LEUCYL-tRNA SYNTHETASE ACTIVITY IN OLD COTYLEDONS: EVIDENCE ON REPRESSOR ACCUMULATION

Previous studies from this laboratory have shown that the complement of tRNALeu isoaccepting species changes during cotyledon senescence. Concomitantly there is a reduction in the capacity of leucyl-tRNA synthetase from 21-day-old cotyledons to acylate tRNA(1-4)(Leu) fully. Leucyl-tRNA synthetase can be fractionated into 3 peaks of activity on hydroxylapatite. All 3 peaks of activity are present in both young (5-day) and old (21-day) cotyledons. This finding eliminates the possibility that 21-day-old cotyledons are specifically deficient in a charging enzyme for tRNA(1-4)(Leu). Other experiments effectively rule out the possibility that tRNA(1-4)(Leu) consists of co-eluting subspecies of tRNA, as was earlier suggested. Inorganic pyrophosphate is inhibitory in the range 10(-5)-10(-3) M and inhibition is overcome by the addition of pyrophosphate, but only to the normal level of charging for the old synthetase preparation. Since no loss in a specific synthetase activity can be demonstrated to account for the deficiency in charging of tRNA(1-4)(Leu) in the older cotyledons, these results are interpreted on the basis of a model of repressor accumulation and its subsequent interaction with aminoacyl-tRNA molecules.