title: Mechanism of downregulation of photosystem I content under high-light conditions in the cyanobacterium Synechocystis sp. PCC 6803.
abstract: Downregulation of photosystem I (PSI) content is an essential process for cyanobacteria to grow under high-light (HL) conditions. In a pmgA (sll1968) mutant of Synechocystis sp. PCC 6803, the levels of PSI content, chlorophyll and transcripts of the psaAB genes encoding reaction-centre subunits of PSI could not be maintained low during HL incubation, although the causal relationship among these phenotypes remains unknown. In this study, we modulated the activity of psaAB transcription or that of chlorophyll synthesis to estimate their contribution to the regulation of PSI content under HL conditions. Analysis of the psaAB-OX strain, in which the psaAB genes were overexpressed under HL conditions, revealed that the amount of psaAB transcript could not affect PSI content by itself. Suppression of chlorophyll synthesis by an inhibitor, laevulinic acid, in the pmgA mutant revealed that chlorophyll availability could be a determinant of PSI content under HL. It was also suggested that chlorophyll content under HL conditions is mainly regulated at the level of 5-aminolaevulinic acid synthesis. We conclude that, upon the shift to HL conditions, activities of psaAB transcription and of 5-aminolaevulinic acid synthesis are strictly downregulated by regulatory mechanism(s) independent of PmgA during the first 6 h, and then a PmgA-mediated regulatory mechanism becomes active after 6 h onward of HL incubation to maintain these activities at a low level.
authors.affiliation: Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Box 101, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8562, Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Muramatsu
authors.affiliation: Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Box 101, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8562, Japan.
authors.forename: Kintake
authors.initials: K
authors.lastname: Sonoike
authors.affiliation: Department of Biochemistry and Molecular Biology, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan.
authors.forename: Yukako
authors.initials: Y
authors.lastname: Hihara
mesh_terms: D000222:Adaptation, Physiological; D000622:Aminolevulinic Acid / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000096:biosynthesis; D015536:Down-Regulation; D015964:Gene Expression Regulation, Bacterial / Q000528:radiation effects; D008027:Light; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D046939:Synechocystis / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D045331:Photosystem I Protein Complex; D002734:Chlorophyll; D000622:Aminolevulinic Acid