INTRODUCTION

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Polyhydroxyalkanoic acids (PHA) represent a rather complex and diverse class of bacterial storage compounds; more than 100 different hydroxyalkanoic acids, which occur as insoluble cytoplasmic inclusions in the cells, have been identified as constituents of these polyesters (30). PHA synthases, the key enzymes of PHA synthesis, catalyze the polymerization of hydroxyalkanoic acids from corresponding coenzyme A (CoA) thioesters to PHA. The PHA synthase gene (phaC) of Ralstonia eutropha is part of the phaCAB operon, which also encodes the -ketothiolase (phaA) and the acetoacetyl-CoA reductase (phaB) (19, 20, 24). There is some evidence that the R. eutropha PHA synthase is posttranslationally modified by 4-phosphopantetheine in Escherichia coli SJ16 (panD), thus presumably providing a second thiol group (7). One thiol group (Cys-319) has been identified by site-specific mutagenesis and covalent labeling of the corresponding PHA synthase peptide fragment to be directly involved in the catalytic mechanism and to be essential for enzymatic activity (7, 38). However, the serine residue, or another amino acid residue, to which 4-phosphopantetheine might be attached has not been identified, nor has it been shown whether the proposed posttranslational modification of the PHA synthase occurs also in R. eutropha. Therefore, the putative modification of the R. eutropha PHA synthase was studied in its natural host in order to gain a better understanding of the reaction mechanism of PHA synthases and to evaluate further requirements for effective expression of PHA synthase genes in other organisms. 4-Phosphopantetheine is used primarily for the synthesis of CoA and acyl carrier protein (ACP), which are the predominant acyl group carriers in the cell (5). The acyl moiety is attached to the terminal sulfhydryl of the 4-phosphopantetheine prosthetic group of these cofactors. 4-Phosphopantetheine is also a prosthetic group of other enzyme systems, such as the entF gene product involved in serine activation in the biosynthesis of E. coli siderophore enterobactin (21). Specific labeling of 4-phosphopantetheinylated proteins occurred in -alanine auxotrophic E. coli (panD) fed with [2-¹⁴C]-alanine (18), since -alanine is a precursor of 4-phosphopantetheine (2). The panD gene encodes the aspartate-1-decarboxylase, which catalyzes the conversion of L-aspartate to CO₂ and -alanine (3, 37). Pantoate is synthesized from ketoisovalerate via two enzymatic steps, catalyzed by ketopantoate hydroxymethyltransferase (panB) and ketopantoate reductase (panE) (13, 36). Pantothenate is then synthesized by an ATP-dependent condensation of pantoate and -alanine catalyzed by the pantothenate synthetase (panC) (11). In this study, we isolated -alanine auxotrophic mutants of R. eutropha in order to (i) investigate whether a posttranslational modification of the PHA synthase occurs in R. eutropha and (ii) clone genes involved in 4-phosphopantetheine synthesis.

	MATERIALS AND METHODS

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Bacterial strains, plasmids, and growth conditions. Strains and plasmids used in this study are listed in Table 1. E. coli cells were grown at 37°C in Luria-Bertani (LB) broth or on LB agar supplemented with an antibiotic(s) (ampicillin [75 µg/ml], kanamycin [50 µg/ml], and/or tetracycline [12.5 µg/ml]) if required. E. coli SJ16 was cultivated in Dex-E-B1-met medium (3) supplemented with antibiotic(s) and -alanine (20 µM) when relevant. R. eutropha was grown at 30°C in nutrient broth (NB) medium or mineral salt medium (MSM) supplemented with an antibiotic(s) (kanamycin [160 µg/ml], streptomycin [500 µg/ml], and/or tetracycline [25 µg/ml]) and/or -alanine (10 mg/liter) when relevant.

