Identification, characterization, and resolution of the in vivo phosphorylated form of the D1 photosystem II reaction center protein.

The chloroplast-encoded D1 protein of oxygenic photosynthetic organisms is a component of the photosystem II reaction center. Previously, we detected an electrophoretic variant of D1 which was generated in vivo in granal-localized reaction centers in a light dependent manner (Callahan, F.E., Ghirardi, M.L., Sopory, S.K., Mehta, A.M., Edelman, M., and Mattoo, A.K. (1990) J. Biol. Chem. 265, 15357-15360). In the present study, we identify this modified form as phosphorylated D1. The in vivo phosphorylation occurs on a threonine residue(s) localized within 1 kDa from the N terminus and is identical to the phosphorylation of D1 catalyzed in vitro by a redox-regulated thylakoid-bound protein kinase. While virtually all of the D1 protein present in thylakoids can be phosphorylated in vitro, the steady-state level of phosphorylated D1 in vivo varies with light intensity and did not exceed 20% of the total D1 under the conditions of this study.