridized at 65°C and 10 rpm for 17 h in a dedicated DNA microarray hybridization oven (Agilent, Santa Clara, CA). The slide was washed in washing buffer 1 (Agilent, Santa Clara, CA) for 1 min with washing buffer 2 (Agilent, Santa Clara, CA), dried, and scanned using the GenePix 4100A scanner (Axon Instruments, Sunnyvale, CA) at 532 nm and 635 nm. Data processing was carried out by the software GeneSpring 10 obtained from Silicon Genetics (Agilent, Santa Clara, CA). To identify statistically significant gene expression changes, the two-sample t test was used. Analysis of acetyltransferase activity. The activity of diaminopentane acetyltransferase was determined in crude cell extract which was prepared from cells grown in minimal medium, as described above. Preparation of crude cell extract using 50 mM Tris-HCl buffer, pH 7.8, and 0.75 mM dithiothreitol (DTT) as a disruption buffer and determination of protein concentration (6) were performed as described previously (2, 13). Assays were performed in a total volume of 1 ml containing 50 mM Tris-HCl buffer (pH 7.8), 0.25 mM acetyl-coenzyme A (CoA), and 200 ␮l cell extract at 30°C. As a substrate, 5 mM 1,5-diaminopentane, 1,4-diaminobutane, or 1,3-diaminopropane was applied. The diamine and acetylcoenzyme A were dissolved in 50 mM Tris-HCl buffer (pH 7.8). The final protein concentration in the assay was in the range of 0.15 to 0.22 mg ml⫺1. In parallel incubations, the reaction was stopped at a particular time (after 0, 2.5, 5, and 20 min) by heating the reaction mixture at 100°C for 10 min. Subsequently, the formed N-acetyldiaminopentane concentration was measured with HPLC and plotted against the stop time of the reaction. The slope of the linear increase was used for calculation of acetyltransferase activity. Negative controls were carried out without substrate. One unit of acetyltransferase activity was defined as the amount of enzyme that formed 1 ␮mol of acetylated product per min at 30°C.

RESULTS Metabolic properties of diaminopentane-producing C. glutamicum DAP-3c. On minimal medium, with glucose as the sole carbon source, C. glutamicum DAP-3c secreted diaminopentane and N-acetyldiaminopentane during the whole cultivation (Fig. 1A). Thus, 20% of the total product was present in the undesired acetylated form. The strain grew exponentially, exhibiting a specific growth rate of 0.25 h⫺1 (Fig. 1B). The wild

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type and the lysine-producing parent strain C. glutamicum 11424 did not accumulate diaminopentane or N-acetyldiaminopentane (data not shown). Enzymatic activity catalyzing diaminopentane acetylation. C. glutamicum DAP-3c revealed enzymatic activity for the acetylation of diaminopentane (Table 2). Boiled cell extract did not catalyze this reaction. The conversion required acetylCoA as the donor of the acetyl group. Based on these results, the next steps focused on the search for transacetylating enzymes utilizing acetyl-CoA as the cosubstrate. The genome of C. glutamicum was thus screened for genes encoding N-acetyltransferases as potential candidates. Overall, 17 N-acetyltransferases were found, including different members of the GNAT superfamily. Gene expression analysis of acetyltransferases. To identify the responsible gene out of the predicted N-acetyltransferase genes, their expression levels were compared between the nonproducing parent strain C. glutamicum 11424 and C. glutamicum DAP-3c, producing diaminopentane and the acetylated variant. This however, did not reveal a statistically significant difference in the expression level of any of the candidates (Fig. 2). Based on this finding, it seemed likely that the observed acetylation of diaminopentane was covered by a constitutively expressed enzyme, probably a side activity in addition to its natural function in the cell. Targeted deletion of acetyltransferase candidates in diaminopentane-producing C. glutamicum. Due to the fact that gene expression analysis did not provide clear evidence for the gene responsible for acetylation of diaminopentane, a targeted deletion of the candidates encoding N-acetyltransferases in the genome of C. glutamicum was carried out. It was expected that a strain lacking the gene encoding the responsible diaminopentane-acetylating enzyme would exhibit at least reduced formation of the acetylated variant. In a first genetic engineering round, six targets were selected. For this purpose, recombinant plasmids, which allow a marker-free replacement of the target gene with a shortened gene fragment, were constructed. These were utilized to delete the NCgl0848, NCgl1208, NCgl1469, NCgl1614, NCgl2090, and NCgl2487 genes, each encoding one of the annotated N-acetyltransferases in C. glutamicum DAP-3c. Physiological properties of acetyltransferase deletion strains. All constructed mutants were found to be viable, so obviously, no essential gene was among the chosen candidates. The corresponding mutants, C. glutamicum ⌬act1 to ⌬act6, were validated for deletion by site-specific PCR. In all cases, the mutant strain revealed a shortened PCR fragment compared to that of the parent strain, which verified the corresponding targeted gene deletion (data not shown). Subsequently the mutants were cultivated in minimal medium on glucose, followed by analysis of the products formed (Table 3). Five mutants revealed unchanged product spectra. Here, the deletion of the corresponding acetyltransferase did not reduce the undesired conversion. In the culture supernatant of the C. glutamicum ⌬act3 strain, however, the level of N-acetyldiaminopentane was below the detection limit (⬍0.1 ␮M). Diaminopentane secretion was even enhanced in this strain. Obviously, the deleted NCgl1469 gene specifically catalyzed the undesired reaction. Through deletion of this gene, the in vitro activity of diaminopentane acetylation was completely elimi-

