Microbial mineral and anode reduction
Central to the mineral (or in anodic) respiratory pathways in Shewanella oneidensis MR-1 is CymA, a cytoplasmic membrane-bound tetraheme c-type cytochrome that functions as the major hydroquinone dehydrogenase and couples electron transport to various periplasmic oxidoreductases. We isolated ΔcymA suppressor mutants, and characterized them biochemically and genetically. Interestingly in the characterized ΔcymA suppressor mutant, respiration of fumarate, ferric citrate, and DMSO, was restored but not of nitrate. The suppression was found to be due to transcriptional activation of a putative periplasmic iron sulfur protein, previously annotated as SirC, and an integral membrane hydroquinone dehydrogenase, SirD, respectively. Analyses of in-frame gene deletions of sirC and sirD indicated that both genes were essential in conferring suppression to a ΔcymA suppressor mutant. Biochemical in vitro reconstitution experiments confirmed electron transport between formate, a physiologically relevant electron donor, and fumarate via fumarate reductase by suppressor membrane fractions. The suppression was found to be caused by insertion of an ISSod1 element upstream of the sirCD transcriptional start site, but downstream of the associated terminal reductase sirA transcriptional start site, generating a novel, constitutively active hybrid promoter which uncoupled expression of sirCD from its associated terminal reductase. This work revealed that an alternative route of electron transfer from quinol to multiple terminal oxidoreductases independent of CymA can evolve in S. oneidensis MR-1.
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