LIGHT REGULATION OF 5-AMINOLEVULINIC ACID-SYNTHESIS SYSTEM IN CUCUMIS-SATIVUS - LIGHT STIMULATES ACTIVITY OF GLUTAMYL-TRANSFER-RNA REDUCTASE DURING GREENING

5-Aminolevulinic acid (ALA) is the common precursor of tetrapyrroles, such as chlorophyll and heme. The stimulatory effects of light on the synthesis of ALA in plastids from greening cucumber (Cucumis sativus) cotyledons were studied by examining the cellular levels of enzymes, t heir cofactors and substrate involved in the ALA-synthesis system. The activity of glutamyl-tRNA synthetase, which catalyzes the formation o f glutamyl-tRNA, was not increased by illumination. The stromal fracti on from 8-h illuminated cotyledons showed about 3-fold higher activity of glutamyl-tRNA reductase of the etiolated cotyledons. The activity of glutamate 1-semialdehyde (GSA) aminotransferase, which catalyzes th e final reaction of ALA synthesis, was not changed by illumination. We stern blot analysis also showed no increase in the levels of glutamyl- tRNA synthetase and GSA aminotransferase. Among the cofactors and the substrates involved in ALA synthesis, light most increased the levels of glutamate in plastids. It seemed that the changes in the levels of other cofactors (i.e., ATP, NADPH and Mg) by light had no influence on the respective reaction. These results indicate that glutamyl-tRNA re ductase is the solely light-regulated enzyme of ALA-synthesis system, and the elevation of glutamate by light may contribute to the stimulat ion of ALA synthesis.