Key words - Chloroplast light-harvesting complex; molecular recognition; photosynthesis; photosystem I; photosystem II; protein kinase/phosphatase; redox sensor/response regulator; redox signalling.

In photosynthesis in chloroplasts and cyanobacteria, redox control of thylakoid protein phosphorylation regulates distribution of absorbed excitation energy between the two photosystems. When electron transfer through chloroplast photosystem II (PS II) proceeds at a rate higher than that through photosystem I (PS I), chemical reduction of a redox sensor activates a thylakoid protein kinase that catalyses phosphorylation of light-harvesting complex II (LHC II). Phosphorylation of LHC II increases its affinity for PS I and thus redistributes light-harvesting chlorophyll to PS I at the expense of PS II. This short-term redox signalling pathway acts by means of reversible, post-translational modification of pre-existing proteins. A long-term equalisation of the rates of light utilisation by PS I and PS II also occur by means of adjustment of the stoichiometry of PS I and PS II. 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 adaptations is proposed. Recent evidence shows that phosphorylation of LHC II causes a change in its 3-D structure, which implies that the mechanism of state transitions in chloroplasts involves control of recognition of PS I and PS II by LHC II. The distribution of LHC II between PS II and PS I is therefore determined the higher relative affinity of phospho-LHC II for PS I, with lateral movement of the two forms of the LHC II being simply a result of their diffusion within the membrane plane. Phosphorylation-induced dissociation of LHC II trimers may induce lateral movement of monomeric phospho-LHC II, which binds preferentially to PS I. After dephosphorylation, monomeric, unphosphorylated LHC II may trimerize at the periphery of PS II.

Key words - Chloroplast light-harvesting complex; molecular recognition; photosynthesis; photosystem I; photosystem II; protein kinase/phosphatase; redox sensor/response regulator; redox signalling.

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