A high light-tolerant mutant of the unicellular cyanobacterium, Anacystis nidulans, was able to tolerate about threefold higher light intensity (30 W/m(2)) when compared with the wild type (10 W/m(2)). The results on the Hill activity and whole chain electron transport in both the mutant and wild-type cells exhibited an opposite response to an increase in the light intensity beyond the normal light condition (10 W/m(2)). Photoinactivation of the electron transport process occurred in both the strains beyond their respective photosynthesis-saturating light intensities. However, mutant cells were able to retain efficiently high photosystem II (PS II) activity (Hill activity) as compared with the wild-type cells. In the wild-type cells, both the oxidizing and reducing sides of the photosynthetic electron transport chain were found to be damaged by the high fluence rate, unlike the mutant cells in which only the oxidizing side of PS II was inactivated. With the help of exogenous electron donors hydroxylamine (NH2OH) and diphenyl carbazide (DPC) photoinactivated sites on the electron transfer chain were delineated. Further, the mutant cells showed active repair of the photoinactivated sites within 48 h after imposition of the normal conditions, whereas in the case of the wild-type cells placed under the same condition, only the oxidizing side was repaired. The active repair process of the damaged sites for photosynthesis was again confirmed by the use of inhibitors of protein synthesis such as chloramphenicol, rifampicin, and L-methionine, DLsulfoximine (MSX), a glutamine synthetase (GS) inhibitor.