Abstract
The green alga Dunaliella salina is able to adapt to long-term changes in irradiance by adjusting the complement of auxiliary antenna complexes associated with each photosystem. This organism is also able to respond to short-term changes in incident light regime by transient adjustments in the absorption cross-section of Photosystem II by the process of state 1–state 2 transitions, which are mediated via redox-dependent phosphorylation of polypeptides of the chlorophyll a b- binding light-harvesting complex of PS II (LHC-II). We have observed protein phosphorylation-dependent state transitions in Dunaliella salina and demonstrate that redox-controlled phosphorylation of LHC-II polypeptides of molecular mass 27, 26 and 25 kDa occurs in vivo during such adaptations. Redox-controlled phosphorylation of the 25 kDa polypeptide also occurred in high irradiance-adapted cells which have a depleted Photosystem II antenna and which are incapable of state transitions. We conclude that this polypeptide may be phosphorylated constitutively and may represent a component of the non-mobile, inner antenna of Photosystem II. State transitions were also observed in high irradiance-grown cells reacclimating to low irradiance, and conclude that the absorption cross-section of only PS II α centres changes as a result of phosphorylation of LHC-II polypeptides. PS II β has an unchanged absorption cross-section. An important conclusion is reached that during incremental assembly the PS II antenna on recovery from high light conditions, phosphorylation of individual LHC-II polypeptides is accompanied by an increase in the effect of Light 1 on variable fluorescence. This suggests that the 26 kDa and 25 kDa LHC-II polypeptides are phosphorylated in a redox-regulated reaction, with a 27 kDa polypeptide becoming phosphorylated finally to decouple a proportion of the antenna of PS II α.
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