Bacterial membrane-bound nickel-dependent hydrogenases [1,2,3] seem to be associated with a b-type cytochrome involved in electron transfer from hydrogen to oxygen. This cytochrome is a protein of about 28 Kd that seems to have four transmembrane regions. The gene coding for this cytochrome is adjacent to that coding for the large subunit of the hydrogenase. It has been assigned a variety of names in different species: hupC, hyaC, hydC or hoxZ.

We developed two signature patterns for this protein. The first corresponds to the first potential transmembrane region. The second signature includes the C-terminal part of a hydrophilic loop between the third and the fourth transmembrane region as well as the first half of the fourth transmembrane region. Both signatures contain conserved histidines that could be involved in binding the heme iron group.