Abstract
Cyanobacteria decrease photosystem I (PSI) content upon acclimation to high-light conditions. In Synechocystis sp. PCC 6803, the pmgA-disrupted mutant that cannot decrease PSI content under high-light conditions exhibits severe growth inhibition under prolonged high-light incubation. In this mutant, the transcript levels of psaAB encoding reaction center subunits of PSI could not be kept at low level under continuous high-light condition unlike those in the WT cells. These findings suggest that large amount of psaAB transcripts may cause the accumulation of PSI content under high-light conditions, leading to growth inhibition. To test this hypothesis, we constructed the psaAB-OX strain in which the psbA2 promoter whose activity is high under high-light was fused to psaAB coding region. The growth of the psaAB-OX strain was inhibited upon a shift to high-light and finally stopped after 2 or 3 days. This indicates the down-regulation of psaAB transcription is indispensable for acclimation to high-light.
