EXPRESSION OF THE PSBA GENE DURING PHOTOINHIBITION AND RECOVERY IN SYNECHOCYSTIS PCC-6714 - INHIBITION AND DAMAGE OF TRANSCRIPTIONAL AND TRANSLATIONAL MACHINERY PREVENT THE RESTORATION OF PHOTOSYSTEM-II ACTIVITY

The D1 reaction center protein of the photosystem II complex is very s ensitive to light. It is continuously being damaged, degraded and resy nthesized. Under high light, photosystem II inactivation is observed. This is because the rate of D1 damage is faster than that of its repla cement. This process can be reversed if exposure to high light is not too long. In this work we study the changes that occur in the transcri ptional and translational machinery that could lead to irreversible ph otoinhibition in Synechocystis PCC 6714. In the first minutes of photo inhibition, high light induced an accumulation of psbA mRNA due to an increase in psbA transcription initiation. Although the transcription rate of other photosynthetic genes (e.g. psaE and cpcB-cpcA) declined, the high turnover of the psbA transcript was maintained for a long ti me. When the light stress was too long, the stability of psbA mRNA inc reased and the psbA transcription rate appeared to decrease. A high le vel of psbA mRNA was maintained even though translation no longer occu rred and the cells were unable to recover. Experiments to measure newl y synthesized D1 incorporation into the thylakoid membranes during rec overy in the presence of rifampicin showed that the initiation of tran scription was not required for translation of psbA mRNA when photoinhi bition was still reversible. Since psbA translation did not depend on the level of psbA transcript or on the initiation of psbA transcriptio n, we propose that damage to the translational machinery also occurred during light stress, leading to the inhibition of D1 synthesis and to irreversible photoinhibition.