LysR-type transcriptional regulators, BenM and CatM, tightly regulate the genes involved in aromatic compound degradation in Acinetobacter baylyi strain ADP1. BenM and CatM share similarities in protein sequence and promoter recognition, yet have striking differences in regulatory function. The experiments described here explore the regulatory role of CatM in the absence of a functional BenM. Spontaneous mutations in the catB promoter were identified in strains that maintained the ability to grow on benzoate as a sole carbon source in the absence of BenM. Characterization of the metabolism, gene expression, enzyme activity, and binding affinity of these mutants ultimately lead to a model of CatM-mediated expression of the benA promoter in response to increased intracellular concentrations of muconate.