DEREPRESSION AND ALTERED FEEDBACK-REGULATION OF VALINE BIOSYNTHETIC-PATHWAY IN ANALOG-RESISTANT MUTANTS OF STREPTOMYCES-CINNAMONENSIS RESULTING IN 2-KETOISOVALERATE EXCRETION

Excretion of 2-ketoisovaleric acid (KIV) was demonstrated in Streptomy ces cinnamonensis mutants resistant to valine analogues 2-amino-3-chlo robutyrate, 2-aminobutyrate and norleucine, respectively. The highest KIV concentrations of 170-230 mg l(-1) were found in cultivation liqui ds of norleucine-resistant strains. Biochemical analyses of the acetoh ydroxyacid synthase (AHAS), valine dehydrogenase (VDH) and branched ch ain amino acid aminotransferase activities revealed the deregulation o f the valine-synthesizing pathway, resulting in KIV excretion. In the 2-amino-3-chlorobutyrate-resistant strain, the activity of AHAS increa sed 23- to 31-fold compared with the parental strain. The norleucine-r esistant mutants combined both a 10- to 23-fold increase in AHAS activ ity and lack of efficient feedback regulation by valine. In the double 2-amino-3-chlorobutyrate plus norleucine-resistant mutant, the AHAS a ctivity was only four to eight-fold higher, but release of feedback re gulation was conserved. Similarly, feedback regulation was inefficient in 2-aminobutyrate-resistant strains, however the AHAS activity was l ower than in the parental strain. A strong induction of VDH was observ ed in all regulatory mutants.