THE CYDD GENE-PRODUCT, COMPONENT OF A HETERODIMERIC ABC TRANSPORTER, IS REQUIRED FOR ASSEMBLY OF PERIPLASMIC CYTOCHROME-C AND OF CYTOCHROME-BD IN ESCHERICHIA-COLI

The cydD gene of Escherichia coli encodes a protein which, together wi th the CydC protein, probably constitutes a heterodimeric, ABC-family membrane transporter, necessary for biosynthesis of the cytochrome bd quinol oxidase. Here, we demonstrate that a cydD mutant also fails to synthesise periplasmic c-type cytochrome(s), suggesting that the trans porter exports haem or some other component involved in assembly of cy tochromes that are found in, or exposed to, the periplasm. The CydDC s ystem appears to be the first example of a transporter required for pe riplasmic cytochrome assembly processes requiring more than one type o f haem. A mutant defective in trxB (adjacent to the cydDC operon, and encoding thioredoxin reductase) was unaffected in cytochrome c or bd a ssembly.