Sci. STKE, 14 February 2006 PHOTOSYNTHESIS Red Light Signals
Repair 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, wi