| Sumario: | In the presence of m-xylene, the protein XyIR encoded by the TOL plasmid of Pseudomonas putida, activates the σ54-dependent promoter Pu. Early activation stages involve the release of the intramolecular repression caused by the signal reception N-terminal (A domain) of XyIR, on the central module of the protein. A genetic approach has been followed to locate the specific segment within A domain of XyIR that is directly 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 transcriptional fusion of Pu to lacZ was transformed with a collection of plasmids encoding equivalent truncated varieties of XyIR, 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 responsible for the specific repression of XyIR activity in the absence of m-xylene.
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