An knockout mutant has been constructed using a gene replacement method and verified using both Southern hybridization and PCR. The mutant, which was unable to synthesize ubiquinone (Q), showed severely diminished growth yields aerobically but not anaerobically with either nitrate or fumarate as terminal electron acceptors. Low oxygen uptake rates were demonstrated in membrane preparations using either NADH or lactate as substrates. However, these rates were greatly stimulated by the addition of ubiquinone-1 (Q-1). The rate of electron transfer to those oxidase components observable by photodissociation of their CO complexes was studied at sub-zero temperatures. In the mutant, the reduced form of haemoproteins - predominantly cytochrome -was reoxidized significantly faster in the presence of oxygen than in a Ubi strain, indicating the absence of Q as electron donor. Continuous multiple-wavelength recordings of the oxidoreduction state of cytochrome(s) during steady-state respiration showed greater reduction in membranes from the mutant than in wild-type membranes. A scheme for the respiratory electron-transfer chain in is proposed, in which Q functions downstream of cytochrome(s)


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