[Effect of 6-benzylaminopurine on (14C)leucine incorporation into protein in a cell-free system from isolated squash cotyledons].

Polysomal preparations from isolated pumpkin cotyledons treated with cytokinin [6-benzylaminopurine (BAP), 10 mg/l] were about two-fold more active in the cell-free system of protein synthesis as compared to polysomes from control cotyledons. The time course of 14C-leucine incorporation into protein and its dependence on polysome concentration were studied; sucrose density fractionation have revealed significant differences in polysome distribution between the treated and control cotyledons. All polysomal fractions from BAP-treated cotyledons were more active in protein synthesis than corresponding fractions from control cotyledons. Mixing of BAP-treated and untreated cotyledons before polysome isolation showed that the difference in their activity did not result from isolation procedure. Factors of polysome activation and/or inhibition were tightly bound to polysomes. Treatment of polysomes with 0.175 M KCl reduced markedly their protein-synthesizing activity and abolished the difference between polysomes of BAP-treated and control cotyledons. The initial level of polysome activity could be restored by addition of proteins isolated from the salt wash, but these proteins were not specific in their action. Possible mechanisms of phytohormone action on ribosome activity are discussed. BAP activation of ribosomes in protein synthesis in vitro is fully eliminated by addition of natural inhibitor--abcisic acid--to BAP solution during cotyledons incubation.