Regulation of CO2 fixation in non-sulfur purple photosynthetic bacteria

Our laboratory studies the regulation Of CO2 fixation in phototrophic microorganisms. The expression of genes encoding enzymes responsible for assimilating CO2 is primarily regulated by a LysR-type transcriptional activator, CbbR, that functions by binding specific sites in the promoter region of both the cbb(I) and cbb(II) CO2 fixation operons under appropriate growth conditions. This straightforward model of regulation has a further level of complexity in the non-sulfur photosynthetic bacterium Rhodopseudomonas palustris. The recent genome sequencing project of this organism identified a unique two-component system interposed between the structural genes of the cbb(I) operon (encoding form I RubisCO) and CbbR. Two-component systems are a paradigm of gene regulation in Bacteria. In general, they consist of a sensor kinase and a response regulator, that upon stimulation, transduces an external stimulus in a His-Asp phospho-relay, ultimately modulating gene expression. The system identified in R. palustris, in addition to its peculiar modular architecture, also has multiple PAS motifs, suggesting a complex redox sensing activity coordinating a multi-step phospho-relay which regulates CO2 fixation under specific growth conditions.