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 (Ci) which is 1000 times lower than that of the wild type, yet it accumulates more Ci than the wild type. The mutant appears to be defective in its ability to utilize the intracellular Ci 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 Ci 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.