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.

Abstract

The D1 reaction center protein of the photosystem II complex is very sensitive to light. It is continuously being damaged, degraded and resynthesized. Under high light, photosystem II inactivation is observed. This is because the rate of D1 damage is faster than that of its replacement. 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 transcriptional and translational machinery that could lead to irreversible photoinhibition in Synechocystis PCC 6714. In the first minutes of photoinhibition, 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 time. When the light stress was too long, the stability of psbA mRNA increased and the psbA transcription rate appeared to decrease. A high level of psbA mRNA was maintained even though translation no longer occurred and the cells were unable to recover. Experiments to measure newly synthesized D1 incorporation into the thylakoid membranes during recovery in the presence of rifampicin showed that the initiation of transcription was not required for translation of psbA mRNA when photoinhibition was still reversible. Since psbA translation did not depend on the level of psbA transcript or on the initiation of psbA transcription, we propose that damage to the translational machinery also occurred during light stress, leading to the inhibition of D1 synthesis and to irreversible photoinhibition.