In-vivo evidence for metabolic control of amino-acid and protein synthesis by exogenous lysine and threonine in Mimulus cardinalis.

Young developing seedlings of Mimulus cardinalis Douglas (Scrophulariaceae) were grown in Hoagland's medium supplemented with lysine (LYS; 1.0 mM), threonine (THR; 1.0 mM), LYS+THR (1.0 mM ea.) or LYS+THR+methionine (MET; 1.0-1.0-0.1 mM, respectively). LYS or THR reduced the total amount of protein in the developing seedlings and when added in combination completely inhibited the accumulation of protein after 7 days. The combination of LYS+THR+MET enhanced the accumulation of protein. When MET was added to seedlings grown on LYS+THR for 9 days, the rate of protein accumulation reverted to normal. Studies using [(14)C]leucine demonstrated similar inhibitory patterns on protein synthesis. The effects of LYS and THR on the distribution of [(14)C]aspartate into the aspartate amino acids including MET were studied. LYS and THR, singly and synergistically when in combination, inhibited the synthesis of MET. Thus the simultaneous application of LYS and THR can be interpreted as producing an artificially induced MET auxotroph. These results indicate that LYS and THR act singly and together in vivo to inhibit the accumulation of soluble LYS, MET and THR in a manner indicative of end-product inhibition. Indirect evidence for end-product control of alanine, arginine, glycine, phenylalanine, proline, tryptophan, tyrosine and valine is also given.