Derepression of valine-sensitive acetohydroxy acid synthase by threonine, .ALPHA.-ketobutyrate or isoleucine in Escherichia coli K12.

After nutritional shift-down, threonine, α-ketobutyrate, or isoleucine with- in certain concentration range stimulated derepression of the synthesis of acetohydroxy acid synthase in strain W3110-215, a low-thymine requiring derivative of Escherichia coli K12. The acetohydroxy acid synthase induced by isoleucine was valine-sensitive. Since chloramphenicol inhibited the increase of the enzyme activity, the increase seemed to be due to enhancement of enzyme synthesis and not to the activation of the enzyme. From these results, it was suggested that not only limitation of leucine or valine, but also the presence of excess threonine or isoleucine is necessary for efficient derepression of the synthesis of acetohydroxy acid synthase.On the other hand, threonine did not stimulate the derepression of the synthesis of the enzyme in strains W2252 and 58-161, methionine-auxotrophic mutants of E. coli K12, growing in minimal medium containing methionine. When a prototrophic strain with regard to methionine requirement was used, methionine or threonine added in minimal medium derepressed synthesis of acetohydroxy acid synthase, but the simultaneous addition of the two compounds did not give any synergistic effect on the enzyme synthesis. The mechanisms of these phenomena are unknown.