Allosteric regulation of branched-chain amino acid biosynthesis by potassium, valine, leucine, and isoleucine. Exogenous addition of potassium, valine, isoleucine, or leucine limits isoleucine biosynthetic flux in strains without ilvGM-encoded AHASII (whose reaction is indicated by an orange dotted line). All other enzyme-catalyzed reactions appear as black arrows. Three pairs of genes, ilvB with ilvN, ilvG with ilvM, and ilvH with ilvI, are each cotranscribed and, in tandem, form the enzymes AHASI (ilvBN; green), II (ilvGM; orange), and III (ilvHI; blue), respectively. These three AHAS isozymes catalyze either the reaction between two pyruvate molecules to produce 2-acetolactate or between pyruvate and 2-ketobutyrate to produce 2-aceto-2-hydroxybutyrate, although the ilvHI gene product is generally insufficient to support growth alone. For simplicity, only relevant gene and metabolite names are shown. Black lines with flat ends represent allosteric inhibition. Nutrients provided exogenously in this work appear in shaded boxes.