Intracellular poly[D-(؊)-3-hydroxybutyrate] (PHB) depolymerases degrade PHB granules to oligomers and monomers of 3-hydroxybutyric acid.Recently an intracellular PHB depolymerase gene (phaZ1) from Ralstonia eutropha was identified. We now report identification of candidate PHB depolymerase genes from R. eutropha, namely, phaZ2 and phaZ3, and their characterization in vivo. phaZ1 was used to identify two candidate depolymerase genes in the genome of Ralstonia metallidurans. phaZ1 and these genes were then used to design degenerate primers. These primers and PCR methods on the R. eutropha genome were used to identify two new candidate depolymerase genes in R. eutropha: phaZ2 and phaZ3. Inverse PCR methods were used to obtain the complete sequence of phaZ3, and library screening was used to obtain the complete sequence of phaZ2. PhaZ1, PhaZ2, and PhaZ3 share ϳ30% sequence identity. The function of PhaZ2 and PhaZ3 was examined by generating R. eutropha H16 deletion strains (⌬phaZ1, ⌬phaZ2, ⌬phaZ3, ⌬phaZ1⌬phaZ2, ⌬phaZ1⌬phaZ3, ⌬phaZ2⌬phaZ3, and ⌬phaZ1⌬phaZ2⌬phaZ3). These strains were analyzed for PHB production and utilization under two sets of conditions. When cells were grown in rich medium, PhaZ1 was sufficient to account for intracellular PHB degradation. When cells that had accumulated ϳ80% (cell dry weight) PHB were subjected to PHB utilization conditions, PhaZ1 and PhaZ2 were sufficient to account for PHB degradation. PhaZ2 is thus suggested to be an intracellular depolymerase. The role of PhaZ3 remains to be established.Polyhydroxyalkanoates (PHAs) are polyoxoesters produced by a wide range of bacteria when they find themselves in an environment with an available carbon source but limited in additional nutrient(s) required for growth (9). The shortchain-length PHAs, where R is a methyl or ethyl, have properties of thermoplastics and are biodegradable (Fig. 1). Much effort has focused on understanding the biology of PHA homeostasis for several reasons. First, this understanding could lead to expression of the appropriate gene set in heterologous systems to make PHA production economically competitive with oil-based polymers. Second, understanding PHA homeostasis serves as a paradigm for understanding the mechanism of homopolymerization reactions in which the product undergoes a phase transition during its formation, generating insoluble inclusions (granules). The intracellular PHAs can be degraded when the bacteria require carbon but are in otherwise nutrientreplete conditions, and the monomers and energy released can be reused to allow the bacteria to grow (17). The insoluble PHA granules must, therefore, be biosynthesized in a controlled fashion to facilitate enzymatic degradation. A variety of proteins associated with PHA homeostasis have been identified and are being characterized (3,8,10,12,20,22). As part of our research to understand the mechanisms that control polymer size and reuse, we have been interested in identifying the intracellular depolymerases that degrade poly[D-(Ϫ)-3-hydroxybutyrate] (PHB) w...

This review focuses on nontemplate-dependent polymerases that use water-soluble substrates and convert them into water-insoluble polymers that form granules or inclusions within the cell. The initial part of the review summarizes briefly the current knowledge of polymer formation catalyzed by starch and glycogen synthases, polyphosphate kinase (a polymerase), cyanophycin synthetases, and rubber synthases. Specifically, our current understanding of their mechanisms of initiation, elongation (including granule formation), termination, remodeling, and polymer reutilization will be presented. General underlying principles that govern these types of polymerization reactions will be enumerated as a paradigm for all nontemplate-dependent polymerizations. The bulk of the review then focuses on polyhydroxyalkanoate (PHA) synthases that generate polyoxoesters. These enzymes are of interest as they generate biodegradable polymers. Our current knowledge of PHA production and utilization in vitro and in vivo as well as the contribution of many proteins to these processes will be reviewed.

