Studies on the specificities of the phenylalanyl- and tyrosyl-sRNA synthetases from plants

Abstract

The phenylalanyl- and tyrosyl-sRNA synthetases of mung bean seed have been partially purified and their ability to utilize a range of amino acid analogues as substrates determined. The substrate specificity of the phenylalanyl-enzyme seems to be less exacting than that of the tyrosyl-enzyme, a finding in agreement with previous observations using animal and microbial enzymes. Mimosine, a toxic amino acid present in Mimosa and Leucaena species, served as a substrate for the phenylalanyl-, but not the tyrosyl-sRNA synthetase from mung bean. However, the transfer of 14C-phenylalanine to tRNA catalysed by its synthetase enzyme was not impaired by the presence of high concentrations of mimosine; Leucaena phenylalanyl-sRNA synthetase also activated mimosine, but it is concluded that mimosine-induced toxicities do not arise primarily by interference with phenylalanine incorporation into protein molecules. 2-Amino-4-methylhex-4-enoic acid, a natural product of Aesculus californica, behaved as a phenylalanine analogue, being activated by the phenylalanyl-sRNA synthetase of mung bean at rates comparable to that determined for phenylalanine itself.