In vitro and in vivo analyses of the role of the carboxysomal β-type carbonic anhydrase of the cyanobacterium Synechococcus elongatus in carboxylation of ribulose-1,5-bisphosphate

The carboxylase activities of crude carboxysome preparations obtained from the wild-type Synechococcus elongatus strain PCC 7942 strain and the mutant defective in the carboxysomal carbonic anhydrase (CA) were compared. The carboxylation reaction required high concentrations of bicarbonate and was not even saturated at 50 mM bicarbonate. With the initial concentrations of 50 mM and 25 mM for bicarbonate and ribulose-1,5-bisphosphate (RuBP), respectively, the initial rate of RuBP carboxylation by the mutant carboxysome (0.22 mu mol mg(-1) protein min(-1)) was only 30 % of that observed for the wild-type carboxysomes (0.71 mu mol mg(-1) protein min(-1)), indicating the importance of the presence of CA in efficient catalysis by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While the mutant defective in the ccmLMNO genes, which lacks the carboxysome structure, could grow under aeration with 2 % (v/v) CO2 in air, the mutant defective in ccaA as well as ccmLMNO required 5 % (v/v) CO2 for growth, indicating that the cytoplasmically localized CcaA helped utilization of CO2 by the cytoplasmically localized Rubisco by counteracting the action of the CO2 hydration mechanism. The results predict that overexpression of Rubisco would hardly enhance CO2 fixation by the cyanobacterium at CO2 levels lower than 5 %, unless Rubisco is properly organized into carboxysomes.