PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Photosystem II mobilisation and redistribution induced by high light: visualising the PSII repair cycle in vivo? Mary Sarcina¹, Conrad Mullineaux^*,1, ¹ Department of Biology, London, UK

ABSTRACT- We use laser-scanning confocal microscopy and Fluorescence Recovery after Photobleaching (FRAP) to probe the distribution and mobility of components of the photosynthetic apparatus in vivo (1,2). The cyanobacterium Synechococcus sp. PCC7942 is an excellent model system for this work because of its elongated cells and regular thylakoid membrane conformation (2,3). Under normal conditions Photosystem II is evenly distributed in the membrane, and FRAP measurements show that it completely immobile: no diffusion can be detected even on long time-scales (1,3). This is in contrast to other membrane components, suggesting that PSII is immobilised by specific interactions. Here we use FRAP to show, for the first time, that PSII can be mobilised under some conditions. After pre-treatment with intense light, a proportion of PSII centres begin to diffuse in the membrane. We observe comparable effects in Synechococcus cells and in maize chloroplasts. Fluorescence imaging shows that the distribution of PSII in Synechococcus changes under these conditions. Within 4-6 minutes, PSII becomes concentrated in specific regions of the thylakoid membrane. The effect is wavelength-specific: in Synechococcus, PSII is mobilised by exposure to red (633 nm) light, but not blue (457 nm) light. The mobilisation is not a direct result of photodamage, since high-light pre-treatment specifically at one end of a cell leads to PSII mobility at the other end of the cell. This suggests that a specific photoreceptor and a signal transduction pathway are involved. Mobilisation and redistribution of PSII may play a key role in the PSII repair cycle and resistance to high-light stress. References: 1. C.W. Mullineaux et al. (1997) Nature 390,421-424; 2. C.W. Mullineaux and M. Sarcina (2002) Trends Plant Sci 7, 237-240; 3. M. Sarcina et al. (2001) J. Biol. Chem. 276, 46830-46834

KEY WORDS: thylakoid membrane, photosystem II, cyanobacterium, photoinhibition