Protein Kinase Activity Associated with Isolated Ribosomes from Peas and Lemna

Protein kinase (ATP:protein phosphokinase) has been found associated with ribosomes prepared from 5-day-old etiolated pea shoots and sterile cultures of Lemna minor. The enzyme catalyzes the phosphorylation of ribosomal proteins in vitro and may be involved in the formation of phosphoproteins in the ribosomes in vivo. Protein kinase sediments with ribosomes during sucrose density gradient centrifugation through a buffer containing 0.02 m KCl. Seventy per cent of the enzymic activity dissociates from the ribosomes in 0.3 m KCl, but the remaining protein kinase remains firmly bound in 0.7 m KCl. Cyclic AMP does not modify the activity of these protein kinases in vitro. Benzyladenine inhibits the protein kinase, but only at high concentrations of this cytokinin.