ELECTRON-TRANSPORT REGULATES EXCHANGE OF 2 FORMS OF PHOTOSYSTEM-II D1PROTEIN IN THE CYANOBACTERIUM SYNECHOCOCCUS

Synechococcus sp, PCC 7942 modulates photosynthetic function by transi ently replacing the constitutive D1 photosystem II protein, D1:1, with an alternate form, D1:2, to help counteract photoinhibition under exc ess light, We show that a temperature drop from 37 to 25 degrees C als o drives D1:1/D1:2 exchange under constant, moderate light, Chilling o r light-induced D1 exchange results from rapid loss of psbAI message c oding for D1:1 and accumulation of psbAII and psbAIII messages coding for D1:2, During chilling, a large pool of a novel form, D1:2, transi ently accumulates, distinguishable from normal D1 by an increase in ap parent molecular mass, D1: is not phosphorylated and is probably a fu nctionally inactive, incompletely processed precursor, After acclimati on to 25 degrees C, D1:2 disappears and D1:1 again predominates, alth ough substantial D1:2 remains, Partial inhibition of electron transpor t under constant, moderate light also triggers the D1 exchange process , These treatments all increase excitation pressure on photosystem II relative to electron transport, Therefore, information from photosynth etic electron transport regulates D1 exchange without any requirement for a change in light intensity or quality, possibly via a redox sensi ng mechanism proximal to photosystem II.