The XylS protein is the positive regulator of the promoter controlling the meta-cleavage pathway (Pm) for catabolism of certain alkylbenzoates on the TOL plasmid of Pseudomonas. Transcription from Pm is mediated by XylS either in the presence of benzoate effectors or through XylS hyperproduction. Two regions of the NH2 terminus of XylS (residues 37-45) had been predicted to be involved in effector control of XylS transcriptional activation. Different methods were used to induce mutations in this region, including genetic selections (where Pm controlled a tetracycline resistance gene), bisulfite mutagenesis at a unique restriction site, and extensive oligonucleotide mutagenesis at residues 41 and 45. The mutants fell into four classes based on their phenotypes with respect to effector-mediated activation of a Pm-lacZ fusion: (a) effector profiles similar to wild type, (b) no Pm stimulation with benzoates, (c) altered effector specificity, and (d) higher basal Pm activities, in some cases including changes in effector specificity. In some mutants, higher basal Pm activity was apparently due to mutations that increased XylS stability. Substitutions at Arg-41 resulted in all four mutant phenotypes, indicating that this is a critical residue in XylS for effector stimulation of transcription activity.