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Papers In Press, published online ahead of print December 21, 2001
J. Biol. Chem, 10.1074/jbc.M110226200
Submitted on October 24, 2001
Revised on December 21, 2001
Accepted on December 21, 2001
Estacion Experimental del Zaidin, CSIC, Granada 18008
Corresponding Author: jlramos{at}eez.csic.es
The 321-residue XylS and XylS1 proteins, encoded by the pWW0 and pWW53 plasmids respectively, differ in only 5 residues at positions 4, 53, 90, 137 and 153. As a result, the effector profile of XylS is wider than that of XylS1, and XylS mediates higher levels of transcription from its cognate-regulatable Pm promoter than does XylS1. We generated a series of XylS-pWW0 mutants and found that single mutants Asp137->Glu and His153->Asn exhibited an activation pattern different to that of the wild-type regulator. In the double-mutant XylSAsp137Glu,His153Asn the effector profile for benzoates was similar to that of XylS1. This suggests that these two residues are crucial for effector recognition and regulator activation to stimulate transcription. XylS-dependent transcription from its cognate promoter is mediated by RNA-polymerase with sigma 32 or sigma 38, whereas XylS1 uses RNA-polymerase with sigma 32 or sigma 70. We also found that point mutations at positions 137 and 153 of XylS led RNA polymerase to mediate transcription from Pm with sigma 70 rather than with sigma 38, as demonstrated by primer extension analysis in a sigma 70-thermosensitive background proficient and deficient in sigma 38. This suggests that a positive transcriptional regulator can choose the RNA polymerase complex that mediates transcription from a given promoter.
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