CONTROL OF ISOLEUCINE, VALINE AND LEUCINE BIOSYNTHESIS .7. ROLE OF VALINE TRANSFER RNA IN REPRESSION

These studies support the hypothesis that charging of valine tRNA is a step required for repression, mediated by the valyl-tRNA synthetase. Consistent with this hypothesis were the observations that (a) dl-threo-α-amino-β-chlorobutyric acid (aminochlorobutyrate) which was activated and attached to valine tRNA blocked the synthesis of the isoleucine-valine biosynthetic enzymes, while dl-α-aminobutyric acid which was activated but not transferred to tRNA had no effect on the formation of these enzymes; (b) the effect of aminochlorobutyrate was due to repression and not to the production of false proteins, since the synthesis of numerous biosynthetic and inducible enzymes was not blocked by the analogue; (c) there was a direct correlation between the intracellular charging of valine tRNA by valine or aminochlorobutyrate and repression of the isoleucine-valine enzymes; (d) the steady-state charging of valine tRNA in vivo was virtually the same in a valine auxotroph grown with added valine or aminochlorobutyrate. © 1969.