Improvement of an l -Leucine-Producing Mutant of Brevibacterium lactofermentum 2256 by Genetically Desensitizing It to α-Acetohydroxy Acid Synthetase

Abstract

Genetic improvement of l -leucine productivity in strain 218, an ile − 2-thiazolealanine-resistant mutant of Brevibacterium lactofermentum 2256, was attempted. In strain 218, which produced 28 mg of l -leucine per ml from 13% glucose, α-isopropylmalate synthetase was genetically desensitized and derepressed to the effect of l -leucine, whereas α-acetohydroxy acid synthetase remained unaltered, although it could be derepressed phenotypically by limiting the isoleucine concentration in the culture. From strain 218 we isolated 103 mutants resistant to β-hydroxyleucine (4 mg/ml). Among these, three were found to produce mere l -leucine than the parent. The α-acetohydroxy acid synthetase of all three mutant strains was found to be genetically desensitized to all of the branched-chain amino acids l -isoleucine, l -valine, and l -leucine. The repression mechanism in α-acetohydroxy acid synthetase formation was the same as in the parent strain. The improved strains typically produced 34 mg of l -leucine per ml, the highest productivity ever reported.