The Calvin cycle enzyme phosphoribulokinase has been localized in terms of catalytic activity and enzyme protein in the cyanobacterium In contrast to the CO-fixing enzyme of the Calvin cycle, D-ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which occurs in the cytoplasm and in the carboxysomes, phosphoribulokinase is essentially (95%) cytoplasmic in extracts from cells grown photoautotrophically to late exponential phase. Total phosphoribulokinase accounted for 0.6% of total cell protein in these cells. Immunochemical identity was found between the phosphoribulokinase located in the cytoplasm and the remaining enzyme (5%) associated with the particulate fraction of the cell obtained following differential centrifugation. On density gradient centrifugation of the particulate fraction into Percoll plus sucrose the RuBisCO activity was located in a carboxysome band in the lower half of the gradient while the phosphoribulokinase activity essentially remained concentrated at the top of the gradient. Comparison of RuBisCO and phosphoribulokinase distributions suggests that the latter is not a carboxysomal enzyme in late exponential phase cells of .


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