Sulfate reducing bacteria of the genus Desulfovibrio harbor a wide var
iety of redox proteins. Three different c-type cytochromes, cytochrome
c-553, cytochrome c(3) and the high molecular mass cytochrome have be
en isolated from these bacteria. The high molecular mass cytochrome is
part of an operon that encodes a transmembrane protein complex that m
ediates electron transfer across the cytoplasmic membrane. The physiol
ogical function of the other two cytochromes is less clear. They are e
ncoded by monocistronic genes and their redox partners can thus not be
identified by gene sequencing. Expression of genes for c-type cytochr
omes in a foreign host art complicated due to the requirement for cova
lent heme insertion. Cytochrome c-553 is readily expressed in Escheric
hia coli in funtional form, but cytochrome c(3) and the high molecular
mass cytochrome are not for reasons that are presently not clear.