DEMONSTRATION OF Q(B)-REDUCING COMPLEXES OF PHOTOSYSTEM-II IN STROMA-EXPOSED REGIONS OF THYLAKOIDS AFTER MEMBRANE-PROTEIN PHOSPHORYLATION

Characteristics of the induction curves of chlorophyll fluorescence we re measured in isolated thylakoids and thylakoid particles from stroma -exposed regions. After the phosphorylation of membrane proteins, vari able fluorescence of chlorophyll decreased, whereas the fraction of PS II complexes, which contained Q(b)-nonreducing reaction centers incre ased. The extent of these changes differed in thylakoids isolated from pea (Pisum sativum L.) plants grown at the optimum and an elevated te mperatures. The thylakoid particles isolated from phosphorylated thyla koid membranes exhibited a higher yield of variable fluorescence of ch lorophyll. In these preparations, methylviologen reduced the yield and slowed the rise of the variable fluorescence signal. In the presence of DCMU, signal rise was accelerated. It is concluded that stroma-expo sed regions of thylakoids, isolated from plants grown at optimum tempe rature, contained Q(b)-nonreducing PS II complexes. In plants grown at an elevated temperature, these regions also contained Q(b)-reducing P S II complexes which cannot transfer electrons to PS I. In both treatm ents, Q(b)-reducing complexes, which can transfer electrons to PS I, a ppealed in stroma-exposed regions after phosphorylation of thylakoid m embrane proteins. After phosphorylation of membrane proteins, Q(b)-red ucing complexes of PS II, which transfer electrons to PS I, could be d etected in thylakoid particles isolated from plants grown at both opti mum and elevated temperatures. This fact provides support for the noti on that protein phosphorylation stabilizes PS II complexes.