In vivo role of catalase-peroxidase in Synechocystis sp. Strain PCC 6803

The katG gene coding for the only catalase-peroxidase in the cyanobacterium Synechocystis sp, strain PCC 6803 was deleted in this organism. Although t he rate of H2O2 decomposition was about 30 times lower in the Delta katG mu tant than in the wild type, the strain had a normal phenotype and its doubl ing time as well as its resistance to H2O2 and methyl viologen were indisti nguishable from those of the wild type. The residual H2O2-scavenging capaci ty was more than sufficient to deal with the rate of H2O2 production by the cell, estimated to be less than 1% of the maximum rate of photosynthetic e lectron transport in vivo, We propose that catalase-peroxidase has a protec tive role against environmental H2O2 generated by algae or bacteria in the ecosystem (for example, in mats), This protective role is most apparent at a high cell density of the cyanobacterium. The residual H2O2-scavenging act ivity in the Delta katG mutant was a light-dependent peroxidase activity. H owever, neither glutathione peroxidase nor ascorbate peroxidase accounted f or a significant part of this H2O2-scavenging activity. When a small thiol such as dithiothreitol was added to the medium, the rate of H2O2 decomposit ion in the Delta katG mutant increased more than 10-fold, indicating that a thiol-specific peroxidase, for which thioredoxin may be the physiological electron donor, is present. Oxidized thioredoxin is likely to be reduced ag ain by photosynthetic electron transport. Therefore, under laboratory condi tions, there are only two enzymatic mechanisms for H2O2 decomposition prese nt in Synechocystis sp. strain PCC 6803. One is catalyzed by a catalase-per oxidase, and the other is catalyzed by thiol-specific peroxidase.