Protein tyrosine phosphorylation in the transition to light state 2 of chloroplast thylakoids

Redox dependent protein phosphorylation in chloroplast thylakoids regulates distribution of excitation energy between the two photosystems of photosyn thesis, PS I and PS II. Several thylakoid phosphoproteins are known to be p hosphorylated on N-terminal threonine residues exposed to the chloroplast s troma. Phosphorylation of light harvesting complex II (LHC II) on Thr-6 is thought to account for redistribution of light energy from PS II to PS I du ring the transition to light state 2. Here, we present evidence that a prot ein tyrosine kinase activity is required for the transition to light state 2. With an immunological approach using antibodies directed specifically to wards either phospho-tyrosine or phospho-threonine, we observed that LHC II became phosphorylated on both tyrosine and threonine residues. The specifi c protein tyrosine kinase inhibitor genistein, at concentrations causing no direct effect on threonine kinase activity, was found to prevent tyrosine phosphorylation of LHC II, the transition to light state 2, and associated threonine phosphorylation of LHC II. Possible reasons for an involvement of tyrosine phosphorylation in light state transitions are proposed and discu ssed.