The flow of carbon to α-acetolactate in K12 is shown to involve the endogenous pool of α-ketobutyrate (α-KB). In vivo, the acetohydroxy acid synthase (AHAS) isoenzymes have an affinity for α-KB sufficiently high that α-acetolactate production is severely limited when α-KB is supplied exogenously. The ability of threonine deaminase to make α-KB is correlated with the synthesis of the AHAS isoenzymes. Mutations in that alter the catalytic and allosteric properties of threonine deaminase affect α-KB production and the expression of the AHAS isoenzymes in a direct way. The mutation results in a feedback-hypersensitive threonine deaminase and slow α-KB and AHAS production. A spontaneous revertant of an strain expressing a feedback-resistant threonine deaminase produces α-KB and AHAS more quickly. A physiological role for the activator (valine) site on threonine deaminase is proposed and valine is shown to increase α-KB production in vivo. Valine can thus regulate its own biosynthetic pathway without jeopardizing the production of isoleucine. The physiological implications of the role of α-KB in the biosynthesis of acetolactate are discussed.


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