PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Characterization of factors related to photosystem stoichiometry in response to environmental changes in Synechocystis sp. PCC 6803. Hanayo Sato^*,1, Kintake Sonoike¹, ¹ Department of Integrated Bioscience, Kashiwa city, Chiba, Japan

ABSTRACT- Photosynthetic organisms have developed various response mechanisms for acclimating to their environmental changes including light intensity and nutrient availability. One of acclimation responses is the alteration of light harvesting apparatus. Such alterations enable these organisms to optimize the balance between the absorption of light energy and the production of NADPH and ATP required for cell maintenance and growth. In oxygen-evolving photosynthetic organisms, photosystem stoichiometry (PSI/PSII) is modulated in order to optimize electron transport in acclimation to various environmental changes. Under high light condition, PSI/PSII is reduced by decreasing PSI amount to avoid excess electron transport. In the cyanobacterium, Synechocystis sp. PCC 6803, pmgA and sll1961 were characterized as factors responsible for this regulation. These mutants lost the ability to decrease PSI amount and consequently could not modulate PSI/PSII. In the present study, we observed the response of both mutants to high light, glucose and nitrogen starvation. The growth of sll1961 mutant and pmgA mutant were severely suppressed by the consecutive exposure to high light whereas no phenotype was observed under low light. These mutants could not grow and could not regulate PSI/PSII under photomixotrophic condition. However, the growth of sll1961 mutant was also inhibited severely in nitrogen-deplete medium but that of pmgA mutant was not. Nitrogen starvation generally causes the degradation of light harvesting pigment-protein complex, phycobilisome (PBS). This process is called bleaching which accompanies color change from blue-green to yellow. sll1961 mutant exhibited non-bleaching but pmgA mutant did not. The PBS content in sll1961 mutant was greater than that in WT and pmgA mutant under nitrogen-starvation conditions, suggesting that Sll1961 is involved in the degradation of PBS. These data imply that Sll1961 acts independently of PmgA on PSI/PSII regulation.

KEY WORDS: nutrient change, photosystem stoichiometry, stress response, non bleaching