DNA manipulations and cloning of panD. Standard recombinant DNA procedures were performed as specified by Sambrook et al. (22). Tn5-induced -alanine auxotrophic mutants of R. eutropha HF39 were generated by using the suicide vector pSUP5011 (26), which was transferred to R. eutropha by conjugation. Tn5 insertion sites were mapped after subcloning of SalI restriction fragments of the Tn5 fragment (specifying kanamycin resistance) plus adjacent chromosomal DNA from the corresponding mutant genomic DNA into pBluescript SK by DNA sequencing using a Tn5-specific sequencing primer (5'-GTTCAGGACGCTACTTG-3').

Enzymatic assay of L-aspartate-1-decarboxylase. The activity of L-aspartate-1-decarboxylase was determined as described by Williamson and Brown (37). The reaction mixture contained 100 mM potassium phosphate buffer (pH 7.5), 5 mM EDTA (dipotassium salt), 3 mM L-[U-¹⁴C]aspartate (220 mCi/mmol), and 50 µg of protein (crude extract) in a total volume of 100 µl. After 2 h of incubation at 42°C, the reaction was stopped by the addition of 10 µl of 50% trichloroacetic acid. Precipitated protein was sedimented by centrifugation, and the reaction products in the supernatant were analyzed by thin-layer chromatography (TLC), using cellulose TLC plates and 1-propanol-water-28% ammonium (80:19:1) as the solvent; the spots were identified by autoradiography. L-[U-¹⁴C]aspartate and [1-¹⁴C]-alanine (Sigma, Deisenhofen, Germany) served as reference compounds.

In vitro PHA synthase activity. PHA synthase activity, at substrate concentrations of up to 130 µM, was measured spectrophotometrically at 412 nm in 25 mM Tris-HCl (pH 7.5) containing 1 mM 5,5'-dithiobis(2-nitrobenzoic acid) as described by Valentin and Steinbüchel (35).

Polyester analysis. Three to 5 mg of lyophilized cell material was subjected to methanolysis in the presence of 15% (vol/vol) sulfuric acid. The resulting methyl esters of the constituent 3-hydroxyalkanoic acids were assayed by gas chromatography by the method of Brandl et al. (1) as described in detail recently (33).

SDS-PAGE and Western immunoblotting. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) was performed as described by Sambrook et al. (22). Proteins were separated in 12.5 or 15% (wt/vol) SDS-polyacrylamide gels and stained with Coomassie brilliant blue R-250. On Western blots using polyvinylidene difluoride membranes (34), PhaC1 from Pseudomonas aeruginosa and PhaC from R. eutropha were detected with anti-PhaC1 and anti-PhaC antisera, respectively, and an alkaline phosphatase-conjugated secondary antibody. Bound antibodies were detected with nitroblue tetrazolium chloride and the toluidine salt of 5-bromo-4-chloro-3-indolylphosphate.

¹⁴C-labeling of 4-phosphopantetheinylated proteins. The procedure of Rusnak et al. (21) was followed, with the modifications indicated below. R. eutropha was cultivated in MSM containing 0.05% (wt/vol) NH₄Cl and 0.5% (wt/vol) sodium gluconate, whereas E. coli was cultivated in Dex-E-B1-met medium containing 0.5% (wt/vol) glucose, 1 mM thiamine, and 0.002% (wt/vol) methionine. Media contained the appropriate antibiotic, 1 mM isopropyl--D-thiogalactopyranoside (IPTG), and 20 µM [U-¹⁴C]-alanine (220 mCi/mmol). Cells were cultivated for 24 h. Crude extracts were prepared, and proteins were separated by SDS-PAGE. Autoradiography was performed to visualize 4-phosphopantetheinylated proteins. Immunoblotting was conducted to identify the PHA synthases. Cells were also analyzed with respect to PHA accumulation to obtain evidence for in vivo activity of PHA synthases.

Nucleotide sequence accession number. The panD gene nucleotide sequence data were deposited in the GenBank database under the accession no. AF061246.

