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J Bacteriol, February 1998, p. 600-604, Vol. 180, No. 3
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Modulation of the Function of the Signal Receptor Domain of XylR, a Member of a Family of Prokaryotic Enhancer-Like Positive Regulators

Rafael Salto,* Asunción Delgado,dagger Carmen Michán,Dagger Silvia Marqués,§ and Juan L. Ramos

Department of Biochemistry, Molecular and Cellular Biology of Plants, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, E-18008 Granada, Spain

Received 12 June 1997/Accepted 20 November 1997

The XylR protein controls expression from the Pseudomonas putida TOL plasmid upper pathway operon promoter (Pu) in response to aromatic effectors. XylR-dependent stimulation of transcription from a Pu::lacZ fusion shows different induction kinetics with different effectors. With toluene, activation followed a hyperbolic curve with an apparent K of 0.95 mM and a maximum beta -galactosidase activity of 2,550 Miller units. With o-nitrotoluene, in contrast, activation followed a sigmoidal curve with an apparent K of 0.55 mM and a Hill coefficient of 2.65. m-Nitrotoluene kept the XylR regulator in an inactive transcriptional form. Therefore, upon binding of an effector, the substituent on the aromatic ring leads to productive or unproductive XylR forms. The different transcriptional states of the XylR regulator are substantiated by XylR mutants. XylRE172K is a mutant regulator that is able to stimulate transcription from the Pu promoter in the presence of m-nitrotoluene; however, its response to m-aminotoluene was negligible, in contrast with the wild-type regulator. These results illustrate the importance of the electrostatic interactions in effector recognition and in the stabilization of productive and unproductive forms by the regulator upon aromatic binding. XylRD135N and XylRD135Q are mutant regulators that are able to stimulate transcription from Pu in the absence of effectors, whereas substitution of Glu for Asp135 in XylRD135E resulted in a mutant whose ability to recognize effectors was severely impaired. Therefore, the conformation of mutant XylRD135Q as well as XylRD135N seemed to mimic that of the wild-type regulator when effector binding occurred, whereas mutant XylRD135E seemed to be blocked in a conformation similar to that of wild-type XylR and XylRE172K upon binding to an inhibitor molecule such as m-nitrotoluene or m-aminotoluene.


* Corresponding author. Present address: Department of Biochemistry and Molecular Biology, School of Pharmacy, University of Granada, E-18071 Granada, Spain. Phone: 34-58-246363. Fax: 34-58-248960. E-mail: rsalto{at}goliat.ugr.es.

dagger Present address: Department of Microbiology and Molecular Genetics, University of California, Los Angeles, CA 90095-1489.

Dagger Present address: Department of Biochemistry, School of Veterinary Sciences, University of Córdoba, 14071 Córdoba, Spain.

§ Present address: Department of Genetics, School of Biology, University of Seville, 41012 Seville, Spain.




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Copyright © 1998 by the American Society for Microbiology. All rights reserved.