The stimulation of a casein kinase II from yeast by polyamines occurs with endogenous substrates at cytosolic salt levels.

A casein kinase II that is tightly bound to yeast ribosomes was partially purified and used to phosphorylate YL 44 and an unidentified 36 kDa protein in purified ribosomes. At typical cytosolic salt concentrations the phosphorylation was strongly stimulated by moderate concentrations (200 microM) of spermine or spermidine. The lowest effective concentration of spermine (20 microM, causing less than 50% stimulation) was close to that of total spermine reported in nongrowing yeast. Increases in free polyamines accompanying the 10-fold increase in total spermine and spermidine in growing yeast may therefore significantly stimulate this phosphorylation.