A Cyanobacterial Mutant Requiring the Expression of Ribulose Bisphosphate Carboxylase from a Photosynthetic Anaerobe

Ribulose bisphosphate carboxylase is essential for both photoautotrophic and photoheterotrophic growth of the cyanobacterium Synechocystis 6803. However, a mutant lacking cyanobacterial carboxylase could be obtained by replacing the natural carboxylase gene with the corresponding gene from Rhodospirillum rubrum, a photosynthetic anaerobe. This treatment produced an organism whose growth depended on the activity of the structurally and functionally dissimilar foreign carboxylase. As a further consequence of this mutagenic replacement, the mutant also lacked microscopically observable carboxysomes, the subcellular inclusion bodies in which the wild-type carboxylase naturally resides. The mutant, dependent on a carboxylase with an inferior relative specificity for CO2 versus O2 and apparently lacking carboxysomes, is extremely sensitive to the CO2/O2 ratio supplied during growth and is unable to grow at all in air. This response to the gas composition should prove useful for selection of various R. rubrum carboxylase mutants with altered specificities for CO2 and O2.