Control of poly-beta-hydroxybutyrate synthase mediated by acetyl phosphate in cyanobacteria

Process optimization of the polyhydroxybutyrate production in the cyanobacteria Synechocystis sp. and Synechococcus sp

The effect of four parameters (acetate, NaCl, inorganic carbon and days in darkness) affecting the polyhydroxybutyrate (PHB) production were tested and optimized for Synechococcus sp. and Synechocystis sp. using a Box-Behnken design. The optimal conditions for Synechocystis sp. were found to be 1.2 g L^-1 of acetate, 4 gC L^-1 of NaHCO₃, 18 g L^-1 of NaCl and 0 days in darkness. For Synechococcus sp., equal acetate concentration and days in darkness, and lower inorganic carbon and NaCl concentrations than those for Synechocystis sp. were needed (0.05 g L^-1 inorganic carbon and 9 g L^-1 NaCl). Optimal conditions were scaled up to 3 L photobioreactors. Using Synechocystis sp., 5.6 %_dcw of PHB was obtained whether adding or not acetate. In opposition, a maximum of 26.1 %_dcw by using acetate was reached with Synechococcus sp. These results provide an easy method to optimize the cultivation conditions to enhance PHB production with cyanobacteria.

Metabolic circuits and gene regulators in polyhydroxyalkanoate producing organisms: Intervention strategies for enhanced production

Worldwide worries upsurge concerning environmental pollutions triggered by the accumulation of plastic wastes. Biopolymers are promising candidates for resolving these difficulties by replacing non-biodegradable plastics. Among biopolymers, polyhydroxyalkanoates (PHAs), are natural polymers that are synthesized and accumulated in a range of microorganisms, are considered as promising biopolymers since they have biocompatibility, biodegradability, and other physico-chemical properties comparable to those of synthetic plastics. Consequently, considerable research have been attempted to advance a better understanding of mechanisms related to the metabolic synthesis and characteristics of PHAs and to develop native and recombinant microorganisms that can proficiently produce PHAs comprising desired monomers with high titer and productivity for industrial applications. Recent developments in metabolic engineering and synthetic biology applied to enhance PHA synthesis include, promoter engineering, ribosome-binding site (RBS) engineering, development of synthetic constructs etc. This review gives a brief overview of metabolic routes and regulators of PHA production and its intervention strategies.

Production of Poly-3-Hydroxybutyrate (P3HB) with Ultra-High Molecular Weight (UHMW) by Mutant Strains of Azotobacter vinelandii Under Microaerophilic Conditions

Poly-3-hydroxybutyrate (P3HB) is a biopolymer, which presents characteristics similar to those of plastics derived from the petrochemical industry. The thermomechanical properties and biodegradability of P3HB are influenced by its molecular weight (MW). The aim of the present study was to evaluate the changes of the molecular weight of P3HB as a function of oxygen transfer rate (OTR) in the cultures using two strains of Azotobacter vinelandii, a wild-type strain OP, and PhbZ1 mutant with a P3HB depolymerase inactivated. Both strains were grown in a bioreactor under different OTR conditions. An inverse relationship was found between the average molecular weight of P3HB and the OTR_max, obtaining a polymer with a maximal MW (8000-10,000 kDa) from the cultures developed at OTR_max of 5 mmol L^-1 h^-1 using both strains, with respect to the cultures conducted at 8 and 11 mmol L^-1 h^-1, which produced a P3HB between 4000 and 5000 kDa. The increase in MW of P3HB was related to the activity of enzymes involved in the synthesis and depolymerization. Overall, our results show that it is possible to modulate the average molecular weight of P3HB by manipulating oxygen transfer conditions with both strains (OP and PhbZ1 mutant) of A. vinelandii.

