THE HMC OPERON OF DESULFOVIBRIO-VULGARIS SUBSP VULGARIS HILDENBOROUGHENCODES A POTENTIAL TRANSMEMBRANE REDOX PROTEIN COMPLEX

The nucleotide sequence of the hmc operon from Desulfovibrio vulgaris subsp. vulgaris Hildenborough indicated the presence of eight open rea ding frames, encoding proteins Orf1 to Orf6, Rrf1, and Rrf2. Orf1 is t he periplasmic, high-molecular-weight cytochrome (Hmc) containing 16 c -type hemes and described before (W. B. R. Pollock, M. Loutfi, M. Brus chi, B. J. Rapp-Giles, J. D. Wall, and G. Voordouw, J. Bacteriol. 173: 220-228, 1991). Orf2 is a transmembrane redox protein with four iron-s ulfur clusters, as indicated by its similarity to DmsB from Escherichi a coli. Orf3, Orf4, and Orf5 are all highly hydrophobic, integral memb rane proteins with similarities to subunits of NADH dehydrogenase or c ytochrome c reductase. Orf6 is a cytoplasmic redox protein containing two iron-sulfur clusters, as indicated by its similarity to the ferred oxin domain of [Fe] hydrogenase from Desulfovibrio species. Rrf1 belon gs to the family of response regulator proteins, while the function of Rrf2 cannot be derived from the gene sequence. The expression of indi vidual genes in E. coli with the T7 system confirmed the open reading frames for Orf2, Orf6, and Rrf1. Deletion of 0.4 kb upstream from orf1 abolished the expression of Hmc in D. desulfuricans G200, indicating this region to contain the hmc operon promoter. The expression of two truncated hmc genes in D. desulfuricans G200 resulted in stable peripl asmic c-type cytochromes, confirming the domain structure of Hmc. We p ropose that Hmc and Orf2 to Orf6 form a transmembrane protein complex that allows electron flow from the periplasmic hydrogenases to the cyt oplasmic enzymes that catalyze the reduction of sulfate. The domain st ructure of Hmc may be required to allow interaction with multiple hydr ogenases.