Transcriptional and Translational Adjustments of Psba Gene Expression in Mature Chloroplasts During Photoinhibition and Subsequent Repair of Photosystem II

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Abstract
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 (FV/FM). 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 psbA transcripts and by the accumulation of psbA mRNA · 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 · 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 FV/FM, 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.