The Modification of Regulatory Circuits Involved in the Control of Polyhydroxyalkanoates Metabolism to Improve Their Production

Poly-(3-hydroxyalkanoates) (PHAs) are bacterial carbon and energy storage compounds. These polymers are synthesized under conditions of nutritional imbalance, where a nutrient is growth-limiting while there is still enough carbon source in the medium. On the other side, the accumulated polymer is mobilized under conditions of nutrient accessibility or by limitation of the carbon source. Thus, it is well known that the accumulation of PHAs is affected by the availability of nutritional resources and this knowledge has been used to establish culture conditions favoring high productivities. In addition to this effect of the metabolic status on PHAs accumulation, several genetic regulatory networks have been shown to drive PHAs metabolism, so the expression of the PHAs genes is under the influence of global or specific regulators. These regulators are thought to coordinate PHAs synthesis and mobilization with the rest of bacterial physiology. While the metabolic and biochemical knowledge related to the biosynthesis of these polymers has led to the development of processes in bioreactors for high-level production and also to the establishment of strategies for metabolic engineering for the synthesis of modified biopolymers, the use of knowledge related to the regulatory circuits controlling PHAs metabolism for strain improvement is scarce. A better understanding of the genetic control systems involved could serve as the foundation for new strategies for strain modification in order to increase PHAs production or to adjust the chemical structure of these biopolymers. In this review, the regulatory systems involved in the control of PHAs metabolism are examined, with emphasis on those acting at the level of expression of the enzymes involved and their potential modification for strain improvement, both for higher titers, or manipulation of polymer properties. The case of the PHAs producer Azotobacter vinelandii is taken as an example of the complexity and variety of systems controlling the accumulation of these interesting polymers in response to diverse situations, many of which could be engineered to improve PHAs production.

Generation of a functional, semisynthetic [FeFe]-hydrogenase in a photosynthetic microorganism

[FeFe]-Hydrogenases are hydrogen producing metalloenzymes with excellent catalytic capacities, highly relevant in the context of a future hydrogen economy. Here we demonstrate the synthetic activation of a heterologously expressed [FeFe]-hydrogenase in living cells of Synechocystis PCC 6803, a photoautotrophic microbial chassis with high potential for biotechnological energy applications. H2-Evolution assays clearly show that the non-native, semi-synthetic enzyme links to the native metabolism in living cells.

Novel quantitative insights into carbon sources for synthesis of poly hydroxybutyrate in Synechocystis PCC 6803

Many freshwater cyanobacteria accumulate polyhydroxybutyrate (PHB) under nitrogen or phosphorus deprivation. While prior literature has shed lights on transcriptomic and metabolomic changes in the model cyanobacterium Synechocystis PCC 6803 cells, the quantitative contributions of the newly fixed carbon following nitrogen deprivation or the externally added acetate to PHB synthesis are not clear. Similarly, it is not clear how photomixotrophy affects precursor contributions. In this study, we show that (i) the pre-growth mode (photoautotrophic or photomixotrophic), while significantly impacting glycogen levels, does not have any significant effect on PHB levels, (ii) the carbon fixed following nitrogen deprivation contributes 26% of C for PHB synthesis in photoautotrophically pre-grown cells and its contribution to the PHB synthesis goes down with the addition of acetate at the resuspension phase or with photomixotrophic pre-growth, (iii) the acetate added at the start of nitrogen deprivation, doubles the intracellular PHB levels and contributes 44-48% to PHB synthesis and this value is not greatly affected by how the cells were pre-grown. Indirectly, the labeling studies also show that the intracellular C recycling is the most important source of precursors for PHB synthesis, contributing about 74-87% of the C for PHB synthesis in the absence of acetate. The addition of acetate significantly reduces its contribution. In photoautotrophic pre-growth followed by acetate addition under nitrogen starvation, the contribution of intracellular C reduces to about 34%. Thus, our study provides several novel quantitative insights on how prior nutritional status affects the precursor contributions for PHB synthesis.

