Rubisco but not Rubisco activase is clustered in the carboxysomes of the cyanobacterium Synechococcus sp. PCC 7942: Mud‐induced carboxysomeless mutants

The Mud technology of Groisman and Casadaban was adapted to the cyanobacterium Synechococcus sp. PCC 7942. A new high‐CO₂‐requiring (hcr) mutant, hcr Mu28 was isolated following the integration of the Mud element 89 bp upstream of ORFI, at the 5′‐flanking region of the rbc operon, which encodes RuBP carboxylase/oxygenase (Rubisco). The integration involved a 7 bp duplication that formed a direct repeat at the integration site, as previously shown in Escherichia coli The mutant was devoid of apparent carboxysome bodies, which are considered to be important for the availability of CO₂ for Rubisco. Immunolabelling studies demonstrated that Rubisco was distributed throughout hcr Mu28 cells, while in the wild type (WT) and in the carboxysome aberrant mutant hcr 0221, Rubisco was markedly associated with the carboxysomes. Rubisco activase, however, was evenly distributed throughout the cytosol of the hcr and WT cells, without any preferential association with the apparent carboxysomes.