Abstract : Analysis of genes for the Beta-ketoadipate pathway revealed mechanisms underlying evolutionary divergence of controls governing biodegradation in bacteria. Transcriptional regulators that respond to muconate in Acinetobacter calcoaceticus and Pseudomonas putida diverged recently from a common ancestor. This divergence produced the A. calcoaceticus catM repressor gene and the P. putida activator catR gene. Thus a single ancestor gave rise to one gene that exercises negative control and another gene that exerts positive control over transcription. Independently transcribed genes for related physiological functions are clustered in the A calcoaceticus chromosome, and the evolutionary basis for selection of this supraoperonic clustering is unknown. Advances in the genetics of this organism will make it possible to explore the genetic and physiological consequences of engineered transpositions which alter the structure of supraoperonic clusters.