Cluster-Level Relationships of Genes Involved in Carbon Metabolism in Synechocystis sp. PCC 6803: Development of a Novel Succinate-Producing Strain

We quantified the transcript levels of 44 genes related to sugar catabolism in strains with altered primary carbon metabolism and discovered a consistent expression pattern among succinate-producing mutants. To identify factors that determine the expression pattern, we calculated Pearson's correlation coefficients, using the transcript data. Correlation analysis revealed positive and negative correlations among genes encoding sugar catabolic enzymes. On the basis of this analysis, we found that the mutant overexpressing both rre37 (encoding an OmpR-type response regulator) and sigE (encoding an RNA polymerase sigma factor) produced increased levels of succinate under dark, anaerobic conditions, with a maximum productivity of 420 mg l-1.

Polyhydroxyalkanoates (PHA) production from synthetic waste using Pseudomonas pseudoflava: PHA synthase enzyme activity analysis from P. pseudoflava and P. palleronii

Synthetic wastewater (SW) at various carbon concentrations (5-60g/l) were evaluated for polyhydroxyalkanoates (PHA) production using the bacteria Pseudomonas pseudoflava. Bacteria showed highest PHA production with 20g/l (57±5%), and highest carbon removal at 5g/l (74±6%) concentrations respectively. Structure, molecular weight, and thermal properties of the produced PHA were evaluated using various analytical techniques. Bacteria produced homo-polymer [poly-3-hydroxybutyrate (P3HB)] when only acetate was used as carbon source; and it produced co-polymer [poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV)] by addition of co-substrate propionate. PHA synthase, the enzyme which produce PHA was extracted from two bacterial strains i.e., P. pseudoflava and P. palleronii and its molecular weight was analysed using SDS-PAGE. Protein concentration, and PHA synthase enzyme activity of P. pseudoflava and P. palleronii was carried out using spectrophotometer. Results denoted that P. pseudoflava can be used for degradation of organic carbon persistent in wastewaters and their subsequent conversion into PHA.

The Multiple Functions of Common Microbial Carbon Polymers, Glycogen and PHB, during Stress Responses in the Non-Diazotrophic Cyanobacterium Synechocystis sp. PCC 6803

Classical microbial carbon polymers such as glycogen and polyhydroxybutyrate (PHB) have a crucial impact as both a sink and a reserve under macronutrient stress conditions. Most microbial species exclusively synthesize and degrade either glycogen or PHB. A few bacteria such as the phototrophic model organism Synechocystis sp. PCC 6803 surprisingly produce both physico-chemically different polymers under conditions of high C to N ratios. For the first time, the function and interrelation of both carbon polymers in non-diazotrophic cyanobacteria are analyzed in a comparative physiological study of single- and double-knockout mutants (ΔglgC; ΔphaC; ΔglgC/ΔphaC), respectively. Most of the observed phenotypes are explicitly related to the knockout of glycogen synthesis, highlighting the metabolic, energetic, and structural impact of this process whenever cells switch from an active, photosynthetic 'protein status' to a dormant 'glycogen status'. The carbon flux regulation into glycogen granules is apparently crucial for both phycobilisome degradation and thylakoid layer disassembly in the presence of light. In contrast, PHB synthesis is definitely not involved in this primary acclimation response. Moreover, the very weak interrelations between the two carbon-polymer syntheses indicate that the regulation and role of PHB synthesis in Synechocystis sp. PCC 6803 is different from glycogen synthesis.

