The stationary-phase-exit defect of cydC (surB) mutants is due to the lack of a functional terminal cytochrome oxidase

The surB gene was identified as a gene product required for Escherichia coli cells to exit stationary phase at 37°C under aerobic conditions. surB was shown to be the same as cydC, whose product is required for the proper assembly and activity of cytochrome d oxidase. Cytochrome d oxidase, encoded by the cydAB operon, is one of two alternate terminal cytochrome oxidases that function during aerobic electron transport in E. coli. Mutations inactivating the cydAB operon also cause a temperature-sensitive defect in exiting stationary phase, but the phenotype is not as severe as it is far surB mutants. In this study, we examined the phenotypes of surB1 ΔcydAB double mutants and the ability of overexpression of cytochrome o oxidase to suppress the temperature-sensitive stationary-phase-exit defect of surB1 and ΔcydAB mutants and analyzed spontaneous suppressors of surB1. Our results indicate that the severe temperature-sensitive defect in exiting stationary phase of surB1 mutants is due both to the absence of terminal cytochrome oxidase activity and to the presence of a defective cytochrome d oxidase. Membrane vesicles prepared from wild-type, surB1, and ΔcydAB strains produced superoxide radicals at the same rate in vitro. Therefore, the aerobic growth defects of the surB1 and ΔcydAB strains are not due to enhanced superoxide production resulting from the block in aerobic electron transport.