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FIG. 1. Physiological characteristics of diaminopentane-producing C. glutamicum DAP-3c (A, B) and DAP-4 (C, D) in batch cultures on glucose. The linear correlation among growth, production of lysine, diaminopentane (DAP), and N-acetyldiaminopentane (N-Ac-DAP), and consumption of glucose indicates a metabolic steady state during the cultivation. The data represent the values of three biological replicates for each strain. OD660, optical density at 660 nm.

nated (Table 2). This underlines that this enzyme was exclusively responsible for the undesired reaction. In contrast to diaminopentane, 1,4-diaminobutane (putrescine) and 1,3-diaminopropane were not utilized as substrates (Table 2). The promising deletion mutant C. glutamicum ⌬act3 was desigTABLE 2. Specific in vitro activity of diaminopentane acetyltransferase (NGcl1469) in the diaminopentaneproducing strains C. glutamicum DAP-3c and DAP-4 Addition of acetyl-CoA

Strain

Sp act (mU mg⫺1)a

1,5-Diaminopentane

⫹ ⫺ ⫹ ⫺

DAP-3c DAP-3c DAP-4 DAP-4

38.12 ⫾ 0.53 ⬍0.01 ⫾ 0.00 ⬍0.01 ⫾ 0.00 ⬍0.01 ⫾ 0.00

1,4-Diaminobutane

⫹ ⫹

DAP-3c DAP-4

⬍0.01 ⫾ 0.00 ⬍0.01 ⫾ 0.00

1,3-Diaminopropane

⫹ ⫹

DAP-3c DAP-4

⬍0.01 ⫾ 0.00 ⬍0.01 ⫾ 0.00

Substrate

a The data comprise mean values and standard deviations of three replicates, with corresponding standard deviations.

FIG. 2. Comparative expression analysis of selected genes, functionally annotated as N-acetyltransferases, in the lysine-producing strain C. glutamicum 11424 and the diaminopentane-producing strain C. glutamicum DAP-3c. The data are given as ratios of the expression of 11424 to that of DAP-3c. The chosen cutoffs for significantly different expression ratios are indicated by the hatched areas. The data originate from the values of at least four biological replicates for each strain.

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TABLE 3. Product spectra of acetyltransferase deletion mutants after 24 h on minimal glucose medium Change in product spectra

Mutant genotypea

Diaminopentane

N-Acetyldiaminopentane

Lysine

⌬act1 ⌬act2 ⌬act3 ⌬act4 ⌬act5 ⌬act6

⫹⫹ ⫹⫹ ⫹⫹⫹ ⫹⫹ ⫹⫹ ⫹⫹

⫹ ⫹ ⫺ ⫹ ⫹ ⫹

⫺ ⫺ ⫺ ⫺ ⫺ ⫺

a The mutants were constructed on the basis of the diaminopentane producer C. glutamicum DAP-3c.

nated C. glutamicum DAP-4 and studied in detail concerning its production performance. Production performance of C. glutamicum DAP-4. Cultivation of C. glutamicum DAP-4 was performed in minimal medium, with glucose as the sole carbon source (Fig. 1C). The secretion of N-acetyldiaminopentane and of lysine was negligible. The diaminopentane yield (223 mmol mol glucose⫺1) was 11% higher than that of the parent strain DAP-3c (Table 4), indicating that the available carbon from elimination of the by-product was, at least partly, channeled into diaminopentane. The specific growth rates were rather similar in DAP-3c (0.25 h⫺1) and in DAP-4 (0.26 h⫺1) (Fig. 1B and D). The specific glucose uptake rates were even slightly enhanced in response to the deletion. DISCUSSION The production of the monomer building block diaminopentane by C. glutamicum strains with extended lysine pathways is one of the most promising biotechnological developments toward bio-based polyamides. Clearly, high-yield and high-selectivity conversions of the raw material into diaminopentane are crucial prerequisites to establish a competitive process. The diaminopentane yields of first-generation mutants overexpressing lysine decarboxylase were, however, rather low in important pioneering studies, underlining that this modification can only be a first step (19). In addition to a substantially increased product yield, systems metabolic engineering toward a tailor-made production strain of C. glutamicum recently discovered N-acetyldiaminopentane, a previously unknown side product of recombinant diaminopentane producers (13). This study further showed that accumulation of N-acetyldiaminopentane is significant, meaning that every fifth diaminopentane molecule is acetylated prior to secretion. In this regard, the identification and elimination of the responsible acetyltransferase gene, as presented here, displays a key contribution