Genetic engineering and metabolite profiling for overproduction of polyhydroxybutyrate in cyanobacteria

Genetic engineering and metabolite profiling for the overproduction of polyhydroxybutyrate (PHB), which is a carbon material in biodegradable plastics, were examined in the unicellular cyanobacterium Synechocystis sp. PCC 6803. Transconjugants harboring cyanobacterial expression vectors that carried the pha genes for PHB biosynthesis were constructed. The overproduction of PHB by the engineering cells was confirmed through microscopic observations using Nile red, transmission electron microscopy (TEM), or nuclear magnetic resonance (NMR). We successfully recovered PHB from transconjugants prepared from nitrogen-depleted medium without sugar supplementation in which PHB reached approximately 7% (w/w) of the dry cell weight, showing a value of 12-fold higher productivity in the transconjugant than that in the control strain. We also measured the intracellular levels of acetyl-CoA, acetoacetyl-CoA, and 3-hydroxybutyryl-CoA (3HB-CoA), which are intermediate products for PHB. The results obtained indicated that these products were absent or at markedly low levels when cells were subjected to the steady-state growth phase of cultivation under nitrogen depletion for the overproduction of bioplastics. Based on these results, efficient factors were discussed for the overproduction of PHB in recombinant cyanobacteria.

Increased bioplastic production with an RNA polymerase sigma factor SigE during nitrogen starvation in Synechocystis sp. PCC 6803

Because cyanobacteria directly harvest CO₂ and light energy, their carbon metabolism is important for both basic and applied sciences. Here, we show that overexpression of the sigma factor sigE in Synechocystis sp. PCC 6803 widely changes sugar catabolism and increases production of the biodegradable polyester polyhydroxybutyrate (PHB) during nitrogen starvation. sigE overexpression elevates the levels of proteins implicated in glycogen catabolism, the oxidative pentose phosphate pathway, and polyhydroxyalkanoate biosynthesis. PHB accumulation is enhanced by sigE overexpression under nitrogen-limited conditions, yet the molecular weights of PHBs synthesized by the parental glucose-tolerant and sigE overexpression strain are similar. Although gene expression induced by nitrogen starvation is changed and other metabolites (such as GDP-mannose and citrate) accumulate under sigE overexpression, genetic engineering of this sigma factor altered the metabolic pathway from glycogen to PHB during nitrogen starvation.

Metabolic Changes in Synechocystis PCC6803 upon Nitrogen-Starvation: Excess NADPH Sustains Polyhydroxybutyrate Accumulation

Polyhydroxybutyrate (PHB) is a common carbon storage polymer among heterotrophic bacteria. It is also accumulated in some photoautotrophic cyanobacteria; however, the knowledge of how PHB accumulation is regulated in this group is limited. PHB synthesis in Synechocystis sp. PCC 6803 is initiated once macronutrients like phosphorus or nitrogen are limiting. We have previously reported a mutation in the gene sll0783 that impairs PHB accumulation in this cyanobacterium upon nitrogen starvation. In this study we present data which explain the observed phenotype. We investigated differences in intracellular localization of PHB synthase, metabolism, and the NADPH pool between wild type and mutant. Localization of PHB synthase was not impaired in the sll0783 mutant; however, metabolome analysis revealed a difference in sorbitol levels, indicating a more oxidizing intracellular environment than in the wild type. We confirmed this by directly measuring the NADPH/NADP ratio and by altering the intracellular redox state of wild type and sll0783 mutant. We were able to physiologically complement the mutant phenotype of diminished PHB synthase activity by making the intracellular environment more reducing. Our data illustrate that the NADPH pool is an important factor for regulation of PHB biosynthesis and metabolism, which is also of interest for potential biotechnological applications.

Direct dissolution of wet and saliferous marine microalgae by polar ionic liquids without heating

We successfully dissolved wet and saliferous microalgae (WSM) in polar ionic liquids (ILs) under mild conditions. The Kamlet-Taft parameters, especially β for the ILs, were good predictors of the ability to dissolve WSM. 1-Ethyl-3-methylimidazolium methylphosphate ([C2mim][MeO(H)PO(2)]) was the IL that best dissolved WSM without heating. WSM (containing 95 wt% water) was mixed with [C2mim][MeO(H)PO(2)]; the WSM had dissolved completely within 30 min at room temperature with gentle stirring. The IL maintained its chemical structure after removal of the microalgae component, suggesting recyclable use. The concentration of contaminant mineral salts in the [C2mim][MeO(H)PO(2)] did not increase with increasing recycle number. The recycled [C2mim][MeO(H)PO(2)] maintained its ability to dissolve WSM regardless of the number of recycling studied here.

