AN ALTERNATIVE CYTOCHROME-OXIDASE OF PARACOCCUS-DENITRIFICANS FUNCTIONS AS A PROTON PUMP

Deletion of the genes encoding subunit I of cytochrome aa(3) in P.dentrificans yields mutants that lack cytochrome aa(3) but which are still capable of oxidizing ubiquinol by O-2 via the cytochrome bc(1) complex, as revealed by myxothiazol-sensitive respiration on succinate. An alternative terminal oxidase must be responsible for the myxothiazol-sensitive activity. The H+/e(-) ratio of proton translocation by mutant cells approached 3.0 for oxidation of succinate by O-2, while it was near 2.0 either with ferricyanide as electron acceptor (engaging the bc(1) complex only), or with O-2 as acceptor in the presence of myxothiazol (engaging the ubiquinol oxidase). However, proton translocation coupled to oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) plus ascorbate amounted to much less than 1.0 H+/e(-). This may be due to interference by another TMPD-oxidising activity that is not engaged during electron flux to oxygen via the cytochrome bc(1), complex. We conclude that Paracoccus can express an alternative cytochrome b-containing terminal oxidase, which translocates protons at least when operating in conjunction with the cytochrome bc(1) complex.