Sequence of b cytochromes relative to ubiquinone in the electron transport chain of Escherichia coli

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Abstract
A ubiquinone-deficient mutant, carrying mutations in 2 genes affecting ubiquinone biosynthesis, was used, in comparison with a normal strain, to determine the sequence of some of the components of the electron transport chain of E. coli. The amounts of cytochromes reduced during aerobic steady-state conditions were estimated by comparing low-temperature difference spectra of normal or ubiquinone-deficient membranes with D-lactate or NAPH as substrate. The amounts of cytochromes reduced showed that ubiquinone functions at 2 sites, 1 site being between the dehydrogenases and cytochromes and the 2nd site being after cytochromes b562 and b556 but before cytochromes b558, d and o. The scheme proposed is discussed in relation to the Mitchell protonmotive ubiquinone cycle.