Reversible phosphorylation and turnover of the D1 protein under various redox states of Photosystem II induced by low temperature photoinhibition

Reversible phosphorylation and turnover of the D1 protein in vivo were stud ied under low-temperature photoinhibition of pumpkin leaves and under subse quent recovery at low light at 4 degrees C or 23 degrees C. The inactivatio n of PS II and photodamage to D1 were not enhanced during low-temperature p hotoinhibition when compared to that at room temperature. The PS II repair cycle, however, was completely blocked at 4 degrees C at the level of D1 de gradation. Both the recovery of the photochemical activity of PS II and the degradation of the damaged D1 protein at low light at 23 degrees C were de layed about 1 hour after low-temperature photoinhibition, suggesting that i n addition to the decrease in catalytic turnover of the enzyme, the proteas e was specifically inactivated in vivo at low temperature. The effect of lo w temperature on the other regulatory enzymes of PS Ii repair, protein kina se and phosphatase [Rintamaki et al. (1996) J Biol Chem 271. 14870-14875] w as variable. The D1 protein kinase was operational at low temperature while dephosphorylation of the D1 protein seemed to be completely inhibited duri ng low temperature treatment. Under subsequent recovery conditions at low l ight and 23 degrees C, the high phosphorylation Level of DI was sustained i n leaf discs photoinhibited at low temperature, despite the recovery of the phosphatase activity. This high phosphorylation level of D1 was due to the persistently active kinase. The D1 kinase, previously shown to get activat ed by reduction of plastoquinone, was, however, found to be maximally activ e already at relatively low redox state of the plastoquinone pool. We sugge st that phosphorylation of PS II centers increases the stability of PS LI c omplexes and concomitantly improves their survival under stress conditions.