Requirement of the nitrogen starvation-induced protein Sll0783 for polyhydroxybutyrate accumulation in Synechocystis sp. strain PCC 6803

Nitrogen is often a limiting nutrient in natural habitats. Therefore, cyanobacteria have developed multiple responses, which are controlled by transcription factor NtcA and the PII-signaling protein, to adapt to nitrogen deficiency. Transcriptional analyses of Synechocystis sp. strain PCC 6803 under nitrogen-deficient conditions revealed a highly induced gene (sll0783) which is annotated as encoding a conserved protein with an unknown function. This gene is part of a cluster of seven genes and has potential NtcA-binding sites in the upstream region. Homologues of this cluster occur in some unicellular, nondiazotrophic cyanobacteria and in several Alpha, Beta-, and Gammaproteobacteria, as well as in some Gram-positive bacteria. Most of the heterotrophic bacteria harboring this gene cluster are able to fix nitrogen and to produce polyhydroxybutyrate (PHB), whereas of the cyanobacteria, only Synechocystis sp. strain PCC 6803 can accumulate PHB. In this work, a Synechocystis sp. strain PCC 6803 sll0783 gene knockout mutant is characterized. This mutant is unable to accumulate PHB, a carbon and energy storage compound. In contrast, the levels of the carbon storage compound glycogen and the PHB precursor acetyl coenzyme A were similar to those of the wild type, indicating that the PHB-deficient phenotype does not likely result from a global deficiency in carbon metabolism. A specific deficiency in PHB synthesis was implied by the fact that the mutant exhibits impaired PHB synthase activity during prolonged nitrogen starvation. However, the expression of PHB synthase-encoding genes was not strongly affected in the mutant, suggesting that the impaired PHB synthase activity observed depends on a posttranscriptional process in which the product of sll0783 is involved.

Identification and characterization of the intracellular poly-3-hydroxybutyrate depolymerase enzyme PhaZ of Sinorhizobium meliloti

Background

S. meliloti forms indeterminate nodules on the roots of its host plant alfalfa (Medicago sativa). Bacteroids of indeterminate nodules are terminally differentiated and, unlike their non-terminally differentiated counterparts in determinate nodules, do not accumulate large quantities of Poly-3-hydroxybutyrate (PHB) during symbiosis. PhaZ is in intracellular PHB depolymerase; it represents the first enzyme in the degradative arm of the PHB cycle in S. meliloti and is the only enzyme in this half of the PHB cycle that remains uncharacterized.

Results

The S. meliloti phaZ gene was identified by in silico analysis, the ORF was cloned, and a S. meliloti phaZ mutant was constructed. This mutant exhibited increased PHB accumulation during free-living growth, even when grown under non-PHB-inducing conditions. The phaZ mutant demonstrated no reduction in symbiotic capacity; interestingly, analysis of the bacteroids showed that this mutant also accumulated PHB during symbiosis. This mutant also exhibited a decreased capacity to tolerate long-term carbon starvation, comparable to that of other PHB cycle mutants. In contrast to other PHB cycle mutants, the S. meliloti phaZ mutant did not exhibit any decrease in rhizosphere competitiveness; however, this mutant did exhibit a significant increase in succinoglycan biosynthesis.

Conclusions

S. meliloti bacteroids retain the capacity to synthesize PHB during symbiosis; interestingly, accumulation does not occur at the expense of symbiotic performance. phaZ mutants are not compromised in their capacity to compete for nodulation in the rhizosphere, perhaps due to increased succinoglycan production resulting from upregulation of the succinoglycan biosynthetic pathway. The reduced survival capacity of free-living cells unable to access their accumulated stores of PHB suggests that PHB is a crucial metabolite under adverse conditions.

