The Role of the Tetraheme Cytochrome c3 in Desulfovibrio vulgaris Hildenborough Metabolism

Abstract

The role of tetraheme cytochrome c3 (CycA) in the metabolism of the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough (DvH) was investigated by deletion of the cycA gene using a marker-exchange deletion strategy. A highly abundant periplasmic cytochrome, CycA has the important function of transferring electrons from periplasmic hydrogenases (Hyd, Hyn, Hys) to transmembrane complexes which transport the electrons to the cytoplasm where sulfate is reduced. Previous studies have indicated that during its interaction with periplasmic hydrogenases, CycA is also involved in the reduction of toxic metals. Growth of the cycA mutant strain on lactate as the electron donor and sulfate as the terminal electron acceptor showed that, despite its abundance, CycA is not essential for DvH growth. However, the rate of growth of the mutant strain was significantly lower, and the extent of growth less, than rates and extents of growth of the wild type and complement strains on lactate/sulfate medium. This indicates that a portion of the electrons generated from cytoplasmic lactate oxidation are transported by CycA for energy production, possibly in a hydrogen cycling mechanism employed to generate ATP. Failure of the mutant strain to grow on either formate or H2, with sulfate or sulfite as electron acceptors, further indicated that CycA may be the only redox partner of periplasmic hydrogenases. The cycA mutant strain also did not grow as well as either the wild type or complement strains on medium supplemented with pyruvate/sulfate. Final growth on pyruvate/sulfate was comparable, but the mutant grew more slowly than the wild type and complement strains. Interestingly, the mutant grew better than the wild type or complement strains on pyruvate alone, possibly due to the release of H2 and/or CO2 in concentrations which may be somewhat inhibitory to wild type growth.