CROSS-REGULATION BY XYLR AND DMPR ACTIVATORS OF PSEUDOMONAS-PUTIDA SUGGESTS THAT TRANSCRIPTIONAL CONTROL OF BIODEGRADATIVE OPERONS EVOLVES INDEPENDENTLY OF CATABOLIC GENES

The Pu promoter of the toluene degradation plasmid pWW0 of Pseudomonas putida drives expression of an operon involved in the sequential oxid ation of toluene and m- and p-xylenes to benzoate and toluates, respec tively. Similarly, the Po promoter of plasmid pV1150 controls expressi on of an operon of Pseudomonas sp. strain CF600 which is required for the complete catabolism of phenol and cresols. These promoters, which both belong to the sigma(54)-dependent class, are regulated by their c ognate activators, XylR and DmpR, respectively. XylR and DmpR are homo logous proteins, and both require aromatic compounds as effector molec ules for activity. However, these two proteins respond to different pr ofiles of aromatic compounds. The activity of each promoter in the pre sence of the heterologous regulator was monitored using lacZ and luxAB reporter systems. Genetic evidence is presented that the two activato rs can functionally substitute each other in the regulation of their c orresponding promoters by binding the same upstream DNA segment. Furth ermore, when coexpressed, the two proteins appear to act simultaneousl y on each of the promoters, expanding the responsiveness of these syst ems to the presence of effecters of both proteins. Potential mechanism s for the occurrence of evolutionary divergence between XylR and DmpR are discussed in view of the DNA sequence similarities among Pu, Po, a nd a third XylR-responsive promoter, Ps.