The intracellular concentration of acetyl phosphate in Escherichia coli is sufficient for direct phosphorylation of two-component response regulators

Acetyl phosphate, the intermediate of the AckA-Pta pathway, acts as a global signal in Escherichia coli. Although acetyl phosphate clearly signals through two-component response regulators, it remains unclear whether acetyl phosphate acts as a direct phospho donor or functions through an indirect mechanism. We used two-dimensional thin-layer chromatography to measure the relative concentrations of acetyl phosphate, acetyl coenzyme A, ATP, and GTP over the course of the entire growth curve. We estimated that the intracellular concentration of acetyl phosphate in wild-type cells reaches at least 3 mM, a concentration sufficient to activate two-component response regulators via direct phosphoryl transfer.

Poly-beta-hydroxybutyrate accumulation in Nostoc muscorum: effects of metabolic inhibitors

Poly-beta-hydroxybutyrate (PHB) accumulation in Nostoc muscorum was studied in presence of various metabolic inhibitors. Supplementation of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was found to suppress PHB accumulation in phosphate-limited N. muscorum under photoautotrophic growth condition. PHB accumulation increased up to 21% and 17% from an initial PHB content of 8.5% of dry weight, respectively, under carbonylcyanide m-chlorophenylhydrazone (CCCP) and dicyclohexylcarbodiimide (DCCD) treatment, whereas 2,4 -dinitrophenol (DNP) supplementation depicted insignificant effect on PHB pool of the test cyanobacterium. Supplementation of l-methionine-dl-sulfoximine (MSX) and azaserine was also found to increase PHB accumulation in N(2) -fixing and NH(4)(+) -grown N. muscorum, but not in NO(3)(-) -grown cells. The stimulatory action of monofluoroacetate on PHB accumulation was suppressed in presence of alpha-ketoglutarate and DCMU. Interestingly, 2,3 -butanedione supplementation was not only found inhibitory for accumulation of PHB in P-deficient, N-deficient and chemoheterotrophically grown N. muscorum but suppression of PHB synthesis was also evident in control cultures in presence of 2,3 -butanedione. The possible mechanisms are discussed.

Studies on poly-beta-hydroxybutyrate synthase activity of Nostoc muscorum

This study compares the PHB synthase activity of Nostoc muscorum, a N(2)-fixing cyanobacterium under control (grown in usual BG-11 medium), nitrogen (N) and phosphorus (P) deprivation and chemoheterotrophic conditions. Specific activity of PHB synthase did not depict significant variations in the latter three types of cultures, except for the control one, where a significantly lower activity was recorded. PHB synthase activity was detected only in the soluble fractions of both the control as well as cells incubated under chemoheterotrophic conditions. A K(m) of 80.2 microM DL-beta-hydroxybutyryl-CoA and V(max) of 197.5 nmol thiobenzoate (TNB) mg protein(-1)min(-1) were observed for the enzyme. PHB synthase remained insensitive to acetyl-CoA, ATP, NADP, NADPH supplementation under in vitro condition. Addition of acetyl phosphate was found to activate the enzyme and the level of activation was dependent on the concentration of acetyl phosphate supplementation. Inhibition of PHB synthase in 2,3-butanedione supplemented cultures and reactivation following acetyl phosphate addition proved the post-translational control of acetyl phosphate over PHB synthase.

