RAPID TURNOVER OF THE D1 REACTION-CENTER PROTEIN OF PHOTOSYSTEM-II AS A PROTECTION MECHANISM AGAINST PHOTOINHIBITION IN A MOSS, CERATODON PURPUREUS (HEDW) BRID

Susceptibility of a moss, Ceratonon purpureus (Hedw.) Brid., to photoinhibition and subsequent recovery of the photochemical efficiency of PSII was studied in the presence and absence of the chloroplast-encoded protein-synthesis inhibitor lincomycin. Ceratodon had a good capacity for repairing the damage to PSII centers induced by strong light. Tolerance against photoinhibition was associated with rapid turnover of the D1 protein, since blocking of D1 protein synthesis more than doubled the photoinhibition rate measured as the decline in the ratio of variable fluorescence to maximal fluorescence (F-v/F-max). Under exposure to strong light in the absence of lincomycin a net loss of D1 protein occurred, indicating that the degradation of damaged D1 protein in Ceratodon was rapid and independent of the resynthesis of the polypeptide. The result suggests that synthesis is the limiting factor in the turnover of D1 protein during photoinhibition of the moss Ceratodon, The level of initial fluorescence (F-o) correlated with the production of inactive PSII centers depleted of D1 protein. The higher the F-o level, the more severe was the loss of D1 protein seen in the samples during photoinhibition. Restoration of F-v/F-max at recovery light consisted of a fast and slow phase. The recovery of fluorescence yield in the presence of lincomycin, which was added at different times in the recovery, indicated that the chloroplast-encoded protein-synthesis-dependent repair of damaged PSII centers took place during the fast phase of recovery. Pulse-labelling experiments with [S-35]methionine supported the conclusion drawn from fluorescence measurements, since the rate of D1 protein synthesis after photoinhibition exceeded that of the control plants during the first hours under recovery conditions.