Overproduction of the MotA protein of Escherichia coli and estimation of its wild-type level.

The motA gene of Escherichia coli was placed under the control of a high-level promoter, that of the tryptophan operon of Serratia marcescens. In the presence of the inducer beta-indoleacrylic acid, MotA was synthesized at greatly elevated levels and inserted without apparent limit into the inner membrane. Growth and motility were impaired, but not drastically so, indicating that MotA by itself does not act as a proton ionophore. Antibody raised against the overproduced protein was used to estimate that a wild-type cell contained 600 +/- 250 copies of MotA. This number is more than would be needed to surround each flagellar basal body with a single circlet of MotA protein; possible interpretations of the result are discussed. The antibody was also used to establish that the MotA protein of Salmonella typhimurium has a similar molecular weight to that of E. coli and is immunologically cross-reactive with it; functional complementation of S. typhimurium motA mutants by the E. coli gene was established.