Characterization of the Synechocystis PCC 6803 Fluorescence Recovery Protein involved in photoprotection

Under high irradiance, most cyanobacteria induce a photoprotective mechanism that decreases the energy arriving at the photosynthetic reaction centers to avoid the formation of dangerous species of oxygen. This mechanism which rapidly increases the heat dissipation of excess energy at the level of the cyanobacterial antenna, the phycobilisomes, is triggered by the photoactivation of the Orange Carotenoid Protein (OCP). Under low light conditions, the Fluorescence Recovery Protein (FRP) mediates the recovery of the full antenna capacity by accelerating the deactivation of the OCP. Several FRP Synechocystis mutants were constructed and characterized in terms of the OCP-related photoprotective mechanism. Our results demonstrate that Synechocystis FRP starts at Met26 and not at Met1 (according to notation in Cyanobase) as was previously suggested. Moreover, changes in the genomic region upstream the ATG encoding for Met26 influenced the concentration of OCP in cells. A long FRP (beginning at Met1) is synthesized in Synechocystis cells when the frp gene is under the control of the psbA2 promoter but it is less active than the shorter protein. Overexpression of the short frp gene in Synechocystis enabled short FRP isolation from the soluble fraction. However, the high concentration of FRP in this mutant inhibited the induction of the photoprotective mechanism by decreasing the concentration of the activated OCP. Therefore, the amplitude of photoprotection depends on not only OCP concentration but also on that of FRP. The synthesis of FRP and OCP must be strictly regulated to maintain a low FRP to OCP ratio to allow efficient photoprotection.