ENERGETIC EFFICIENCY OF ESCHERICHIA-COLI - EFFECTS OF MUTATIONS IN COMPONENTS OF THE AEROBIC RESPIRATORY-CHAIN

The aerobic respiratory chain of Escherichia coli can function with ei ther of two different membrane-bound NADH dehydrogenases (NDH-1 and ND H-2) and with either of two ubiquinol oxidases (bd-type and bo-type). The amounts of each of these enzymes present in the E. coli membrane d epend on growth conditions in general and particularly on the dissolve d oxygen concentration. Previous in vitro studies have established tha t NDH-1 and NDH-2 differ in the extent to which they are coupled to th e generation of an energy-conserving proton motive force. The same is true for the two ubiquinol oxidases. Hence, the bioenergetic efficienc y of the aerobic respiratory chain must depend on the electron flux th rough each of the specific enzyme components which are being utilized. In this work, the specific rates of oxygen consumption for cells grow ing under glucose-limited conditions are reported for a series of isog enic strains in which one or more respiratory components are genetical ly eliminated. The results are compatible with the proton translocatio n values of the various components reported from in vitro measurements . The data show that (i) the bd-type oxidase is less efficient than is the bo-type oxidase, but the former is still a coupling site in the r espiratory chain; and (ii) under the conditions employed, the wild-typ e strain uses both the NDH-1 and NDH-2 NADH dehydrogenases to a signif icant degree, but most of the electron flux is directed through the bo -type oxidase.