The regulation of the ilvADGE operon: Evidence for positive control by threonine deaminase

Abstract

We isolated a strain (PS187) containing a mutation in the ilvA structural gene (threonine deaminase) affecting the regulation of the isoleucine and valine biosynthetic enzymes. We studied the effect of the mutation, ilvA538, on the ability of the ilvADGE operon to respond to limitations of branched chain amino acids. We found that this regulatory mutation prevents a derepression response to limitations of branched chain amino acids. The mutation is epistatic to constitutive mutations which are deficient in co-repressor synthesis. In addition, the ilvA + allele is dominant to ilvA538 in trans, while a cis regulatory mutation, vlr-2005, is dominant in cis. We suggest a positive regulatory role for the ilvA gene product and present evidence that the apo-dimer form of threonine deaminase is the molecular species essential for an efficient depression response of the ilvADGE operon.