Photosynthesis and photorespiration in a mutant of the cyanobacterium Synechocystis PCC 6803 lacking carboxysomes

A mutant of the cyanobacterium Synechocystis PCC 6803 was obtained by replacing the gene of the carboxylation enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) with that of the photosynthetic bacterium Rhodospirillum rubrum. This mutant consequently lacks carboxysomes — the protein complexes in which the original enzyme is packed. It is incapable of growing at atmospheric CO₂ levels and has an apparent photosynthetic affinity for inorganic carbon (C_i) which is 1000 times lower than that of the wild type, yet it accumulates more C_i than the wild type. The mutant appears to be defective in its ability to utilize the intracellular C_i pool for photosynthesis. Unlike the carboxysomal carboxylase activity of Rubisco, which is almost insensitive to inhibition by O₂ in vitro, the soluble enzyme is competitively inhibited by O₂. The photosynthetic rate and C_i compensation point of the wild type were hardly affected by low O₂ levels. Above 100 μM O₂, however, both parameters became inhibited. The CO₂ compensation point of the mutant was linearly dependent on O₂ concentration. The higher sensitivity of the mutant to O₂ inhibition than that expected from in-vitro kinetics parameters of Rubisco, indicates a low capacity to recycle photorespiratory metabolites to Calvin-cycle intermediates.