A hybridization assay using Escherichia coli K-12 DNA isolated from the specialized transducing bacteriophage λCI857St68h80 dilv was used to examine the rate of synthesis of the messenger RNA's (mRNA) derived from the isoleucine-valine (ilv) gene cluster of Salmonella typhimurium. In all cases examined, changes in ilv enzyme levels could be correlated with changes in the rate of synthesis of ilv mRNA. Several well characterized regulatory mutants of S. typhimurium had rates of synthesis of ilv mRNA 3 to 8-fold higher than the repressed wildtype strain. The increased rates of ilv mRNA synthesis found in a hisT strain as well as in isoleucyl-and leucyl-tRNA synthetase mutants, strongly suggests a role for branched-chain aminoacyl-tRNA's in transcriptional control.
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