Biosynthesis of Polyhydroxyalkanoates and their Regulation in Rhizobia
During free living reproductive growth Rhizobium spp. is capable to accumulate poly-ß-hydroxybutyrate (PHB) and to synthesize intracellular glycogen. These bacteria can also produce and excrete exopolysaccharides and ß-1,2-glucan. Rhizobia provide an excellent model to investigate the connection between cellular metabolism and polyester accumulation. We have shown that under oxygen-limiting conditions, free-living cells of Sinorhizobium meliloti 41 can use intracellular glycogen to generate ATP, while maintaining their PHB content. PHB synthesis serves as an alternative pathway for storage/regeneration of reducing equivalents. We have described genes involved in PHB biosynthesis in S. meliloti encoding for ß-ketothiolase (phaA), acetoacetyl-CoA reductase (phaB) and PHA-synthase (phaC) together with an open reading frame, referred to as aniA. Under oxygen-limiting conditions (such as conditions in the bacteroid state) aniA is actively expressed, and a mutation in this gene generates an overproduction of extracellular polymeric substances (EPS). This finding suggests that the production of EPS could be directly or indirectly regulated by aniA. Therefore, in S. meliloti, aniA is likely to be involved in carbon/energy flux regulation that, in turn, is dependent upon oxygen availability. By hybridization studies we revealed, in various soil bacteria the presence of genes with sequence similarity to aniA of S. meliloti 41. These results will be important to gain a deeper insight into aniA function in the control of PHB (more generally PHA) and EPS biosynthesis.
KeywordsExtracellular Polymeric Substance Malic Enzyme Extracellular Polymeric Substance Production Free Life Intracellular Glycogen
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