Polyhydroxybutyrates (PHBs) are polyoxoesters generated from (R)3-hydroxybutyryl coenzyme A by PHB synthase. During the polymerization reaction, the polymers undergo a phase transition and generate granules. Wautersia eutropha can transiently accumulate PHB when it is grown in a nutrient-rich medium (up to 23% of the cell dry weight in dextrose-free tryptic soy broth [TSB]). PHB homeostasis under these growth conditions was examined by quantitative Western analysis to monitor the proteins present, their levels, and changes in their levels over a 48-h growth period. The proteins examined include PhaC (the synthase), PhaP (a phasin), PhaR (a transcription factor), and Polyhydroxybutyrates (PHBs) are biodegradable polymers with properties of thermoplastics synthesized by PHB synthases (PhaC) using (R)3-hydroxybutyryl coenzyme A as a substrate. In times of nutrient limitation, many bacteria generate these polymers when an appropriate carbon source is available (1). The soluble (R)3-hydroxybutyryl coenzyme A is transformed into insoluble polymers, called granules, in a nontemplate-dependent polymerization process (18). When the bacteria find themselves in a more hospitable environment, they degrade this polymer to generate building blocks and reducing equivalents for anabolism. However, bacteria in a nutrient-rich medium can also make and degrade PHB. The biology of PHB production and utilization under nutrient-rich conditions, however, remains to be elucidated.The widespread detection of genes in many microorganisms that carry out PHB synthesis and degradation suggests that these pathways have evolved to be a general mechanism for bacterial cell survival in times of stress. The organism that we have chosen to study PHB homeostasis is Wautersia eutropha H16. This organism contains a class I synthase to generate PHB (15).In this paper, we describe studies of PHB production and utilization by W. eutropha H16 in a nutrient-rich medium (dextrose-free tryptic soy broth [TSB]). Using antibodies (Abs) to most of the proteins identified as being involved in PHB homeostasis thus far, we carried out Western blotting as a function of growth time to measure the rates of appearance and disappearance of PhaC, PhaP (a phasin protein), and PhaR (a putative transcription factor), as well as PhaZ1 a , PhaZ1 b , PhaZ1 c , and PhaZ2 (formerly known as PhaZ1, PhaZ2, PhaZ3, and oligomer hydrolase, respectively); all of these proteins are thought to be involved in PHB degradation (10, 23). The average cell volume (20) and the average number of granules per cell at 4 and 24 h in TSB were determined by stereology analyses of transmission electron microscopy (TEM) images. These results and knowledge concerning the M w of PHB allowed us to estimate the amount of each protein per cell, the percentage of the granule surface that is covered by PhaC and PhaP, and the number of PhaC molecules per PHB polymer. A comparison of these results with similar studies performed under nitrogen-limited growth conditions (unpublished data) provided insight int...

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Poly-(R)-3-hydroxybutyrate (PHB) homeostasis in Ralstonia eutropha takes place at the interface of the cytosol and the hydrophobic PHB granule. PHB synthesis and degradation are therefore intimately linked to the process of granule assembly and breakdown. Unraveling this time-dependent three-dimensional process requires an understanding of the kinetics of synthesis of relevant proteins. Reverse transcriptase quantitative PCR and quantitative Western blotting were carried out on batch cultures of R. eutropha H16 in order to gain insight into how expression of the PHB-related genes phaA, phaB, phaC, phaP, phaR, phaZ1a, phaZ1b, and phaZ1c changed during a cell growth phase, a PHB production phase, and a PHB utilization phase. phaA, phaB, phaC, phaR, and phaZ1a were transcribed throughout cell growth, PHB production, and PHB degradation. PHB-mediated induction of PhaP expression was shown to occur at the transcriptional level, with transcript levels increasing during PHB production and decreasing during PHB utilization. Levels of PhaP correlated strongly with levels of PHB. Levels of phaZ1b transcript and protein increased sharply during production and decreased during degradation, but transcript accumulation did not depend on PHB production as in the case of phaP. No evidence of phaZ1c expression was found under the experimental conditions used in this study.