STIMULATION OF GLUTAMYL-TRANSFER-RNA REDUCTASE-ACTIVITY BY BENZYLADENINE IN GREENING CUCUMBER COTYLEDONS

The effects of benzyladenine (BA) on the synthesis of 5-aminolevulinic acid (ALA) in plastids were studied by analyzing the BA-induced modul ation of the activities of the three enzymes involved in the ALA-synth esis system. The activities of enzymes were assayed in stromal fractio ns from plastids isolated from BA-treated and untreated cotyledons. BA doubled the overall rate of synthesis of ALA as compared to rates in untreated dark and light-illuminated control cotyledons. BA stimulated the formation of glutamyl-tRNA, the product of the aminoacylation rea ction catalyzed by glutamyl-tRNA synthetase. An RNA digestion assay sh owed, however, that the stimulation was due to an increase in levels o f endogenous tRNA(Glu), which is the substrate of the synthetase, and not to an increase in the activity of the synthetase itself. The strom al fraction from BA-treated cotyledons had about twice the glutamyl-tR NA reductase activity of untreated dark and light-illuminated control cotyledons. The activity of the reductase was almost the same as the o verall ALA-synthetic activity under all conditions tested. RNA gel blo t analysis showed that BA increased the levels of the transcript for t he reductase in darkness and in the light. The activity of glutamate l -semialdehyde aminotransferase, which catalyzes the final reaction in the synthesis of ALA, was unchanged by treatment with BA. The present results indicate that stimulation of the synthesis of ALA by BA was ca used by increased levels of glutamyl-tRNA reductase and that the reduc tase is the regulatory and rate-determining enzyme in the ALA-synthesi s system except in untreated etioplasts, in which the level of glutamy l-tRNA may be a rate-determining factor.