Cytochrome Complex Essential for Photosynthetic Oxidation of both Thiosulfate and Sulfide in Rhodovulum sulfidophilum

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Abstract
Many photosynthetic bacteria use inorganic sulfur compounds as electron donors for carbon dioxide fixation. A thiosulfate-induced cytochrome c has been purified from the photosynthetic α-proteobacterium Rhodovulum sulfidophilum . This cytochrome c 551 is a heterodimer of a diheme 30-kDa SoxA subunit and a monoheme 15-kDa SoxX subunit. The cytochrome c 551 structural genes are part of an 11-gene sox locus. Sequence analysis suggests that the ligands to the heme iron in SoxX are a methionine and a histidine, while both SoxA hemes are predicted to have unusual cysteine-plus-histidine coordination. A soxA mutant strain is unable to grow photoautotrophically on or oxidize either thiosulfate or sulfide. Cytochrome c 551 is thus essential for the metabolism of both these sulfur species. Periplasmic extracts of wild-type R. sulfidophilum exhibit thiosulfate:cytochrome c oxidoreductase activity. However, such activity can only be measured for a soxA mutant strain if the periplasmic extract is supplemented with purified cytochrome c 551 . Gene clusters similar to the R. sulfidophilum sox locus can be found in the genome of a green sulfur bacterium and in phylogenetically diverse nonphotosynthetic autotrophs.

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