Malate:quinone oxidoreductase is essential for growth on ethanol or acetate in Pseudomonas aeruginosa

Pseudomonas aeruginosa ATCC 17933 growing aerobically on ethanol uses a pyrroloquinoline quinone-dependent ethanol oxidation system. A mutant with an interrupted putative mqo gene, in which malate:quinone oxidoreductase (MQO), an enzyme involved in the citric acid cycle/glyoxylate cycle, was defective, showed a severe growth defect on ethanol and was unable to grow on acetate. Glucose, lactate, succinate or malate supported growth of the mutant. However, an NAD-dependent malate dehydrogenase activity could not be detected. Complementation of the mutant by the wild-type allele of the mqo gene restored wild-type behaviour. The wild-type expressed the dye-dependent MQO and NAD(P)-dependent malic enzymes (MEs). Pyruvate carboxylase (PC) was found upon growth of the wild-type and the mutant on all substrates studied. PC activity in the wild-type was induced on glucose and lactate and was always higher on all substrates in the mqo mutant. In P. aeruginosa ATCC 17933, an active MQO is required for growth on ethanol or acetate, while with glucose, lactate, succinate or malate an apparent bypass route operates, with MEs using malate for generating pyruvate, which is carboxylated to oxaloacetate by PC. To the authors' knowledge, this is the first time that a specific mutant MQO phenotype has been observed, caused by the inactivation of a gene encoding MQO activity. mqo of P. aeruginosa ATCC 17933 corresponds to mqoB (PA4640) of the P. aeruginosa PAO1 genome project.