The CYD subfamily implicated in cytochrome bd biogenesis. The CydC and CydD proteins are important for the formation of cytochrome bd terminal oxidase of E. coli and it has been proposed that they were necessary for biosynthesis of the cytochrome bd quinol oxidase and for periplasmic c-type cytochromes. CydCD were proposed to determine a heterooligomeric complex important for heme export into the periplasm or to be involved in the maintenance of the proper redox state of the periplasmic space. In Bacillus subtilis, the absence of CydCD does not affect the presence of halo-cytochrome c in the membrane and this observation suggests that CydCD proteins are not involved in the export of heme in this organism.
Jiyao W et al.(2023). "The conserved domain database in 2023.",