CYTOCHROME BD BIOSYNTHESIS IN ESCHERICHIA-COLI - THE SEQUENCES OF THECYDC AND CYDD GENES SUGGEST THAT THEY ENCODE THE COMPONENTS OF AN ABCMEMBRANE TRANSPORTER

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 chr omosome. We report here the cloning of both genes on a 5.3 kb ClaI-Hin dIII restriction fragment, which, when used to transform either a cydC or cydD mutant, restored the ability of these mutants to grow on a se lective medium containing azide and zinc ions and also restored the sp ectral signals associated with the cytochrome components of the oxidas e complex. A subcloned 1.8kb Ddel fragment similarly restored growth a nd cytochrome content of a cydD mutant, but not a cydC mutant. The com plete nucleotide sequence of the ClaI-HindIII fragment reveals three o pen reading frames, one being trxB (1 9.3 min on the E. coli chromosom e map, encoding thioredoxin reductase), confirming the mapping positio n of cydD previously established by P1-mediated transduction. Two ORFs identified by complementation experiments as cydD and cydC encode pro teins with predicted molecular masses, respectively, of 65103 and 6294 6 Da. The hydropathy profile of each protein reveals an N-terminal hyd rophobic domain and a C-terminal hydrophilic domain containing a putat ive nucleotide-binding site. The gene products probably constitute an ABC (ATP-binding cassette) family membrane transporter, the function o f which is necessary for the formation of the cytochrome bd quinol oxi dase. The CydDC system appears to be the first prokaryotic example of a heterodimeric ABC transport system in which each polypeptide contain s both hydrophobic and ATP-binding domains.