Abstract
A ubiA − menA − double quinone mutant of Escherichia coli K12 was constructed together with other isogenic strains lacking either ubiquinone or menaquinone. These strains were used to study the role of quinones in electron transport to oxygen and nitrate. Each of the four oxidases examined (NADH, d-lactate, α-glycerophosphate and succinate) required a quinone for activity. Ubiquinone was active in each oxidase system while menaquinone gave full activity in α-glycerophosphate oxidase, partial activity in d-lactate oxidase but was inactive in NADH and succinate oxidation. The aerobic growth rates, growth yields and products of glucose metabolism of the quinone-deficient strains were also examined. The growth rate and growth yield of the ubi + menA − strain was the same as the wild-type strain, whereas the ubiA − men + strain grew more slowly on glucose, had a lower growth yield (30% of wild type) and accumulated relatively large quantities of acetate and lactate. The growth of the ubiA − menA − strain was even more severely affected than that of the ubiA − men + strain. Electron transport from formate, d-lactate, α-glycerophosphate and NADH to nitrate was also highly dependent on the presence of a quinone. Either ubiquinone or menaquinone was active in electron transport from formate and the activity of the quinones in electron transport from the other substrates was the same as for the oxidase systems. In contrast, quinones were not obligatory carriers in the anaerobic formate hydrogenlyase system. It is concluded that the quinones serve to link the various dehydrogenases with the terminal electron transport systems to oxygen and nitrate and that the dehydrogenases possess a degree of selectivity with respect to the quinone acceptors.
Published Version
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