Characterization of a protein-kinase activity associated with phytochrome from etiolated oat (Avena sativa L.) seedlings.

A protein-kinase activity which is co-purified with phytochrome from etiolated oat seedlings was investigated in some detail. Whereas phytochrome was always phosphorylated in solution (together with some contaminating protein bands), radioactive phosphate was not found in the phytochrome band after native gel electrophoresis and incubation of the entire gel with labeled ATP. Since protein kinases are usually autophosphorylated under these conditions, the result shows that the kinase activity does not reside in the phytochrome molecule itself. Radioactivity was exclusively detected in a band with the apparent molecular weight 450 kDa; sodium-dodecyl-sulfate gel electrophoresis revealed an apparent molecular weight of 60 kDa for the phosphorylated subunit. The N-terminal amino-acid sequence A L E S A (G) K Q (L) V P W was determined for this subunit which is a potential candidate for the protein kinase. The optimum conditions (pH, metal ion concentration) and kinetics of the phosphorylation reaction were determined. The presumed connection between proteinkinase activity and the signal chain leading from the far-red-absorbing form of phytochrome to physiological responses still awaits elucidation.