A mutant of Rhizobium etli, that

did not accumulate PHB, was shown to significantly fix more nitrogen than the isogenic wild type [10, 11], whereas non-fixing nifH mutants of R. etli[12] and Bradyrhizobium japonicum[13] accumulated more PHB than their isogenic nitrogen-fixing parental strains. There is a conflict between rhizobia AZD0530 ic50 and legumes over the rate of PHB accumulation, due to the metabolic tradeoff between nitrogen fixation and PHB accumulation. Therefore, PHB biosynthesis and accumulation in species of rhizobia may be controlled to balance the tradeoff, but the mechanism underlying this control has not yet been fully explained. One of the best studied microorganisms with respect to PHB biosynthesis and accumulation is the Gram-negative bacterium Ralstonia eutropha[14]. It synthesizes PHB using the three PHB synthetic genes: phbA, which encodes 3-ketoacyl-CoA thiolase; phbB, which encodes acetoacetyl CoA reductase; and phbC, which encodes the enzyme PHB synthase. PHB degradation, however, is performed by PHB depolymerase, which is encoded by phaZ. Phasins, encoded by phaP, are a class of low-molecular-mass amphipathic proteins that form a layer at the surface

BMS-777607 purchase of the PHB granule and stabilize it [15]. The R. eutropha possesses at least four phaP paralogs identified so far [16]. Expression of the major phasin, encoded by phaP1, is regulated by the transcriptional Depsipeptide mouse repressor PhaR [17, 18]. Under conditions less favorable for PHB biosynthesis, PhaR binds to the phaP1 promoter region to repress transcription of this gene. After the onset of PHB biosynthesis, when the nascent PHB granules gradually form, PhaR leaves the promoter and binds to the granules so that phaP1 is transcribed

and translated. During the later stages of PHB accumulation, PhaR is estimated to bind no longer to the granules as it is displaced by PhaP1 phasin. The displaced PhaR returns to bind to the phaP1 promoter and represses transcription again [16]. Most members of the Rhizobiaceae are known to possess single copies of the PHB biosynthesis genes. For instance, strains of Sinorhizobium meliloti, the symbionts of alfalfa, regarded as one of the model organisms to study symbiotic nitrogen fixation, are characterized to have a single set of the genes for PHB metabolism, namely phbA, phbB, phbC, and phaZ[19, 20], whereas two paralogous genes, phaP1 and phaP2, encode functional phasins [21]. On the other hand, strains of B. japonicum, the symbionts of soybean, are known to accumulate a large amount of PHB [22], and the B. japonicum USDA110 genome was found to contain five paralogs of phbC, as well as two paralogs of phbAB[23]. This genetic redundancy may suggest a functional importance that has not yet been fully elucidated. In this study, we examined the expression profiles of the paralogs relevant to PHB metabolism in free-living B.