Hormonal Control of Enzyme Synthesis: On the Mode of Action of Gibberellic Acid and Abscisin

Gibberellic acid (GA) enhances the synthesis otf a-amylase andl ribonutclease in isolated aleurone layers andl this process is inhibited by abscisiin. Removal of gibberellic acid in mid-course of a-amylase productioni results in a slowing down of a-amylase synthesis, suggesting a continued requirement of GA for eiizyme synthesis. This is paralleled by a continuous requirement for RNA synthesis. Addition of 6-methylpurine or 8-azaguanine in mid-course results in ani inhibition of a-amylase synthesis within 3 to 4 hours. However, adtinomycin D added in inid-course is almost without effect. 'rhis is not due to its failuire to enter the cells, because it does inhibit 1'C-uridinie incorporation at this stage. Addition of abscisin to aleuroine layers which are synthesizing a-amylase results in an inhibition o'f this synthesis within 2 to 3 hours. Cycloheximide oIn the other hand inhibits enzyme synthesis immediately upon its addition. These data are consistent with the hyp.othesis that the expression of the GA effect requires the synthesis of enzyme-specific RNA molecules. The similarity in the kinetics of inhilbition between abscisin on the one haind and 8-azaguanine or 6-meth'yl;purine exert its action by inhibiting the synthesis or by preventiing their incorporation into