In plant and photosynthesizing cyanobacteria, light is not only essential for photosynthesis but also damages the photosystem II (PSII) reaction centers. A repair cycle replenishes the PSII with newly synthesized subunits, and this process appears to require movement of the PSII from the grana (stacks of thylakoid membranes) to the stroma lamellae (membranous connections between the grana). Sarcina et al. used the cyanobacterium Synechococcus sp PCC7942 as a model system to study the mobility of PSII to different wavelengths of light. Chlorophyll is naturally fluorescent, with 80% of the fluorescence from PSII and 20% from PSI, so the mobility of PSII was monitored using fluorescence recovery after photobleaching (FRAP) of the chlorophyll. Exposure of the cells to red light stimulated the movement of chlorophyll, which in no light, blue light, or green light conditions was immobile. The movement in response to red light was dose dependent, with 3-second exposure triggering movement of 20% of the chlorophyll and 15-second exposure triggering a maximum movement of 60% of the chlorophyll. When entire cells were exposed to red light, the fluorescence became concentrated in discrete areas. Both red light and blue light cause photodamage to PSII; however, red light appears to engage the repair mechanism more quickly than does blue light. PSII damage was monitored by measuring oxygen production in response to blue or red light in the presence or absence of lincomycin, which inhibits protein synthesis and blocks the repair process. Blue light-treated cells exhibited sustained oxygen production (no loss in production for the first 20 minutes after exposure) in the absence of lincomycin, indicating rapid initiation of the repair process. Red light-treated cells showed loss of oxygen production within the first 10 minutes, and the repair cycle was engaged after 20 minutes. Thus, red light appears to produce a specific signal that alters PSII mobility, allowing it to effectively engage the repair cycle and maintain photosynthesis under conditions that cause photodamage. Exactly how the red light is perceived and how it stimulates PSII mobility remain to be determined.
M. Sarcina, N. Bouzovitis, C. W. Mullineaux, Mobilization of photosystem II induced by intense red light in the cyanobacterium Synechococcus sp PCC7942. Plant Cell 18, 457-464 (2006). [Abstract] [Full Text]