Involvement of a d‐type oxidase in the Na+‐motive respiratory chain of Escherichia coli growing under low Δ\̄gmH+ conditions

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Abstract
An attempt has been made to find out which of the two terminal oxidases, the d-type or the o-type, operates as a Na+ pump in Escherichia coli grown at low Δ\̄gmH+ conditions. For this purpose, mutants lacking either d or o oxidase have been studied. It is shown that a d−,o+ mutant grows slowly or does not grow at all under low Δ\̄gmH+ conditions (alkaline or protonophore-containing growth media were used). Inside-out subcellular vesicles from the d−,o+ mutant cannot oxidize ascorbate and TMPD, and cannot transport Na+ when succinate is oxidize in the presence of a protonophore. The same vesicles are found to transport Na+ when NADH is oxidized as if the Na+-motive NADH-quinone oxidase were operative. On the other hand, a mutant lacking o oxidase (d+,o−) grows at low Δ\̄gmH+ conditions as fast as the maternal E. coli strain containing both d and o oxidases. Corresponding vesicles oxidize ascorbate and TMPD as well as succinate, the oxidations being coupled to the protonophore-stimulated Na+ transport. Growth in the presence of a protonophore is found to induce a strong increase in the d oxidase level in the maternal d+,o+E. coli strain. It is concluded that oxidase of the d-type, rather than of the o-type, operates as a Na+ pump in E. coli grown under conditions unfavorable for the H+ cycle