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 more 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.