DISRUPTION OF THE PSEUDOMONAS-AERUGINOSA DIPZ GENE, ENCODING A PUTATIVE PROTEIN-DISULFIDE REDUCTASE, LEADS TO PARTIAL PLEIOTROPIC DEFICIENCY IN C-TYPE CYTOCHROME BIOGENESIS

The Pseudomonas aeruginosa dipZ gene has been cloned and sequenced. Wh ereas disruption of Escherichia coli dipZ (dsbD), the hydrophilic C-te rminal domain of which has been deduced to be periplasmic and to funct ion as a protein-disulfide reductase, leads to the absence of c-type c ytochromes, disruption of P. aeruginosa dipZ attenuated, but did not a bolish, holo-c-type cytochrome biosynthesis. Comparison of the P. aeru ginosa DipZ sequence with three other DipZ sequences indicated that th ere are not only two conserved cysteine residues in the C-terminal hyd rophilic domain, but also two more in the central highly hydrophobic d omain. The latter would be located toward the centre of two of the eig ht membrane-spanning alpha-helices predicted to compose the hydrophobi c central domain of DipZ. Both these cysteine residues, plus other tra nsmembrane helix residues, notably prolines and glycines, are also con served in a group of membrane proteins, related to Bacillus subtilis C cdA, which lack the N- and C-terminal hydrophilic domains of the DipZ proteins. It is proposed that DipZ of P. aeruginosa and other organism s transfers reducing power from the cytoplasm to the periplasm through reduction and reoxidation of an intramembrane disulfide bond, or othe r mechanism involving these cysteine residues, and that this function can also be performed by B. subtilis CcdA and other CcdA-like proteins . The failure of dipZ disruption to abolish c-type cytochrome synthesi s in P. aeruginosa suggests that, in contrast to the situation in E. c oli, the absence of DipZ can be compensated for by one or more other p roteins, for example a CcdA-like protein acting in tandem with one or more thioredoxin-like proteins.