PR-SPECIFIC PHYTOCHROME PHOSPHORYLATION IN-VITRO BY A PROTEIN-KINASE PRESENT IN ANTI-PHYTOCHROME MAIZE IMMUNOPRECIPITATES

Protein kinase activity has repeatedly been found to co-purify with th e plant photoreceptor phytochrome, suggesting that light signals recei ved by phytochrome may be transduced or modulated through protein phos phorylation. In this study immunoprecipitation techniques were used to characterize protein kinase activity associated with phytochrome from maize (Zea mays L.). A protein kinase that specifically phosphorylate d phytochrome was present in washed anti-phytochrome immunoprecipitate s of etiolated coleoptile proteins. No other substrate tested was phos phorylated by this kinase. Adding salts or detergents to disrupt low-a ffinity protein interactions reduced background phosphorylation in imm unoprecipitates without affecting phytochrome phosphorylation, indicat ing that the protein kinase catalytic activity is either intrinsic to the phytochrome molecule or associated with it by high-affinity intera ctions. Red irradiation (of coleoptiles or extracts) sufficient to app roach photoconversion saturation reduced phosphorylation of immunoprec ipitated phytochrome. Subsequent far-red irradiation reversed the red- light effect. Phytochrome phosphorylation was stimulated about 10-fold by a co-immunoprecipitated factor. The stimulatory factor was highest in immunoprecipitates when Mg2+ was present in immunoprecipitation re actions but remained in the supernatant in the absence of Mg2+. These observations provide strong support for the hypothesis that phytochrom e-associated protein kinase modulates light responses in vivo. Since o nly phytochrome was found to be phosphorylated, the co-immunoprecipita ted protein kinase may function to regulate receptor activity.