	RESULTS

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Isolation and characterization of -alanine auxotrophic Tn5 mutants of R. eutropha. To investigate the posttranslational covalent modification of the PHA synthase in R. eutropha, we isolated four -alanine auxotrophic Tn5-induced mutants (C3, M30, O22, and Q20), using suicide vector pSUP5011 (26). In all four mutants, wild-type growth could be recovered when -alanine was added to the medium (data not shown). The DNA regions of all four mutants harboring the Tn5 insertions were subcloned via SalI digestion and selection for Tn5-mediated kanamycin resistance. DNA sequence analysis mapped two Tn5 insertions (mutants Q20 and O22) in an open reading frame (ORF) at positions 2235 and 2279, respectively, with strong similarity to the panD gene from E. coli, encoding the aspartate-1-decarboxylase (4); two Tn5 insertions (mutants C3 and M30) occurred in an ORF at positions 1470 and 1399, respectively, with strong similarity to the pntAA gene from E. coli, encoding the NAD(P)⁺ transhydrogenase subunit alpha 1 (8) (Fig. 1).

View larger version (12K): [in this window] [in a new window]   FIG. 1.   Partial physical map of the genomic 15-kb EcoRI fragment and comparison of the localization of panD (A) and pntAA(B) with the localization of panD and pntAA in E. coli (accession no. AE000122 and AE000255) and panD in B. subtilis (accession no. Z99115) as well as pntAA in Mycobacterium tubercolosis (accession no. Z92770). ORFs from R. eutropha were designated according to the strongest similarity of the derived amino acid sequences to databases. Tn5 insertions sites are labeled with Tn5 and the corresponding mutant designation (M30, C3, Q20, or O22).

Cloning and DNA sequencing of the panD gene region. The panD gene region was cloned by phenotypic complementation to -alanine prototrophy of the panD mutant R. eutropha Q20 and by transferring recombinant cosmids (pVK100) of a genomic library of strain HF39 to this mutant by conjugation. The four recombinant cosmids (C5, C9, C10, and C20) isolated harbored a common 15-kb EcoRI fragment beside the cosmid and other genomic fragments and complemented the R. eutropha mutant Q20. Subcloning of the 15-kb EcoRI fragment into pBluescript SK resulted in plasmid pSKE15; DNA sequencing revealed strong similarity to panD from E. coli at one end and strong homology of about 44% identity to SecF (integral inner membrane protein involved in protein translocation) from E. coli at the other end (Fig. 1). Analysis of subfragments of the 15-kb EcoRI fragment revealed the nucleotide sequence of an approximately 3.3-kb region comprising five ORFs including the entire panD gene (Fig. 1).

Physical organization of the panD gene in R. eutropha and DNA sequence analysis. The panD derived amino acid sequence revealed the strongest similarity, about 63 and 67%, to panD-encoded L-aspartate-1-decarboxylases from E. coli and Bacillus subtilis, respectively (Fig. 2). In contrast to E. coli and B. subtilis, the 4-phosphopantetheine biosynthesis genes in R. eutropha are not colocalized (4). panD is localized 55 bp downstream of ORF3, the derived amino acid sequence of which exhibited strong similarity (42%) to the pntAA-encoded transhydrogenase subunit alpha 1 from E. coli (Fig. 1). Interestingly, ORF5 and incomplete ORF6 were identified directly 273 bp downstream of panD, which on the amino acid sequence level share strong similarity (about 60 and 57%) with PntAB (transhydrogenase subunit alpha 2) and PntB (transhydrogenase subunit beta) from E. coli, respectively (Fig. 1). Upstream of ORF3 and putatively transcribed in the opposite direction, we identified ORF2. The ORF2-encoded amino acid sequence revealed about 44% similarity to 7,8-dehydro-8-oxoguanine-triphosphatase (MutT) from E. coli (32). Downstream of ORF2 an incomplete ORF1 was identified, possessing 63 and 48% amino acid sequence similarity to hypothetical proteins in the purB 5' regions of Haemophilus influenzae and E. coli, respectively. Therefore, in R. eutropha, panD is separated from other genes required for the synthesis of 4-phosphopantetheine and localized within a cluster of genes encoding the transhydrogenase. Such an organization of pan or pnt genes has not previously been described.

