The gene has been cloned and sequenced. Whereas disruption of (), the hydrophilic C-terminal domain of which has been deduced to be periplasmic and to function as a protein-disuifide reductase, leads to the absence of -type cytochromes, disruption of attenuated, but did not abolish, holo--type cytochrome biosynthesis. Comparison of the DipZ sequence with three other DipZ sequences indicated that there are not only two conserved cysteine residues in the C-terminal hydrophilic domain, but also two more in the central highly hydrophobic domain. The latter would be located toward the centre of two of the eight membrane-spanning α-helices predicted to compose the hydrophobic central domain of DipZ. Both these cysteine residues, plus other transmembrane helix residues, notably prolines and glycines, are also conserved in a group of membrane proteins, related to CcdA, which lack the N- and C-terminal hydrophilic domains of the DipZ proteins. It is proposed that DipZ of and other organisms transfers reducing power from the cytoplasm to the periplasm through reduction and reoxidation of an intramembrane disulfide bond, or other mechanism involving these cysteine residues, and that this function can also be performed by CcdA and other CcdA-like proteins. The failure of disruption to abolish -type cytochrome synthesis in suggests that, in contrast to the situation in , the absence of DipZ can be compensated for by one or more other proteins, for example a CcdA-like protein acting in tandem with one or more thioredoxin-like proteins.


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