REDOX SIGNALING AND THE STRUCTURAL BASIS OF REGULATION OF PHOTOSYNTHESIS BY PROTEIN-PHOSPHORYLATION

In photosynthesis in chloroplasts and cyanobacteria, redox control of thylakoid protein phosphorylation regulates distribution of absorbed e xcitation energy between the two photosystems. When electron transfer through chloroplast photosystem II (PSII) proceeds at a rate higher th an that through photosystem I (PSI), chemical reduction of a redox sen sor activates a thylakoid protein kinase that catalyses phosphorylatio n of light-harvesting complex II (LHCII). Phosphorylation of LHCII inc reases its affinity for PSI and thus redistributes light-harvesting ch lorophyll to PSI at the expense of PSII. This short-term redox signall ing pathway acts by means of reversible, post-translational modificati on of pre-existing proteins. A long-term equalisation of the rates of light utilisation by PSI and PSII also occurs: by means of adjustment of the stoichiometry of PSI and PSII. It is likely that the same redox sensor controls both state transitions and photosystem stoichiometry. A specific mechanism for integration of these short-and long-term ada ptations is proposed. Recent evidence shows that phosphorylation of LH CII causes a change in its 3-D structure, which implies that the mecha nism of state transitions in chloroplasts involves control of recognit ion of PSI and PSII by LHCII. The distribution of LHCII between PSII a nd PSI is therefore determined by the higher relative affinity of phos pho-LHCII for PSI, with lateral movement of the two forms of the LHCII being simply a result of their diffusion within the membrane plane. P hosphorylation-induced dissociation of LHCII trimers may induce latera l movement of monomeric phospho-LHCII, which binds preferentially to P SI. After dephosphorylation, monomeric, unphosphorylated LHCII may tri merize at the periphery of PSII.