Defective Transamination, a Mechanism for Resistance to Ketomycin in Escherichia coli

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Abstract
A spontaneous mutant of a derivative of Escherichia coli strain K-12 resistant to 50 μg of ketomycin per ml was selected. The mutant displayed a two- to threefold derepression of the isoleucine-valine biosynthetic enzymes and a reduced growth rate in minimal medium. The lesion was found to lie in the gene ( ilvE ) specifying transaminase B and resulted in an isoleucine limitation. The presence of exogenous isoleucine during growth in minimal medium restored normal phenotypic properties. The reduced transaminase B activity is responsible for the resistance to ketomycin. An unusual derepression of the acetohydroxy acid synthetase in response to an isoleucine limitation was noted.