In Escherichia coli, the three
branched-chain amino acid activating
enzymes appear to be essential for multivalent repression of the
isoleucine- and
valine-forming
enzymes. The results of experiments with a mutant, strain CU18, having an altered
threonine deaminase, indicate that free
isoleucine and some form of
threonine deaminase (the product of the ilvA gene) are also involved in multivalent repression. This strain exhibits abnormally high derepressibility but normal repressibility of its ilv gene products, and its
threonine deaminase is inhibited only by high concentrations of
isoleucine. In strain CU18, the
isoleucine analogue, thiaisoleucine, is incapable of replacing
isoleucine in the multivalent repression of the ilv genes, whereas the analogue can fully replace the natural
amino acid in repression in other strains examined. The
dipeptide, glycyl-
leucine, which, like
isoleucine, is a heterotropic negative effector of
threonine deaminase but is not a substrate for isoleucyl-transfer
ribonucleic acid synthetase, can completely prevent the accumulation of
threonine deaminase-forming potential during
isoleucine starvation in strains with normal
threonine deaminases. It can not, however, prevent such accumulation in strain CU18 or in other strains in which
threonine deaminase is insensitive to any concentration of
isoleucine.