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toward a superior C. glutamicum strain for diaminopentane. The encoding gene NCgl1469 is so far assigned as histone acetyltransferase HPA2 and related acetyltransferase. Based on the present findings, we propose a functional assignment as diaminopentane acetyltransferase. The obtained mutant, C. glutamicum DAP-4, secreted only the target product (Fig. 1B). The accumulation of N-acetyldiaminopentane could be completely avoided by the deletion of diaminopentane acetyltransferase (NCgl1469). First, this resulted in a yield increase by 11%, allowing a more efficient conversion of the sugar into the desired product (Table 4). Moreover, the created mutant strain appears to be advantageous with respect to a facilitated downstream process. The subsequent usage of diaminopentane as a polymer building block paces high demands on product purity, much higher than that of, e.g., applications of lysine in the feed market. NAcetyldiaminopentane is similar to the product and therefore difficult to be separated, so that its elimination facilitates product purification and provides a further benefit with regard to production costs. It is important to note that C. glutamicum DAP-4 also did not secrete lysine, so that diaminopentane could be obtained by-product free. Convincingly, the deletion of diaminopentane acetyltransferase did not cause the negative effects on strain physiology being reflected, e.g., by maintained growth rate or substrate uptake rate (Table 4). So far, only very little is known about the function of NGcl1469 in C. glutamicum, which probably comprises more than the catalytic activity observed here. The enzyme did react with 1,5-diaminopentane but not with other diamines such as 1,4-diaminobutane (putrescine) or 1,3-diaminopropane. The encoding gene was found constitutively expressed and nonessential for growth under the conditions chosen. In C. glutamicum, it is activated upon deletion of lexA, the key repressor of the SOS repair pathway against DNA damage (10), but its role here has not been elucidated. Generally, the hundreds of existing N-acetyltransferases show substantial functional diversity with a broad substrate range from small molecules to proteins, including self-acetylation (5, 17, 18). In eukaryotes, they regulate gene expression via acetylation of lysine residues in histones (16). Among bacteria, they are prevalent in aminoglycoside-resistant clinical strains, catalyzing detoxification of the drugs via regioselective N-acetylation (25). In addition, they play a variety of anabolic and catabolic roles in, e.g., the biosynthesis of UDP-N-acetylglucosamine, an essential compound in prokaryotes and eukaryotes, or the acetylation of polyamines, targeting them for export out of the cell or degradation (8). In summary, the obtained production strain C. glutamicum DAP-4 displays the next important step toward an industrially

TABLE 4. Growth and production characteristics of diaminopentane-producing C. glutamicum DAP-3c and DAP-4a Strain

␮ (h⫺1)

YDap/S (mmol mol⫺1)

YN-Ace/S (mmol mol⫺1)

YLys/S (mmol mol⫺1)

YX/S (g mol⫺1)

qS (mmol g⫺1 h⫺1)

DAP-3c DAP-4

0.25 ⫾ 0.00 0.26 ⫾ 0.01

200 ⫾ 5 223 ⫾ 6

52 ⫾ 3 ⬍0.1

2⫾0 1⫾0

64.2 ⫾ 0.8 60.3 ⫾ 0.2

4.2 ⫾ 0.1 4.4 ⫾ 0.1

a The data given comprise the specific growth rate (␮), specific glucose uptake rate (qS), and yields for diaminopentane (YDap/S), N-acetyldiaminopentane (YN-Ace/S), lysine (YLys/S), and biomass (YX/S). The yields were determined as slopes of the linear fit between biomass or product formation and substrate consumption (compare to data shown in Fig. 1). The values given here are mean values from three parallel cultivation experiments, with corresponding standard deviations.

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attractive cell factory. The metabolic properties of the mutants created here suggest promising strategies for further improvement. One relevant target emerges from the fact that the excess carbon from elimination of N-acetyldiaminopentane production could only be partially directed toward diaminopentane. This probably relates to a limited export of diaminopentane, as similarly observed for lysine production in C. glutamicum (26), so that the identification and subsequent overexpression of the unknown diaminopentane export protein display a further possibility to optimize production. Due to the structural similarity of lysine and diaminopentane, the lysine exporter lysE might be an interesting candidate. ACKNOWLEDGMENTS We gratefully acknowledge support from the BMBF grant “Biobased Polyamides through Fermentation” (0315239A), funding our research in a consortium with the companies BASF SE, Daimler AG, Fischerwerke GmbH, and Robert Bosch GmbH within the initiative Bioindustry21. S. Kind acknowledges financial support from the Max Buchner Research Foundation (grant MBFSt 2816). We further thank Andrea Michel for support in genetic engineering work. Additionally, we thank Jeroen Dickschat (Organic Chemistry Institute, Technische Universita¨t Braunschweig) for the synthesis and provision of N-acetyldiaminopentane.

10.

11. 12.

13.

14.

15. 16. 17.

18.

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