Protein phosphorylation and excitation energy distribution in normal, intermittent-light-grown, and a chlorophyll b-less mutant of barley

Abstract

We have analyzed the low-temperature (77 K) fluorescence induction curves from chloroplasts isolated from normal barley, a chlorophyll b-less mutant, and intermittent-light-grown barley. Using the equations derived from the bipartite model of photosynthesis we have shown an increase in both the proportion of absorbed quanta distributed to photosystem I (a) and the amount of energy transferred from photosystem II to photosystem I ( ψT ( II → I) ), concomitant with phosphorylation of the light-harvesting chlorophyll-protein complex serving photosystem II (LHC-II) in normal barley chloroplasts. No changes in these energy distribution parameters were observed in chloroplasts isolated from the chlorophyll b-less mutant or intermittent-light-grown barley, both of which lack the LHC-II. The 25,000-dalton LHC-II polypeptide was virtually the only phosphorylated protein in the wild-type barley and the incorporation of labeled phosphate could account for a single phosphorylation of up to 80–90% of the LHC-II in the thylakoid membrane. Simultaneous measurement of room-temperature fluorescence indicates that the thylakoid protein phosphorylation-induced quenching of fluorescence is indicative of a State II transition. The state transition is accompanied by small changes in the extent of grana stacking.