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Journal of Bacteriology, March 1999, p. 1875-1882, Vol. 181, No. 6
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

In Vivo Role of Catalase-Peroxidase in Synechocystis sp. Strain PCC 6803

Martin Tichy and Wim Vermaas*

Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1601

Received 28 September 1998/Accepted 13 January 1999

The katG gene coding for the only catalase-peroxidase in the cyanobacterium Synechocystis sp. strain PCC 6803 was deleted in this organism. Although the rate of H2O2 decomposition was about 30 times lower in the Delta katG mutant than in the wild type, the strain had a normal phenotype and its doubling time as well as its resistance to H2O2 and methyl viologen were indistinguishable from those of the wild type. The residual H2O2-scavenging capacity 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 electron transport in vivo. We propose that catalase-peroxidase has a protective 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 activity in the Delta katG mutant was a light-dependent peroxidase activity. However, neither glutathione peroxidase nor ascorbate peroxidase accounted for a significant part of this H2O2-scavenging activity. When a small thiol such as dithiothreitol was added to the medium, the rate of H2O2 decomposition 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 again by photosynthetic electron transport. Therefore, under laboratory conditions, there are only two enzymatic mechanisms for H2O2 decomposition present in Synechocystis sp. strain PCC 6803. One is catalyzed by a catalase-peroxidase, and the other is catalyzed by thiol-specific peroxidase.

* Corresponding author. Mailing address: Department of Plant Biology, Arizona State University, Box 871601, Tempe, AZ 85287-1601. Phone: (602) 965-3698. Fax: (602) 965-6899. E-mail: wim{at}asu.edu.

Journal of Bacteriology, March 1999, p. 1875-1882, Vol. 181, No. 6
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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