Physiological regulation of ribulose 1,5 bisphosphate carboxylase/oxygenase in Rhodospirillum rubrum

Rhodospirillum rubrum employs several mechanisms for regulating the activity of ribulose 1,5 bisphosphate carboxylase/oxygenase (RuBPC/O). The activity of RuBPC/O is modulated at the level of synthesis of the enzyme and also post-transcriptionally by inactivation of the preformed enzyme. The synthesis of RuBPC/O in R. rubrum is greatly influenced by the conditions of culture. Under conditions of carbon dioxide limitation, R. rubrum derepresses the synthesis of RuBPC/O. When growing autotrophically under low CO2 (1.5-2%) concentrations, the synthesis of the enzyme is derepressed such that RuBPC/O represents up to 50% of the soluble protein in the cell. These phenomenal levels of enzyme activity are not observed when the culture is supplied with higher levels of CO2 (5-6%). The increase in enzyme activity observed in derepressed cultures is not paralleled by an increase in vivo CO2 fixation rate. Rhodospirillum rubrum grown in the presence of 32P-orthophosphate, has radiolabel associated with several proteins. One of these proteins co-migrates with RuBPC/O in a SDS-polyacrylamide gel. The phosphoprotein can be separated from RuBPC/O using the technique of two-dimensional gel electrophoresis. A Western blot does not show any immunological cross-reactivity between the two proteins