Regulation of photosynthesis by reversible phosphorylation of the light-harvesting chlorophyll a/b protein

Abstract

Photosynthetic CO2 fixation in chloroplasts is driven by ATP and NADPH generated by electron and proton transport within the thylakoid mem- brane.The movement of an electron from water to NADP+ along the electron transport chain requires two photons and involves two separate photosystems (PS I and PS II) operating in series.Each photosystem contains a reaction centre consisting of one or two chlorophyll a molecules capable of producing across the membrane a separation of reductant and oxidant that will initiate electron transport (Velthuys, 1980;Malkin, 1982).How- ever, less than 1% of chlorophyll molecules possess reaction centre activity.The remainder have a less direct connection with electron transport.Their role is to absorb as much light as possible and channel the resulting excitation energy to the reaction centres.Without these light-harvesting chlorophyll molecules, the electron transport rate would be severely limited under any light intensity less than that of full sunlight.Much of the current interest in the light-harvesting chlorophylls stems from the discovery that most if not all of the chlorophyll molecules of the membrane exist as pigment-protein complexes (Ogawa et al., 1966; Thornber et al., 1967, 1979; Thornber, 1975).The protein components are thought to give to the chlorophyll molecules the correct spectral qualities, orientation and spacing to enable them to perform their light-harvesting role quickly and efficiently.In green plants, the most abundant of the pigmentprotein complexes is the light-harvesting chlorophyll a/b complex (LHC).It comprises about 50% of the total chlorophyll content of the photosynthetic membrane and about 30% of the total protein.The polypeptide component, the light-harvesting chlorophyll a/b protein (LHCP), is encoded in the nucleus, synthesized in the cytoplasm and transported into the chloroplast in precursor form.