regO: a novel locus in the regulation of tetrapyrrole biosynthesis in Rhodospirillum rubrum

Abstract

Purpose A new locus, regO, involved in the regulation of photosynthesis gene expression in response to oxygen and light, has been studied in Rhodosprillum rubrum ATCC1117 (Rsp. rubrum) for identification of its function. Methods Inactivation of regO by interposon mutagenesis resulted in the inability of cells to grow photosynthetically, (i.e. become PS–). Protein domain analysis of RegO using the BLAST engine was also performed. Results The mutant strain was able to grow only anaerobically in the dark in the presence of DMSO as an external electron acceptor. Under these conditions, the mutant strain produced substantially lower amounts of photosynthetic membranes, indicating that regO is involved in the regulation of photosynthetic gene expression in response to anaerobiosis. The Rsp. rubrum REGO–disrupted mutant recovered the synthesis of photosynthetic membranes and retained regulation by light and/or oxygen tension when wild-type regO was provided in-trans. Protein domain analysis of RegO revealed that it encodes a multi-domain sensor histidine kinase (HK). The signal-input domains, or PAS domains, bear strong similarities to putative heme-bound sensors involved in sensing light, redox potential, and/or oxygen. The output HK domain exhibits strong homology to sensor domains from bacterial two-component systems involved in signal transduction in response to the same environmental signals. Conclusion regO is coding for a sensor histidine kinase that belongs to bacterial two-component systems responsible for signal transduction in response to light and oxygen, particularly in the absence of oxygen. It is believed to be involved in the regulation of tetrapyrrole biosynthesis, which was shown as a lack of photosynthetic membranes in the mutant strain REGO– .Unlike other sensor kinase homologues from related anoxygenic phototrophic bacterial species, although functionally similar to RegB and PrrB, RegO is predicted to lack transmembrane domains and is thus expected to be a cytosolic member of a two-component signal transduction system. RegO also differs from its functional homologues, Reg B/PrrB sensor protein kinases, of the two component systems in that it lacks the second component of this two-component signal transduction system found in the neighboring genes. That encouraged us to give it the name RegO, indicating the lack of a cognate response regulator similar to Reg A/PrrA on other closely related anoxygenic Rhodobacter species.