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

Summary: 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 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.