View larger version (61K): [in this window] [in a new window]   FIG. 2.   Comparison of the PanD sequence from R. eutropha with various PanD sequences from other bacteria. Accession numbers for the PanD amino acid sequences used were AE000122 (E. coli), Z99115 (B. subtilis), P56065u (Helicobacter pylori), O06281 (Mycobacterium tuberculosis), and Q55382 (Synechocystis sp.).

Identification of the complementing unit. The putative coding region of panD was amplified by PCR and subcloned into vectors pBluescript SK and pBBR1MCS-3 downstream of and colinear to the lac promoter, leading to plasmids pKSKX0.76 and pBBR1MCS-3KX0.76, respectively (Table 1). A ribosome-binding site was inserted by PCR at a position optimally relative to the putative start codon of panD (25). Plasmid pBBR1MCS-3KX0.76 complemented all four -alanine auxotrophic Tn5 mutants of R. eutropha (Table 1), and plasmid pKSKX0.76 mediated -alanine prototrophy to E. coli SJ16 (panD), whereas plasmid pSKE15 did not enable complementation of E. coli SJ16 (panD). Complementation of panD mutants was demonstrated in growth experiments and by determination of L-aspartate-1-decarboxylase activity (Fig. 3).

View larger version (79K): [in this window] [in a new window]   FIG. 3.   In vitro activity of the panD-encoded L-aspartate-1-decarboxylase from R. eutropha in E. coli SJ16 harboring various plasmids. L-[U-14C]aspartate was used as the substrate for L-aspartate-1-decarboxylase (crude extracts), and the reaction products were separated by TLC. Spots were identified by autoradiography. Reference substances were L-[U-14C]aspartate (lane 1), [2-14C]-alanine (lane 2), and a mixture of L-[U-14C]aspartate and [2-14C]-alanine (1:1) (lane 3). Crude extracts from various E. coli cells were subjected to this assay for L-aspartate-1-decarboxylase activity: lane 4, E. coli SJ16 (plus -alanine); lane 5, E. coli SJ16 harboring plasmid pKSKX0.76 (plus -alanine and IPTG); lane 6, E. coli SJ16 harboring plasmid pKSKX0.76 (plus -alanine, minus IPTG); lane 7, E. coli SJ16 harboring plasmid pSKE15 (plus -alanine).

Determination of L-aspartate-1-decarboxylase activity. L-Aspartate-1-decarboxylase activity was qualitatively analyzed in crude extracts from various E. coli recombinants by using L-[U-¹⁴C]aspartate as the substrate. The reaction product (-alanine) was analyzed by TLC and autoradiography. L-Aspartate was converted to -alanine when E. coli S17-1 was cultivated on NB medium but not when it was cultivated on Dex-E-B1-met medium containing 20 µM -alanine. E. coli SJ16 exhibited no L-aspartate-1-decarboxylase activity when grown in Dex-E-B1-met medium containing 20 µM -alanine, but when harboring plasmid pKSKX0.76 and in the presence of 1 mM IPTG, it showed enzyme activity (Fig. 3). Omission of IPTG significantly decreased L-aspartate-1-decarboxylase activity. No activity was obtained with plasmid pSKE15 in E. coli SJ16 in the presence of -alanine (Fig. 3).

