Evidence for an altered operator specificity: catabolite repression control of the leucine operon in Salmonella typhimurium.

A mutation, GD-1, in the leucine operon imposed unusual growth characteristics upon a leucine auxotrophic strain bearing the leucine operator mutation, leu-500. The strain with the GD-1 mutation was able to grow on a minimal salts medium when citrate was the sole carbon source, but required leucine when glucose was present. Tests with a large number of carbohydrates suggest that in the strain bearing the GD-1 mutation the leucine biosynthetic enzymes are under catabolite repressor control. Recombination studies indicate that the GD-1 mutation is a secondary alteration of the leucine operator at or very close to the site of the leu-500 mutation. Mutations at the supX locus (previously termed su leu 500 and located on the chromosome between the cysteine B and tryptophan gene clusters) result in elimination of the catabolite repression effect. The data are interpreted as an indication that the GD-1 and leu-500 mutations alter the leucine operator with respect to its specificity of response to repressors.