Overproduction of 2-ketoisovalerate and monensin production by regulatory mutants of Streptomyces cinnamonensis resistant to 2-ketobutyrate and amino acids

We have prepared Streptomyces cinnamonensis mutants resistant to 2-ketobutyrate in combination with valine or isoleucine. The selection process was strictly dependent on the presence of nitrate. Sixteen representatives of three groups of mutants were biochemically analyzed as to the activities of enzymes of the branched-chain amino acid metabolism: acetohydroxy acid synthase, transaminase B and valine dehydrogenase. A majority of mutants resistant to 2-ketobutyrate and valine (BVR strains) exerted no feedback regulation of the acetohydroxy acid synthase by valine and proved to be potent producers of 2-ketoisovaleric acid. The BVR-13 strain produced up to 2.4 g l⁻¹ of 2-ketoisovaleric acid during a 72-h cultivation in a nitrate–glucose medium. The excreted valine reached only 2.6% of 2-ketoisovaleric acid concentration. Bentonite present in the culture medium reduced NH₄⁺ concentration and augmented the 2-ketoisovaleric acid production 1.3-fold. Half of the tested strains cultivated in a soya-bean medium exerted a substantially higher production of antibiotic monensins with elevated proportion of the monensin A.