The nitrate respiration complex of Escherichia coli: subunit composition.

The presence of nitrate in the growth medium of Escherichia coli causes the production of a multi-enzyme system capable of mediating electron transport between the existing respiratory chain and nitrate, resulting in its reduction to nitrite [for a review see Payne (1973)l. The point of interaction with the NADH-oxygen respiratory chain is probably at the level of ubiquinone (Enoch &Lester, 1974). This nitrate respiration system consists minimally of nitrate reductase (EC 1.7.99.4) and a 6-type cytochrome (Ruiz-Herrera & DeMoss, 1969). These components are located in the membrane fraction of E. coli and their activity is regulated by the concentration of oxygen in the growth medium (Showe & DeMoss, 1968). The present communication describes the identification of the protein components of this nitrate respiration system by three independent methods. The experiment illustrated in Fig. 1 indicates the relative electrophoretic mobilities in sodium dodecyl sulphatepolyacrylamide gels of inner-membrane subunits of E. coli whose synthesis (or incorporation into the membrane) is induced by nitrate (the term 'induced' is used throughout to include both classical induction and de-repression). Several similar experiments have been performed with different pairs of growth conditions whose common feature, between one experiment and the next, is the presence of nitrate in one-half of each pair and its absence from the other. Inspection of the data