The acetate switch

To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the "acetate switch," occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the "switch" (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl approximately P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl approximately P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl approximately P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl approximately P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to "flip the switch," the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the "acetate switch" as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

Polyhydroxyalkanoate (PHA) biosynthesis in Thermus thermophilus: purification and biochemical properties of PHA synthase

The biosynthesis of polyhydroxyalkanoates (PHAs) was studied, for the first time, in the thermophilic bacterium Thermus thermophilus. Using sodium gluconate (1.5% w/v) or sodium octanoate (10 mM) as sole carbon sources, PHAs were accumulated to approximately 35 or 40% of the cellular dry weight, respectively. Gas chromatographic analysis of PHA isolated from gluconate-grown cells showed that the polyester (Mw: 480,000 g mol(-1)) was mainly composed of 3-hydroxydecanoate (3HD) with a molar fraction of 64%. In addition, 3-hydroxyoctanoate (3HO), 3-hydroxyvalerate (3HV) and 3-hydroxybutyrate (3HB) occurred as constituents. In contrast, the polyester (Mw: 391,000 g mol(-1)) from octanoate-grown cells was composed of 24.5 mol% 3HB, 5.4 mol% 3HO, 12.3 mol% 3-hydroxynonanoate (3HN), 14.6 mol% 3HD, 35.4 mol% 3-hydroxyundecanoate (3HUD) and 7.8 mol% 3-hydroxydodecanoate (3HDD). Activities of PHA synthase, a beta-ketothiolase and an NADPH-dependent reductase were detected in the soluble cytosolic fraction obtained from gluconate-grown cells of T. thermophilus. The soluble PHA synthase was purified 4271-fold with 8.5% recovery from gluconate-grown cells, presenting a Km of 0.25 mM for 3HB-CoA. The optimal temperature of PHA synthase activity was about 70 degrees C and acts optimally at pH near 7.3. PHA synthase activity was inhibited 50% with 25 microM CoA and lost all of its activity when it was treated with alkaline phosphatase. T. thermophilus PHA synthase, in contrary to other reported PHA synthases did not exhibit a lag phase on its kinetics, when low concentration of the enzyme was used. Incubation of PHA synthase with 1 mM N-ethyl-maleimide inhibits the enzyme 56%, indicating that cysteine might be involved in the catalytic site of the enzyme. Acetyl phosphate (10 mM) activated both the native and the dephosphorylated enzyme. A major protein (55 kDa) was detected by SDS-PAGE. When a partially purified preparation was analyzed on native PAGE the major band exhibiting PHA synthase activity was eluted from the gel and analyzed further on SDS-PAGE, presenting the first purification of a PHA synthase from a thermophilic microorganism.

A repressor protein, PhaR, regulates polyhydroxyalkanoate (PHA) synthesis via its direct interaction with PHA

Phasins (PhaP) are predominantly polyhydroxyalkanoate (PHA) granule-associated proteins that positively affect PHA synthesis. Recently, we reported that the phaR gene, which is located downstream of phaP in Paracoccus denitrificans, codes for a negative regulator involved in PhaP expression. In this study, DNase I footprinting revealed that PhaR specifically binds to two regions located upstream of phaP and phaR, suggesting that PhaR plays a role in the regulation of phaP expression as well as autoregulation. Many TGC-rich sequences were found in upstream elements recognized by PhaR. PhaR in the crude lysate of recombinant Escherichia coli was able to rebind specifically to poly[(R)-3-hydroxybutyrate] [P(3HB)] granules. Furthermore, artificial P(3HB) granules and 3HB oligomers caused the dissociation of PhaR from PhaR-DNA complexes, but native PHA granules, which were covered with PhaP or other nonspecific proteins, did not cause the dissociation. These results suggest that PhaR is able to sense both the onset of PHA synthesis and the enlargement of the granules through direct binding to PHA. However, free PhaR is probably unable to sense the mature PHA granules which are already covered sufficiently with PhaP and/or other proteins. An in vitro expression experiment revealed that phaP expression was repressed by the addition of PhaR and was derepressed by the addition of P(3HB). Based on these findings, we present here a possible model accounting for the PhaR-mediated mechanism of PHA synthesis. Widespread distribution of PhaR homologs in short-chain-length PHA-producing bacteria suggests a common and important role of PhaR-mediated regulation of PHA synthesis.

