Pathway for Isoleucine Formation from Pyruvate by Leucine Biosynthetic Enzymes in Leucine-Accumulating Isoleucine Revertants of Serratia marcescens

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Abstract
Leaky revertants isolated from isoleucine auxotrophs of Serratia marcescens mutant resistant to α-aminobutyric acid were previously reported to accumulate leucine in the medium, due to the absence of both feedback inhibition and repression of leucine biosynthesis. Growth of the revertant was accelerated by pyruvate, D(–)-citramalate, citraconate, and α-ketobutyrate, but not by threonine. Extracts of the revertant exhibited high activities of pyruvate-dependent coenzyme A liberation from acetyl-coenzyme A, hydration of citraconate, and conversion of citraconate to α-ketobutyrate, but showed no threonine-deaminating activity. In the leucine accumulating revertants the above three activities were not affected by leucine, but in the wild strain and other revertants accumulating no leucine all or one of these activities was controlled by leucine. A leucine auxotroph isolated from the leucine-accumulating revertant showed isoleucine auxotrophy as well. From these data, it is concluded that, in leucine-accumulating revertants of S. marcescens, isoleucine is synthesized from α-ketobutyrate via citramalate formed from pyruvate and acetyl-coenzyme A by leucine biosynthetic enzymes, as a result of desensitization of α-isopropylmalate synthetase to feedback inhibition.

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