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Plant and Cell Physiology, 1995, Vol. 36, No. 7 1237-1243
© 1995

Stimulation of Glutamyl-tRNA Reductase Activity by Benzyladenine in Greening Cucumber Cotyledons

Tatsuru Masuda1, Hiroyuki Ohta1, Yuzo Shioi1, Hideo Tsuji2 and Ken-ichiro Takamiya1

1 Department of Biological Sciences, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology Nagatsuta 4259, Midori-ku, Yokohama, 226 Japan
2 Department of Biology, Kobe Women's University Aoyama 2-1, Higashi-suma, Suma-ku, Kobe, 654 Japan

The effects of benzyladenine (BA) on the synthesis of 5-aminolevulinic acid (ALA) in plastids were studied by analyzing the BA-induced modulation of the activities of the three enzymes involved in the ALA-synthesis system. The activities of enzymes were assayed in stromal fractions 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 reaction catalyzed by glutamyl-tRNA synthetase. An RNA digestion assay showed, however, that the stimulation was due to an increase in levels of endogenous tRNAGlu, which is the substrate of the synthetase, and not to an increase in the activity of the synthetase itself. The stromal fraction from BA-treated cotyledons had about twice the glutamyl-tRNA reductase activity of untreated dark and light-illuminated control cotyledons. The activity of the reductase was almost the same as the overall ALA-synthetic activity under all conditions tested. RNA gel blot analysis showed that BA increased the levels of the transcript for the reductase in darkness and in the light. The activity of glutamate 1-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 caused by increased levels of glutamyl-tRNA reductase and that the reductase is the regulatory and rate-determining enzyme in the ALA-synthesis system except in untreated etioplasts, in which the level of glutamyl-tRNA may be a rate-determining factor.

(Received April 13, 1995; Accepted July 21, 1995)
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