Effect of increased PHA synthase activity on polyhydroxyalkanoates biosynthesis in Synechocystis sp. PCC6803

Polyhydroxyalkanoate (PHA) synthase activity in Synechocystis sp. PCC6803 was increased two-fold by introducing the PHA biosynthetic genes of Ralstonia eutropha. The resulting recombinant Synechocystis sp. PCC6803 strain was subjected to conditions that favor PHA accumulation and the effects of various carbon sources were studied. In addition, the fine structure of both wild-type and recombinant Synechocystis sp. PCC6803 was examined using freeze-fracture electron microscopy technique. The PHA granules in the recombinant Synechocystis sp. PCC6803 were localised near the thylakoid membranes. Maximum amount of PHA accumulation was obtained in the presence of acetate, where the number of granules in the recombinant cells ranged from 4 to 6 and their sizes were in the range of 70-240 nm. In comparison to wild-type Synechocystis sp. PCC6803, recombinant cells with increased PHA synthase activity showed only a marginal increase in PHA content suggesting that PHA synthase is not the rate limiting enzyme of PHA biosynthesis in Synechocystis sp. PCC6803.

Photosynthetic accumulation of poly-(hydroxybutyrate) by cyanobacteria--the metabolism and potential for CO2 recycling

Regulatory mechanism in PHB [poly-(hydroxybutyrate)] accumulation by cyanobacteria, especially by a thermophilic isolate, Synechococcus MA19 was reviewed in comparison with a genetically engineered strain. The strain, MA19 accumulates PHB under nitrogen starved and photoautotrophic conditions (MA19-N). Little PHB synthase activity was detected in crude extracts from the cells grown in nitrogen sufficient conditions (MA19 + N). The activity was detected exclusively in membrane fractions from MA19 + N. The change of the enzyme activity was insensitive to chloramphenicol, which suggests post-translational activation. In vitro, acetyl phosphate activated PHB synthase in membrane fractions from MA19 + N, and the extent of activation depended on the concentration of acetyl phosphate. Phosphotransacetylase which catalyzes the conversion of acetyl-CoA to acetyl phosphate was detected in crude extracts from MA19-N but not in those from MA19 + N. These results suggested that intracellular acetyl phosphate concentration could be controlled, depending on C-N balance and intracellular acetyl-CoA concentration. On the contrary, in genetically-engineered cyanobacterium (transformant with PHB synthesizing genes from Ralstonia eutropha), it did not seem to be PHB synthase but acetyl-CoA flux that limits PHB synthesis. The closer association of PHB granules with thylakoid membranes in MA19 is suggested than that in the genetically-engineered cyanobacterium, which may reflect the difference of distribution of PHB synthase. Transposon-mutagenesis was used to acquire mutants of its altered PHB regulatory mechanism. PHA production by cyanobacteria was considered from the aspects of photobioreactors.

Nitrogen-starvation-induced chlorosis in Synechococcus PCC 7942: adaptation to long-term survival

When deprived of essential nutrients, the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 undergoes a proteolytic degradation of the phycobiliproteins, its major light-harvesting pigments. This process is known as chlorosis. This paper presents evidence that the degradation of phycobiliproteins is part of an acclimation process in which growing cells differentiate into non-pigmented cells able to endure long periods of starvation. The time course of degradation processes differs for various photosynthetic pigments, for photosystem I and photosystem II activities and is strongly influenced by the illumination and by the experimental conditions of nutrient deprivation. Under standard experimental conditions of combined nitrogen deprivation, three phases of the differentiation process can be defined. The first phase corresponds to the well-known phycobiliprotein degradation, in phase 2 the cells lose chlorophyll a prior to entering phase 3, the fully differentiated state, in which the cells are still able to regenerate pigmentation after the addition of nitrate to the culture. An analysis of the protein synthesis patterns by two-dimensional gel electrophoresis during nitrogen starvation indicates extensive differential gene expression, suggesting the operation of tight regulatory mechanisms.