The effect of leucine on the biosynthesis of leucine in maize root tips.

Abstract

A mixture of 15 amino acids known to promote growth in excised maize root tips inhibited the synthesis of threonine, valine, leucine, lysine, arginine, and proline from acetate-2-C14. This effect was not caused by a reduced uptake of acetate-C14 from the medium or by the exchange of C14-labeled amino acids derived from acetate with the Cl2-amino acids of the medium. The inhibition was more dramatic with the C14-amino acids recovered from the residue than with the alcohol-soluble amino acids. The biosynthesis of leucine from acetate-2-C14 was inhibited specificially by added leucine. With exogenous leucine, discrete pools, i.e. the metabolic pool in addition to the total soluble pool, were expanded. The metabolic pool or some fraction of this pool may be effective in the inhibition of leucine biosynthesis. The results suggest that leucine normally supplied to the intact root tip by the transport system could similarly inhibit the endogenous synthesis of leucine.