MECHANISM OF BENZYLADENINE-INDUCED STIMULATION OF THE SYNTHESIS OF 5-AMINOLEVULINIC ACID IN GREENING CUCUMBER COTYLEDONS - BENZYLADENINE INCREASES LEVELS OF PLASTID TRANSFER RNAGLU

The mechanism of the stimulatory effect of a cytokinin, namely, benzyl adenine (BA), on the synthesis of 5-aminolevulinic acid (ALA) in cucum ber cotyledons was studied. The rate of synthesis of ALA by plastids i solated from BA-treated cotyledons was twice that by plastids from unt reated controls. Western blot analysis of stromal proteins showed that BA did not affect the level of glutamyl-tRNA synthetase or of glutama te 1-semialdehyde (GSA) aminotransferase. Analysis of free amino acids revealed that BA did not increase the level of glutamate in the strom a. However, the amount of total plastidic RNA was doubled in BA-treate d cotyledons. Northern blot analysis showed that the level of plastid TRNA(Glu) was increased by treatment with BA to the same extent as tha t of another plastid tRNA, reflecting an increase in total plastidic R NA. The rate of formation of glutamyl-tRNA was also doubled in plastid s from BA-treated cotyledons. The results indicate that stimulation of the synthesis of ALA by BA is due to an increased level of tRNA(Glu) in plastids.