THE TERMINAL OXIDASES OF PARACOCCUS-DENITRIFICANS

Three distinct types of terminal oxidases participate in the aerobic respiratory pathways of Paracoccus denitrificans. Two alternative genes encoding subunit I of the aa(3)-type cytochrome c oxidase have been isolated before, namely ctaDI and ctaDII. Each of these genes can be expressed separately to complement a double mutant (Delta ctaDI, Delta ctaDII), indicating that they are isoforms of subunit I of the aa(3)-type oxidase. The genomic locus of a quinol oxidase has been isolated: cyoABC. This protohaem-containing oxidase, called cytochrome bb(3), is the only quinol oxidase expressed under the conditions used. In a triple oxidase mutant (Delta ctaDI, Delta ctaDII, cyoB::Km(R)) an alternative cytochrome c oxidase has been characterized; this cbb(3)-type oxidase has been partially purified. Both cytochrome aa(3) and cytochrome bb(3) are redox-driven proton pumps. The proton-pumping capacity of cytochrome cbb(3) has been analysed; arguments for and against the active transport of protons by this novel oxidase complex are discussed.