Enzyme level of enterococcal F sub(1)Fo-ATPase is regulated by pH at the step of assembly

The amount of F sub(1)Fo-ATPase in Enterococcus hirae (formerly Streptococcus faecalis) increases when the cytoplasmic pH is lowered below 7.6, and protons are extruded to maintain the cytoplasmic pH at around 7.6. In the present study, we found that the transcriptional activity of the F sub(1)Fo-ATPase operon was not regulated by pH. The synthesis of F sub(1) subunits was increased 1.65 plus or minus 0.12-fold by the acidification of medium from pH 8.0 to pH 5.3. Western-blot analysis showed that there were F sub(1) subunits in the cytoplasm, and the number of alpha plus beta subunits in the cytoplasm was 50% of the total 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 sub(1)Fo-ATPase. The cytoplasmic F sub(1) subunits were shown to be degraded, and Fo subunits not assembled into the intact F sub(1)Fo complex were suggested to be digested. These data suggest that regulation of the enzyme level of F sub(1)Fo-ATPase by the intracellular pH takes place mainly at the step of enzyme assembly from its subunits.