Labeling of 4-phosphopantetheinylated proteins. To investigate the postulated posttranslational modification of the PHA synthase from R. eutropha in its natural host, we used the R. eutropha panD mutant Q20. This panD mutant was cultivated under conditions permissive for PHA accumulation in the presence of [2-¹⁴C]-alanine, and crude extracts were subjected to SDS-PAGE (autoradiography) and immunoblot analysis. Furthermore, recombinant E. coli SJ16(pBHR68), functionally expressing the wild-type PHA synthase from R. eutropha, was analyzed with respect to 4-phosphopantetheinylation of PHA synthase (Fig. 4) (28). We also analyzed two site-specific mutants (pBHR68S260A and pBHR68S546I) of the PHA synthase from R. eutropha, carrying mutations at the only two highly conserved serine residues which might function as targets for covalent modification by 4-phosphopantetheine. Immunoblot analysis with anti-PHA synthase antibodies demonstrated expression of either PHA synthase gene (Fig. 4C). The corresponding PHA-expressing cells revealed in vivo activity of only the wild-type PHA synthases, whereas neither site-specific mutation caused accumulation of PHA at a detectable level. In addition, the in vitro PHB synthase activity of the two site-specific mutants was almost completely abrogated (Table 2). Autoradiography of SDS-PAGE-separated proteins derived from R. eutropha Q20 (panD) revealed no specific labeling of proteins corresponding in size to PHA synthase proteins (apparent molecular weight of 65,000) (Fig. 4). Use of -alanine by the cells is indicated by very weak labeling of any protein, which became visible only after prolonged exposure of the gels to X-ray films (Fig. 4). In contrast, E. coli SJ16 enabled specific labeling of 4-phosphopantetheinylated proteins, as indicated by the presence of a strongly labeled protein with an apparent molecular weight of 8,000, which presumably corresponds to holo-ACP (Fig. 4). However, no specific labeling of either PHA synthase protein was observed (Fig. 4).

View larger version (54K): [in this window] [in a new window]   FIG. 4.   14C-labeling of 4-phosphopantetheinylated proteins from R. eutropha (-alanine auxotrophic mutants Q20 and M30) and recombinant E. coli SJ16. (A) SDS-PAGE of crude extracts from R. eutropha Q20 and M30 mutants as well as E. coli SJ16 harboring various plasmids, which were cultivated in the presence of [2-14C]-alanine. Lane M, molecular weight standard; lane 1, R. eutropha HF39 (negative control); lane 2, R. eutropha Q20; lane 3, R. eutropha M30; lane 4, E. coli SJ16(pBluescript KS); lane 5, E. coli SJ16(pBHR68, expressing PHA synthase from R. eutropha); lane 6, E. coli SJ16(pBHR68S260A), expressing site-specific mutant of PHA synthase from R. eutropha); lane 7, E. coli SJ16(pBHR68S546I, expressing site-specific mutant of PHA synthase from R. eutropha). (B) Autoradiography of the gel in panel A. (C) Immunoblot of the gel in panel A with polyclonal anti-PhaC (R. eutropha) antibodies. The arrow indicates the position of PHA synthase.

View this table: [in this window] [in a new window]   TABLE 2.   PHB synthase, -ketothiolase, and acetoacetyl-CoA reductase activities in E. coli harboring various plasmids

Effect of -alanine auxotrophy on PHA accumulation in R. eutropha. The four -alanine auxotrophic Tn5 mutants of R. eutropha, recombinant strains of these mutants harboring plasmid pBBR1MCS-3KX0.76 and the wild type were cultivated under conditions permissive for PHA accumulation on MSM containing 1% (wt/vol) gluconate and 0.05% NH₄Cl as well as in the presence of 10 mg of -alanine per liter. In the absence or presence of -alanine, the wild type accumulated PHA to a level of about 55 or 65% of cell dry weight (CDW), respectively (Fig. 5). The panD mutants exhibited a strong decrease in PHA accumulation, to approximately 20% of the wild-type level, when -alanine was omitted. However, wild-type-level PHA accumulation was recovered when the cells harbored plasmid pBBR1MCS-3KX0.76 and when the cells were cultivated in the presence of -alanine (Fig. 5).

