1. Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544-1014, USA
2. Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1014, USA
3. Laboratoire de Spectrometrie de Masse Bio-Organique, Ecole de Chimie, Polymeres et Materiaux, 25 Rue Becquerel, 67087 Strasbourg, France

Correspondence to: Frederick M. Hughson¹ Correspondence and requests for materials should be addressed to F.M.H. (e-mail: Email: hughson@princeton.edu). Atomic coordinates for the LuxP-AI-2 complex have been deposited in the Protein Data Bank under the accession code 1JX6.

Cell–cell communication in bacteria is accomplished through the exchange of extracellular signalling molecules called autoinducers. This process, termed quorum sensing, allows bacterial populations to coordinate gene expression. Community cooperation probably enhances the effectiveness of processes such as bioluminescence, virulence factor expression, antibiotic production and biofilm development^{1, 2, 3, 4}. Unlike other autoinducers, which are specific to a particular species of bacteria, a recently discovered autoinducer (AI-2)⁵ is produced by a large number of bacterial species. AI-2 has been proposed to serve as a 'universal' signal for inter-species communication^{1, 2, 6, 7}. The chemical identity of AI-2 has, however, proved elusive. Here we present the crystal structure of an AI-2 sensor protein, LuxP, in a complex with autoinducer. The bound ligand is a furanosyl borate diester that bears no resemblance to previously characterized autoinducers. Our findings suggest that addition of naturally occurring borate to an AI-2 precursor generates active AI-2. Furthermore, they indicate a potential biological role for boron, an element required by a number of organisms but for unknown reasons.