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The ability of Rhodobacter sphaeroides 2.4.1(T) to respire anaerobically with the alternative electron acceptor dimethyl sulfoxide (DMSO) or trimethylamine N-oxide (TMAO) is manifested by the molybdoenzyme DMSO reductase, which is encoded by genes of the dor locus. Previously, we have demonstrated that dor expression is regulated in response to lowered oxygen tensions and the presence of DMSO or TMAO in the growth medium. Several regulatory proteins have been identified as key players in this regulatory cascade: FnrL, DorS-DorR, and DorX-DorY. To further examine the role of redox potentiation in the regulation of dor expression, we measured DMSO reductase synthesis and beta-galactosidase activity from dor::lacZ fusions in strains containing mutations in the redox-active proteins CcoP and RdxB, which have previously been implicated in the generation of a redox signal affecting photosynthesis gene expression. Unlike the wild-type strain, both mutants were able to synthesize DMSO reductase under strictly aerobic conditions, even in the absence of DMSO. When cells were grown photoheterotrophically, dorC::lacZ expression was stimulated by increasing light intensity in the CcoP mutant, whereas it is normally repressed in the wild-type strain under such conditions. Furthermore, the expression of genes encoding the DorS sensor kinase and DorR response regulator proteins was also affected by the ccoP mutation. By using CcoP-DorR and CcoP-DorY double mutants, it was shown that the DorR protein is strictly required for altered dor expression in CcoP mutants. These results further demonstrate a role for redox-generated responses in the expression of genes encoding DMSO reductase in R. sphaeroides and identify the DorS-DorR proteins as a redox-dependent regulatory system controlling dor expression.

Under anaerobic-dark growth conditions, in the presence of the alternative electron acceptor dimethyl sulfoxide (DMSO) or trimethylamine N-oxide (TMAO), Rhodobacter sphaeroides 2.4.1(T) respires anaerobically using the molybdoenzyme DMSO reductase (DMSOR). Genes encoding DMSOR and associated proteins are encoded by genes of the dor locus. Previously, we demonstrated that the expression of DMSOR is regulated by both the oxygen status of the cell via the FnrL protein and by the presence of DMSO or TMAO, presumably through the DorS-DorR two-component sensor-regulator system. Here we further investigate expression of the dor genes through the use of transcriptional lacZ fusions to the dorS, dorR, and dorC promoters. The expression of dorC::lacZ was strongly induced by the absence of oxygen and presence of DMSO. In accordance with our previous findings of DMSOR activity, dorC::lacZ expression was reduced by up to one-third when cells were grown photosynthetically in the presence of DMSO with medium or high light, compared to the expression observed after anaerobic-dark growth. The induction of dorC::lacZ expression in the presence of DMSO was dependent on the DorS and DorR proteins. Expression of the dorS and dorR genes was also induced in the absence of oxygen. In an FnrL mutant, dorS::lacZ expression was not induced when oxygen tensions in the media were lowered, in contrast to what occurred in the wild-type strain. The expression of dorS::lacZ and dorR::lacZ was dependent on the DorS and DorR proteins themselves, suggesting the importance of autoregulation. These results demonstrate a cascade regulation of dor gene expression, where the expression of the regulatory proteins DorS and DorR governs the downstream regulation of the dorCBA operon encoding the structural proteins of DMSOR.

The ccoNOQP gene cluster of Rhodobacter sphaeroides 2.4.1T encodes a cbb3 cytochrome oxidase which is utilized in oxygen-limited conditions for aerobic respiration. The beta-galactosidase activity of a ccoN::lacZ transcriptional fusion was low under high (30%)-oxygen and anaerobic growth conditions. Maximal ccoN::lacZ expression was observed when the oxygen concentration was lowered to 2%. In an FnrL mutant, ccoN::lacZ expression was significantly lower than in the wild-type strain, suggesting that FnrL is a positive regulator of genes encoding the cbb3 oxidase.

The Azotobacter vinelandii mod locus, which is involved in high-affinity molybdate transport and the early event in Mo metabolism, consists of two divergently transcribed operons, modG and modEABC. modA, modB and modC encode the components of the high-affinity molybdate transporter, and modG encodes a Mo-binding protein. High concentrations of Mo repressed transcription of both operons. The modEABC operon was also repressed by tungstate and to a lesser extent by vanadate. modE, the first gene in the modEABC operon, controlled the Mo-dependent transcription of both operons. It was not involved in the metal regulation of alternative nitrogenase gene expression. Although a modE mutant constitutively expressed genes encoding the molybdate transporter, it had a reduced rate of Mo accumulation.