Kettunen R, Pursiheimo S, Rintamäki E, Van Wijk K J, Aro E M
Department of Biology, University of Turku, Finland.
Eur J Biochem. 1997 Jul 1;247(1):441-8. doi: 10.1111/j.1432-1033.1997.00441.x.
The D1 reaction centre protein of photosystem II (PSII), encoded by the plastid psbA gene, has the highest turnover rate of all thylakoid proteins, due to light-induced damage to D1. The expression of the psbA gene was studied in chloroplasts of fully developed pea (Pisum sativum L.) leaves during high-light photoinhibitory treatment and subsequent restoration of PSII function at low light. psbA transcript levels were determined and the translational activity was followed by in vivo pulse-labelling, by in vitro translations with intact chloroplasts, and by run-off translations on isolated thylakoid membranes. PSII photochemical efficiency was determined in vivo by monitoring the ratio of variable fluorescence to maximal fluorescence (F(V)/F(M)). Enhanced D1 synthesis in pea leaves, upon a shift first from darkness to growth light and subsequently to high light, was accompanied by a substantial increase in the total number of pshA transcripts and by the accumulation of psbA mRNA x initiation complexes on thylakoid membrane. This suggested that high-light illumination increased the transcriptional activity of the psbA gene in mature leaves, and that enhanced translational initiation of psbA mRNA was followed by docking of the initiation complexes to the thylakoid membrane. The high-light-induced increase in the number of thylakoid-associated psbA mRNA x initiation complexes, occurred in parallel with enhanced in vivo D1 synthesis. This, however, did not result in an enhanced accumulation of D1 translation products in in vitro run-off translations when pea leaves were shifted from growth light to high light. This may suggest that at high light only a portion of thylakoid-associated psbA mRNA can be under translational elongation at a given moment. When the leaves were shifted from high light to low light to allow repair of PSII, thylakoid-associated psbA mRNA was rapidly released from the membrane and the high-light-induced pool of psbA transcripts was degraded. The synthesis of the D1 protein decreased on the same time scale. However, the restoration of PSII photochemical function, measured as F(V)/F(M), took a substantially longer time. It is concluded that during changing light conditions, mature leaves are able to adjust psbA gene expression both at the transcriptional and at the translational level.
光系统II（PSII）的D1反应中心蛋白由质体psbA基因编码，由于D1受到光诱导损伤，它是所有类囊体蛋白中周转速率最高的。在高光抑制处理以及随后在低光下恢复PSII功能的过程中，对完全发育的豌豆（Pisum sativum L.）叶片叶绿体中的psbA基因表达进行了研究。通过体内脉冲标记、完整叶绿体的体外翻译以及分离类囊体膜上的连续翻译来测定psbA转录本水平并追踪翻译活性。通过监测可变荧光与最大荧光的比值（F(V)/F(M)）在体内测定PSII光化学效率。豌豆叶片先是从黑暗转移到生长光，随后转移到高光时，D1合成增强，同时pshA转录本总数大幅增加，并且类囊体膜上积累了psbA mRNA x起始复合物。这表明高光照射增加了成熟叶片中psbA基因的转录活性，并且psbA mRNA翻译起始增强之后，起始复合物与类囊体膜对接。高光诱导的类囊体相关psbA mRNA x起始复合物数量增加与体内D1合成增强同时发生。然而，当豌豆叶片从生长光转移到高光时，这并未导致体外连续翻译中D1翻译产物积累增加。这可能表明在高光下，在给定时刻只有一部分类囊体相关psbA mRNA能够进行翻译延伸。当叶片从高光转移到低光以修复PSII时，类囊体相关psbA mRNA迅速从膜上释放，高光诱导的psbA转录本池被降解。D1蛋白的合成在相同时间尺度上减少。然而，以F(V)/F(M)衡量的PSII光化学功能恢复所需时间长得多。得出的结论是，在光照条件变化期间，成熟叶片能够在转录和翻译水平上调节psbA基因表达。
