Department of Biological Sciences, University of Calgary, Canada.
It has been shown [Pierik, A. J., Duyvis, M. G., van Helvoort, J. M. L. M., Wolbert, R. B. G. & Hagen, W. R. (1992) Eur. J. Biochem. 205, 111-115] that desulfoviridin, the dissimilatory sulfite reductase of sulfate-reducing bacteria of the genus Desulfovibrio, contains a third, gamma, subunit (11 kDa), in addition to the well-established alpha (50 kDa) and beta (40 kDa) subunits, and an alpha 2 beta 2 gamma 2 subunit structure has been proposed. Cloning and sequencing of the dsvC gene indicated it to encode a protein of 105 amino acids (11.9 kDa; gamma subunit). The finding that the dsvC gene, located on a 3.5-kb SacII fragment, is transcribed in both Escherichia coli and Desulfovibrio vulgaris as an mRNA of only 400-600 nucleotides, and that both the dsvA and dsvB genes are present on a 7.2-kb SacII fragment, indicates that dsvC forms a separate transcriptional unit. The steady-state level of alpha and beta subunits expressed in D. vulgaris Hildenborough cells is rather constant, while that of the gamma subunit increased strongly in the stationary growth phase. Biochemical analysis of the purified protein, expressed in E. coli, and library comparison of its sequence, have so far failed to establish the function of gamma.