SUMMARY: Preliminary studies demonstrated that synthesized isoleucine by a pathway not involving threonine or threonine dehydratase. Radiotracer experiments with cells grown in a defined carbohydrate-free medium showed that radioactivity from [UC]serine, [3C]pyruvate, [C]NaHCO and [1-], [2-] and [3C]propionate was incorporated into isoleucine. Conversely, there was no detectable incorporation of C into isoleucine during growth with [UC]glutamate, [UC]threonine, [UC]valine, [UC]leucne or [UC]methionine. Crude extracts of the bacteria grown in a minimal medium contained levels of α-acetohydroxyacid synthase activities comparable to those in K12 grown in minimal medium. Stepwise degradation of isoleucine obtained from grown in the presence of specifically-labelled precursors indicated that can make isoleucine via the reductive carboxylation of propionate to yield α-oxobutyrate, which is metabolized to isoleucine in the classical fashion. Isoleucine was also formed by via the reductive carboxylation of α-methylbutyrate to α-oxo-β-methylvalerate.


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