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Effector-Stimulated Single Molecule Protein-DNA Interactions of a Quorum-Sensing System in Sinorhizobium meliloti

Frank Wilco Bartels ^*, Matthew McIntosh , Alexander Fuhrmann ^*, Christoph Metzendorf , Patrik Plattner , Norbert Sewald , Dario Anselmetti ^*, Robert Ros ^* and Anke Becker 

^* Experimental Biophysics and Applied Nanoscience, Department of Physics, Institute for Genome Research and Systems Biology, Center for Biotechnology, and Organic and Bioorganic Chemistry, Department of Chemistry, Bielefeld University, 33615 Bielefeld, Germany

Correspondence: Address reprint requests to Robert Ros, Tel.: 49-(0)521-106-5388; Fax: 49-(0)521-106-2959; E-mail: rros{at}physik.uni-bielefeld.de.

Intercellular communication by means of small signal molecules coordinates gene expression among bacteria. This population density-dependent regulation is known as quorum sensing. The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti Rm1021 possesses the Sin quorum sensing system based on N-acyl homoserine lactones (AHL) as signal molecules. Here, we demonstrate that the LuxR-type regulator ExpR binds specifically to a target sequence in the sinRI locus in the presence of different AHLs with acyl side chains from 8 to 20 carbons. Dynamic force spectroscopy based on the atomic force microscope provided detailed information about the molecular mechanism of binding upon activation by six different AHLs. These single molecule experiments revealed that the mean lifetime of the bound protein-DNA complex varies depending on the specific effector molecule. The small differences between individual AHLs also had a pronounced influence on the structure of protein-DNA interaction: The reaction length of dissociation varied from 2.6 to 5.8 Å. In addition, dynamic force spectroscopy experiments indicate that N-heptanoyl-DL-homoserine lactone binds to ExpR but is not able to stimulate protein-DNA interaction.

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