Derepression and altered feedback regulation of valine biosynthetic pathway in analogue-resistant mutants of Streptomyces cinnamonensis resulting in 2-ketoisovalerate excretion

Excretion of 2-ketoisovaleric acid (KIV) was demonstrated in Streptomyces cinnamonensis mutants resistant to valine analogues 2-amino-3-chlorobutyrate, 2-aminobutyrate and norleucine, respectively. The highest KIV concentrations of 170–230 mg l⁻¹ were found in cultivation liquids of norleucine-resistant strains. Biochemical analyses of the acetohydroxyacid synthase (AHAS), valine dehydrogenase (VDH) and branched chain amino acid aminotransferase activities revealed the deregulation of the valine-synthesizing pathway, resulting in KIV excretion. In the 2-amino-3-chlorobutyrate-resistant strain, the activity of AHAS increased 23- to 31-fold compared with the parental strain. The norleucine-resistant mutants combined both a 10- to 23-fold increase in AHAS activity and lack of efficient feedback regulation by valine. In the double 2-amino-3-chlorobutyrate plus norleucine-resistant mutant, the AHAS activity was only four to eight-fold higher, but release of feedbackregulation was conserved. Similarly, feedback regulation was inefficient in 2-aminobutyrate-resistant strains, however the AHAS activity was lower than in the parental strain. A strong induction of VDH was observed in all regulatory mutants.