Mutations leading to constitutive expression from the TOL plasmid meta-cleavage pathway operon are located at the C-terminal end of the positive regulator protein XylS.

The XylS protein is the positive activator of the TOL plasmid meta-cleavage pathway operon for the metabolism of alkylbenzoates in Pseudomonas putida. The regulator stimulates transcription from the TOL meta pathway operon promoter (Pm) when activated by benzoate effectors or in the absence of effectors when overproduced. xylS mutant alleles that encode regulators which constitutively mediate expression from Pm were isolated and characterized. The mutant proteins all exhibit single amino acid substitutions adjacent to putative alpha-helix-turn-alpha-helix domains at their C-terminal ends. The XylS mutant proteins can still be partially activated by the usual and unusual benzoate effectors for the wild-type regulator and when activated stimulate higher levels of transcription from Pm.