Cytochrome bd biosynthesis in Escherichia coli: the sequences of the cydC and cydD genes suggest that they encode the components of an ABC membrane transporter

Summary At least four genes are known to affect formation of the cytochrome bd‐type terminal oxidase of Escherichia coli. In addition to the genes (cydA and cydB) encoding the two constituent subunits of this complex, a further two genes (cydC and cydD) map near 19 min on the E. coli chromosome. We report here the cloning of both genes on a 5.3 kb ClaI‐HindIII restriction fragment, which, when used to transform either a cydC or cydD mutant, restored the ability of these mutants to grow on a selective medium containing azide and zinc ions and also restored the spectral signals associated with the cytochrome components of the oxidase complex. A subcloned 1.8 kb DdeI fragment similarly restored growth and cytochrome content of a cydD mutant, but not a cydC mutant. The complete nucleotide sequence of the ClaI‐HindIII fragment reveals three open reading frames, one being trxB (19.3 min on the E. coli chromosome map, encoding thioredoxin reductase), confirming the mapping position of cydD previously established by P1‐mediated transduction. Two ORFs identified by complementation experiments as cydD and cydC encode proteins with predicted molecular masses, respectively, of 65103 and 62 946 Da. The hydropathy profile of each protein reveals an N‐terminal hydrophobic domain and a C‐terminal hydrophilic domain containing a putative nucleotide‐binding site. The gene products probably constitute an ABC (ATP‐binding cassette) family membrane transporter, the function of which is necessary for the formation of the cytochrome bd quinol oxidase. The CydDC system appears to be the first prokaryotic example of a heterodimeric ABC transport system in which each polypeptide contains both hydrophobic and ATP‐binding domains.