Processes involved in the regulation of urease levels in Streptococcus salivarius by pH

Urease levels in Streptococcus salivarius are regulated by pH. There is either increased synthesis at low pH (5.5) or enhanced degradation at neutral pH, suggested by urease instability during stationary phase. To establish which mechanism predominates, protein synthesis was inhibited by chloramphenicol (CAP) in cultures grown at pH 5.8, 6.8 and 7.3. There was no significant urease degradation detectable in mid-exponential and early-stationary phase at any pH. Urease degradation occurred in both control and CAP cultures later in stationary phase, but with a faster rate of decline in the low pH culture. After CAP addition there was detectable assembly of urease from pools of post-translational subunits to give an approximately 20% increase in enzyme. Above pH 7, 0.1% cysteine inhibited urease synthesis but not growth, and evidence was obtained for O2 inhibition of growth and urease synthesis. Regulation of urease levels in S. salivarius by pH primarily involves an effect on urease synthesis. There may be cross-regulation between pH control, anaerobiosis-controlled enzyme regulatory circuits and thiols.