Methionine starvation of methionine auxotrophs in the presence of excess branched-chain amino acids results in a partial derepression of the isoleucine and valine enzymes. Reversed-phase chromatography indicated that isoleucine, valine and leucine tRNA were altered during methionine starvation. In addition, the total tRNA isolated from cells under these conditions was undermethylated. The observed derepression may be caused by the inability of methyl-deficient tRNA's to participate adequately in normal regulatory functions.
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