REGULATION OF SYNTHESIS OF PYRUVATE-CARBOXYLASE IN THE PHOTOSYNTHETICBACTERIUM RHODOBACTER-CAPSULATUS

The synthesis of pyruvate carboxylase (PC) was studied by using quanti tative immunoblot analysis with an antibody raised against PC purified from Rhodobacter capsulatus and was found to vary 20-fold depending o n the growth conditions. The PC content was high in cells grown on pyr uvate or on carbon substrates metabolized via pyruvate (lactate, D-mal ate, glucose, or fructose) and low in cells grown on tricarboxylic aci d (TCA) cycle intermediates or substrates metabolized without intermed iate formation of pyruvate (acetate or glutamate). Under dark aerobic growth conditions with lactate as a carbon source, the PC content was approximately twofold higher than that found under light anaerobic gro wth conditions. The results of incubation experiments demonstrate that PC synthesis is induced by pyruvate and repressed by TCA cycle interm ediates, with negative control dominating over positive control. The c ontent of PC in R. capsulatus cells was also directly related to the g rowth rate in continuous cultures. The analysis of intracellular level s of pyruvate and TCA cycle intermediates in cells grown under differe nt conditions demonstrated that the content of PC is directly proporti onal to the ratio between pyruvate and C-4 dicarboxylates. These resul ts suggest that the regulation of PC synthesis by oxygen and its direc t correlation with growth rate may reflect effects on the balance of i ntracellular pyruvate and C-4 dicarboxylates. Thus, this important enz yme is potentially regulated both allosterically and at the level of s ynthesis.