Abstract
A new genetic locus was identified in Rhodobacter sphaeroides which is required for optimal synthesis of the light-harvesting spectral complexes as well as for optimal growth under anaerobic conditions with dimethyl sulfoxide (DMSO) as a terminal electron acceptor. The primary structure of the deduced osp gene product shows significant homology to the receiver domain of known response regulators common to bacterial two-component systems. However, site-directed mutagenesis revealed that the Osp protein appears not to be involved in a phospho-relay signal transduction pathway. Paradoxically, the effect of the Osp protein upon spectral complex levels is exerted at the transcriptional level of photosynthesis gene expression. The absence of the Osp protein does not appear to have a general effect on house-keeping metabolism. In cells lacking Osp, the levels of DMSO reductase appear to be normal. The quaternary structure of the Osp protein was determined to be a homodimer and it was directly demonstrated that Osp does not bind to the promoter region of photosynthesis genes as judged by mobility-shift experiments and primary structure analysis.
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