View larger version (53K): [in this window] [in a new window]   FIG. 5.   PHB accumulation of -alanine auxotrophic Tn5 induced mutants of R. eutropha (M30, C3, Q20, and O22). The parent strain R. eutropha HF39 served as a control. Cells were cultivated in MSM containing 0.05% (wt/vol) NH4Cl plus 1% (wt/vol) sodium gluconate and in the presence or absence of -alanine (10 mg/liter). Each mutant harboring panD-expressing plasmid pBBR1MCS-3KX0.76 (indicated as *) was also analyzed.

	DISCUSSION

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To investigate posttranslational covalent modification of the PHA synthase from R. eutropha by 4-phosphopantetheine in its natural host, we isolated four independent -alanine auxotrophic Tn5-induced mutants of R. eutropha. These mutants, analogous to the E. coli SJ16 panD mutant, should enable specific labeling of 4-phosphopantetheinylated proteins in R. eutropha when fed with [2-¹⁴C]-alanine, a precursor of CoA, which serves as a donor of 4-phosphopantetheine to apo-ACP (5). Subcloning of the DNA regions containing the Tn5 insertions and DNA sequence analysis indicated that two insertions occurred, one in an ORF with strong similarity to the transhydrogenase subunit: alpha 1 and the other in ORFs with strong similarities to L-aspartate-1-decarboxylases from E. coli and B. subtilis. The L-aspartate-1-decarboxylase (encoded by panD) converts L-aspartate to CO₂ and -alanine, which is an intermediate of 4-phosphopantetheine synthesis. A 15-kb EcoRI fragment complementing all four -alanine auxotrophic Tn5 mutants was cloned from genomic DNA of R. eutropha. DNA sequence analysis of a 3.3-kb DNA region of the 15-kb EcoRI fragment revealed that the panD gene was located directly downstream of an ORF with strong homology to pntAA-encoded transhydrogenase subunit alpha 1. No consensus promoter sequence was detected upstream of the panD gene coding region, but upstream of pntAA a weakly conserved ⁷⁰-specific promoter was identified (9). These data indicate that pntAA and panD are cotranscribed, which explains the -alanine auxotrophy of mutants with Tn5 insertions in the pntAA gene. Thus, Tn5 insertions in the putative pntAA gene have negative polar effects on panD expression in mutants M30 and C3. Downstream of panD we identified two further ORFs, presumably encoding transhydrogenase subunits alpha 2 (pntAB) and beta (pntB). No evidence for colocalization of panD with panB and panC, encoding ketopantoate-hydroxymethyltransferase and pantothenate synthetase, respectively, as shown for E. coli and B. subtilis, was found (4). Hybrid plasmids comprising the panD region from R. eutropha complemented the -alanine auxotrophic Tn5 mutants of R. eutropha and E. coli SJ16, and activity of the L-aspartate-1-decarboxylase was demonstrated in E. coli SJ16 harboring plasmid pKSKX0.76 (Fig. 3), strongly suggesting that the panD gene from R. eutropha encodes a L-aspartate-1-decarboxylase (37).

