Genetics and control of CO2 assimilation in the chemoautotroph Ralstoniaeutropha
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The nutritional versatility of facultative autotrophs requires efficient overall control of their metabolism. Most of these organisms are Proteobacteria that assimilate CO2 via the highly energy-demanding Calvin-Benson-Bassham reductive pentose-phosphate cycle. The enzymes of the cycle are encoded by cbb genes organized in cbb operons differing in size and composition, although conserved features are apparent. Transcription of the operons, which may form regulons, is strictly controlled, being induced during autotrophic but repressed to varying extents during heterotrophic growth of the bacteria. The chemoautotroph Ralstoniaeutropha is one of the organisms studied extensively for the mechanisms involved in the expression of cbb gene systems. CbbR is a LysR-type transcriptional regulator and the key activator protein of cbb operons. The cbbR gene is typically located adjacent and in divergent orientation to its cognate operon. The activating function of CbbR seems to be modulated by metabolites signaling the nutritional state of the cell to the cbb system. Phosphoenolpyruvate is such a signal metabolite acting as a negative effector of R. eutropha CbbR, whereas NADPH has been proposed to be a coactivator of the protein in two other chemoautotrophs, Xanthobacterflavus and Hydrogenophilusthermoluteolus. There is evidence for the participation of additional regulators in cbb control. In the photoautotrophs Rhodobactercapsulatus and Rhodobactersphaeroides, response regulator RegA of the global two-component signal transduction system RegBA serves this function. It is conceivable that specific variants of cbb control systems have evolved to ensure their optimal integration into regulatory networks operating in the diverse autotrophs characterized by different metabolic capabilities.
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