IDENTIFICATION OF THE REPRESSOR SUBDOMAIN WITHIN THE SIGNAL RECEPTIONMODULE OF THE PROKARYOTIC ENHANCER-BINDING PROTEIN XYLR OF PSEUDOMONAS-PUTIDA

In the presence of m-xylene, the protein XylR encoded by the TOL plasm id of Pseudomonas putida, activates the sigma(54)-dependent promoter P u. Early activation stages involve the release of the intramolecular r epression caused by the signal reception N-terminal (A domain) of XylR on the central module of the protein. A genetic approach has been fol lowed to locate the specific segment within A domain of XylR that is d irectly responsible for its down-regulation in the absence of inducer, as compared to that involved in effector (m-xylene) binding. For this , a reporter Escherichia coli strain carrying a monocopy transcription al fusion of Pu to lacZ was transformed with a collection of plasmids encoding equivalent truncated varieties of XylR, consisting of nested and internal deletions throughout the entire A domain. Examination of the resulting phenotypes allowed the assignment of the A domain region near the central activation domain, as the portion of the protein res ponsible for the specific repression of XylR activity in the absence o f m-xylene.