Gerngross et al. (7) obtained evidence that the PHA synthase from R. eutropha is posttranslationally modified by 4-phosphopantetheine in E. coli SJ16, identifying radioactively labeled 4-phosphopantetheinylated proteins in the -alanine auxotrophic E. coli SJ16. Since, only one essential cysteine residue (Cys-319) was identified by site-specific mutagenesis in the PHA synthase of R. eutropha and since no other cysteine residue is highly conserved in PHA synthases (7, 16), the second thiol group postulated for the catalytic mechanism might be provided by a 4-phosphopantetheine linked to a conserved serine residue. In this study, we investigated the putative 4-phosphopantetheinylation of PHA synthase in the -alanine auxotrophic R. eutropha Q20 and therefore in the natural host for this PHA synthase. In addition, two site-specific mutants of the PHA synthase from R. eutropha, carrying mutations at the only two conserved serine residues (S260A and S546I), and wild-type PHA synthase were analyzed with respect to 4-phosphopantetheinylation in E. coli SJ16. All of the investigated PHA synthase genes were expressed to similar levels, as demonstrated by immunoblotting (Fig. 4C), but the two site-specific serine mutants of the PHA synthase from R. eutropha exhibited neither in vitro nor in vivo activity (Table 2). However, no specific labeling of the PHA synthase by 4-phosphopantetheine was obtained, whereas only 4-phosphopantetheinylated ACP was detected in the autoradiograms. Gerngross et al. (7) observed in E. coli SJ16, in addition to ACP and the PHA synthase, two 4-phosphopantetheinylated proteins: one unknown 35-kDa protein, which is presumably identical with the recently characterized EntB (isochorismate lyase), and the 140-kDa EntF protein (enterobactin synthase) (6, 21). Both enzymes are involved in enterobactin biosynthesis, and expression of the corresponding genes is strictly dependent on iron-limited growth conditions (21). Under iron starvation, EntB and EntF were identified as 4-phosphopantetheinylated proteins in E. coli SJ16 when [3-³H]-alanine was added to the growth medium (21). In the presence of 2 µM FeSO₄, only ACP was detected as a 4-phosphopantetheinylated protein (21). In addition labeling experiments with [¹⁴C]pantothenic acid clearly indicated that ACP is the predominantly labeled protein in E. coli (18). Since we did not use iron-limited conditions, the observation of only ACP as a 4-phosphopantetheinylated protein is in good agreement with results of previous labeling experiments. Analysis of 4-phosphopantetheinylated proteins in R. eutropha Q20 did not reveal specific labeling of 4-phosphopantetheinylated proteins except ACP but indicated radiolabeling of all proteins detected. This suggests either that external [2-¹⁴C]-alanine, in contrast to the case for E. coli SJ16 (10, 11, 21), is not exclusively used for CoA synthesis in R. eutropha Q20 or intermediates of CoA biosynthesis are degraded and channeled to central metabolism. Since no evidence for 4-phosphopantetheinylation of PHA synthases was obtained and since the PHA synthase of R. eutropha was functionally expressed in various organisms from different kingdoms, 4-phosphopantetheinylation seems not to be required for enzymatic activity of PHA synthases. In addition, so far no pantetheinylated peptide of the PHA synthase from R. eutropha has been isolated (16). Calculations of specific activity and the extent of labeling (about 1%), as previously obtained, makes specific posttranslational modification by 4-phosphopantetheine of PHA synthase very unlikely (7, 16). Instead, during heterologous expression of the PHA synthase gene from R. eutropha in E. coli, most probably a minor fraction of the PHA synthase protein was covalently modified by the action a 4-phosphopantetheine transferase present in E. coli, which is obviously not relevant for PHA metabolism in R. eutropha. This finding is relevant to strategies for expressing PHA synthase genes in organisms, such as plants, (17, 31), which can be used for biotechnological production of PHA.

Based on these observation and on kinetic studies, Sinskey and coworkers are now postulating a new model of the PHA synthase reaction mechanism in which two subunits of PHA synthase from a homodimer, with each subunit providing one thiol group by Cys-319. Thus, the protein dimer is the active form of the enzyme (16). To investigate a metabolic link between CoA biosynthesis and PHA biosynthesis, we cultivated the -alanine auxotrophic R. eutropha Tn5 mutants under conditions permissive for PHA accumulation and in the presence or absence of -alanine. Although growth in the absence of -alanine was weak, all mutants accumulated PHA to a level of about 10% of the CDW (Fig. 5). The weak accumulation of PHA might be due to low concentrations of acetyl-CoA and other essential thioesters of the central metabolism as well as to the physiological state of the cells (12). Wild-type PHA accumulation in the absence of -alanine was restored when panD (pBBR1MCS-3KX0.76) was expressed in the mutants, supporting the review that PHA accumulation relies on an intact CoA biosynthesis.