Cytochrome c550 is an essential component of the quinoprotein ethanol oxidation system in Pseudomonas aeruginosa:: cloning and sequencing of the genes encoding cytochrome c550 and an adjacent acetaldehyde dehydrogenase

Pseudomonas aeruginosa ATCC 17933 grown aerobically on ethanol produces a soluble cytochrome c(550) together with a quinoprotein ethanol dehydrogenase. A 3.2 kb genomic DNA fragment containing the gene encoding cytochrome c(550) was cloned and sequenced. Two other complete and two truncated ORFs were also identified. A truncated ORF encoding the quinoprotein ethanol dehydrogenase (exaA) was found upstream of the cytochrome c(550) gene (exaB) and in reverse orientation. An ORF encoding a NAD(+)-dependent acetaldehyde dehydrogenase (exaC) was located downstream of the cytochrome c(550) gene and in the same orientation. Another ORF showed similarity to the pqqA gene and a truncated ORF similarity to the pqqB gene, both involved in the biosynthesis of the prosthetic group PQQ. The organization of these genes was found to be different from the well-studied methanol oxidation system in methylotrophic bacteria. The deduced amino acid sequence of cytochrome c(550) from P. aeruginosa showed some similarity to cytochrome c(6) of the alga Chlamydomonas reinhardtii and the haem domain of quinohaemoprotein alcohol dehydrogenases of acetic acid bacteria, but no similarity to the soluble cytochrome c(L) of the quinoprotein methanol oxidation system of methylotrophs could be detected. A mutant of P. aeruginosa with an interrupted cytochrome c(550) gene was unable to grow on ethanol, which proves that cytochrome c(550) is an essential component of the ethanol oxidation system in this organism.