The carboxylase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPC/O) decreased when an anaerobic culture of RhodospiriUum rubrum was exposed to atmospheric levels of oxygen. From 70 to 80% of the activity was lost within 12 to 24 h. Inactivation was apparent when the enzyme was assayed in situ (in whole cells) and when activity was measured in dialyzed crude extracts. The quantity of enzyme protein, as estimated from sodium dodecyl sulfate-polyacrylamide gels or as quantified immunologically, did not decrease within 24 h of exposure to air. Following extended exposure to aerobic conditions (48 to 72 h), degradation of enzyme occurred. These results indicate that the inactivation of RuBPC/O in R. rubrum may be due to an alteration or modification of the preformed enzyme, followed by eventual degradation of the inactive enzyme. When shifted back to anaerobic conditions (under an argon atmosphere), the RuBPC/O activity increased rapidly. This increase appeared to be due to de novo synthesis of enzyme. The increase in activity was not observed when the culture was maintained in the dark or in the absence of a suitable carbon source. Thus, the oxygen-mediated inactivation of RuBPC/O appeared to be due to some form of irreversible modification. The cloned R. rubrum RuBPC/O gene, expressed in Escherichia coli, yielded functional enzyme that was not affected by oxygen, indicating that inactivation in R. rubrum is mediated by a gene product(s) not found in E. coli.Ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBPC/O) catalyzes the first reaction of both the reductive and oxidative photosynthetic carbon cycles (10). In the absence of oxygen, RuBPC/O catalyzes the carboxylation and cleavage of RuBP, resulting in the formation of two molecules of 3-phosphoglyceric acid. In the presence of oxygen, RuBPC/O catalyzes the oxygenolysis of RuBP, resulting in the formation of 1 molecule of 3-phosphoglyceric acid and 1 molecule of phosphoglycolate. Usually the phosphoglycolate formed is further metabolized via the glycolate pathway, resulting in the wasteful loss of carbon and nitrogen. The oxygenase function of the enzyme therefore results ultimately in a futile cycle, with loss of CO2 and enormous energy expenditure (5). The ability of RuBPC/O to catalyze these two distinct reactions necessitates careful metabolic regulation of enzyme activity. Rhodospirillum rubrum, like all members of the family Rhodospirillaceae, synthesizes high levels of RuBPC/O when grown under anaerobic photosynthetic conditions with butyrate or hydrogen as the electron donor (1,13,15,16,18). In the present investigation, it is shown that RuBPC/O is metabolically regulated in R. rubrum following exposure to oxygen. The evidence suggests that inactivation occurs by some form of alteration or modification of the enzyme when cells are exposed to atmospheric levels of oxygen. The physiological significance of this response is evident and may be related to the need to inactivate RuBPC/O in order to limit the wasteful loss of carbon a...