RECOGNITION OF SPECIFIC PATTERNS OF AMINO-ACID INHIBITION OF GROWTH IN HIGHER-PLANTS, UNCOMPLICATED BY GLUTAMINE-REVERSIBLE GENERAL AMINO-ACID INHIBITION

The complexity of the regulatory mechanisms that govern amino acid bio synthesis, particularly in multibranched pathways, frequently results in sensitivity to growth inhibition by exogenous amino acids. Usually the inhibition caused by a given amino acid(s) is relieved by another amino acid(s), thus indicating the cause of inhibition to be a specifi c interference with endogenous formation of the latter amino acid(s). We recently summarized the evidence that Nicotiana silvestris (and pro bably most higher plants), in suspension culture, exhibits a separate phenomenon of amino acid mediated growth inhibition called general ami no acid inhibition. Every amino acid provokes general amino acid inhib ition except for L-glutamine. In fact, L-glutamine completely overcome s general amino acid inhibition. We have now demonstrated that specifi c amino acid inhibition can be recognized and characterized al the lev el of growth inhibition without interference caused by general amino a cid inhibition by the simple provision of exogenous L-glutamine. Sever al examples of specific amino acid inhibition of growth were demonstra ted in N. silvestris. In one case, L-threonine inhibits growth partial ly in the presence of L-glutamine. The residual amino acid inhibition was overcome by the additional presence of L-lysine and L-methionine, indicating that L-threonine specifically inhibits the biosynthesis of both L-lysine and L-methionine. As a second example, the L-valine-medi ated inhibition of growth that persisted in the presence of L-glutamin e was overcome by L-isoleucine, indicating that exogenous L-valine inh ibits L-isoleucine biosynthesis. The use of amino acid analogs as expe rimental tools for biochemical-genetic studies in higher plants is als o complicated by general amino acid inhibition. Conditions were demons trated under which p-fluorophenylalanine and m-fluorotyrosine could be used as specific antimetabolites of L-phenylalanine and L-tyrosine bi osynthesis without interference from general amino acid inhibition. We thus present a rigorous basis for recognition of specific relationshi ps between metabolic branches that can guide derailed enzymological an alyses. (C) 1997 Elsevier Science Ireland Ltd.