Muramatsu Masayuki, Sonoike Kintake, Hihara Yukako
Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Box 101, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8562, Japan.
Department of Biochemistry and Molecular Biology, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan.
Microbiology (Reading). 2009 Mar;155(Pt 3):989-996. doi: 10.1099/mic.0.024018-0.
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.
光系统I（PSI）含量的下调是蓝藻在高光（HL）条件下生长的必要过程。在集胞藻6803的pmgA（sll1968）突变体中，尽管这些表型之间的因果关系尚不清楚，但在HL培养期间，PSI含量、叶绿素以及编码PSI反应中心亚基的psaAB基因的转录水平无法维持在低水平。在本研究中，我们调节了psaAB转录活性或叶绿素合成活性，以评估它们在HL条件下对PSI含量调节的贡献。对psaAB-OX菌株（其中psaAB基因在HL条件下过表达）的分析表明，psaAB转录本的量本身不会影响PSI含量。在pmgA突变体中用抑制剂乙酰丙酸抑制叶绿素合成表明，叶绿素的可利用性可能是HL条件下PSI含量的决定因素。还表明HL条件下的叶绿素含量主要在5-氨基乙酰丙酸合成水平上受到调节。我们得出结论，在转变到HL条件后，在最初的6小时内，psaAB转录活性和5-氨基乙酰丙酸合成活性通过独立于PmgA的调节机制被严格下调，然后在HL培养6小时后，一种由PmgA介导的调节机制开始活跃，以将这些活性维持在低水平。
