Role of two forms of the D1 protein in the recovery from photoinhibition of photosystem II in the cyanobacterium Synechococcus PCC 7942

The study of turnover of two distinct forms of the photosystem II (PSII) D1 protein in cells of the cyanobacterium Synechococcus PCC 7942 showed that the 'high-light' form D1:2 is degraded significantly faster at 500 mu E m(- 2) s(-1) as compared with 50 mu E m(-2) s(-1) while the degradation rates o f the 'low-light' form D1:1 under low and high irradiance are not substanti ally different. Consequently, the D1:1 turnover does not match photoinactiv ation of PSII under increased irradiance and therefore the cells containing this D1 form exhibit a decrease in the PSII activity. Monitoring of the co ntent of each D1 form during a recovery from growth-temperature photoinhibi tion showed a good correlation between the synthesis of D1:2 and restoratio n of the PSII activity. In contrast, when photoinhibitory treatment was con ducted at low temperature, a fast recovery was not accompanied by the D1:2 accumulation. The data suggest that photoinactivation at growth temperature results in a modification of PSII that inhibits insertion of D1:1 and, the refore, for restoration of the photochemical activity in the photoinactivat ed PSII complexes the D1:2 synthesis is needed. This may represent the prim ary reason for the requirement of psbAII/psbAIII expression under increased irradiance. (C) 2000 Elsevier Science B.V. All rights reserved.