Enzyme level of enterococcal F1F0-ATPase is regulated by pH at the step ofassembly

The amount of F(1)Fo-ATPase in Enterococcus hirae (formerly Streptococcus f aecalis) increases when the cytoplasmic pH is lowered below 7.6, and proton s are extruded to maintain the cytoplasmic pH at around 7.6. In the present study, we found that the transcriptional activity of the F(1)Fo-ATPase ope ron was not regulated by pH. The synthesis of F-1 subunits was increased 1. 65 +/- 0.12-fold by the acidification of medium from pH 8.0 to pH 5.3, West ern-blot analysis showed that there were F-1 subunits in the cytoplasm, and the number of alpha plus beta subunits in the cytoplasm was 50% of the tot al number of the subunits in cells growing at pH 8.0. This decreased to 22% after shifting the medium pH to 5.3, with a concomitant 5.1-fold increase in the level of membrane-bound F(1)Fo-ATPase. The cytoplasmic F-1 subunits were shown to be degraded, and Fo subunits not assembled into the intact F( 1)Fo complex were suggested to be digested. These data suggest that regulat ion of the enzyme level of F(1)Fo-ATPase by the intracellular pH takes plac e mainly at the step of enzyme assembly from its subunits.