AI-2 is an autoinducer made by many bacteria. LsrB binds AI-2 in the periplasm, and Tsr is the L-serine chemoreceptor. We show that AI-2 strongly attracts Escherichia coli. Both LsrB and Tsr are necessary for sensing AI-2, but AI-2 uptake is not, suggesting that LsrB and Tsr interact directly in the periplasm.Many functions in bacteria are regulated by population density, including formation of biofilms and production of virulence factors (5). Assessment of population density, known as quorum sensing, relies on the ability of cells to determine the concentrations of compounds known as autoinducers (AIs). As the cell density increases, an AI accumulates to a concentration that triggers a quorum-sensing response. Autoinducers activate some genes and repress others. Induced genes typically include those responsible for production of the autoinducer, resulting in a positive feedback loop. Cell densities required to accumulate enough AI for good induction are 10 8 per ml or higher.AIs are of two basic types: species specific and general (22). Species-specific AI-1s are acyl homoserine lactones in Gramnegative bacteria and modified peptides in Gram-positive bacteria. Full induction of bioluminescence in the marine bacterium Vibrio harveyi, which colonizes dead organic matter, requires both a specific AI-1 inducer and a general autoinducer, called AI-2 (6). AI-2 is derived from spontaneous cyclization of 4,5-dihydroxy-2,3-pentanedione (DPD). DPD is made from S-ribosylhomocysteine by the enzyme LuxS (25). S-Ribosylhomocysteine is an intermediate in the breakdown of S-adenosylhomocysteine, the product remaining after methyl group donation by S-adenosylmethionine.AI-2 is produced by a wide range of Gram-negative and Gram-positive bacteria and exists in multiple forms that are in equilibrium with each other (5). The form that is active in V. harveyi is (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran borate (S-THMF borate) (8), which binds to the periplasmic protein LuxP. In S. enterica serovar Typhimurium, a boron-free isomer of AI-2 [(2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetra-hydrofuran (R-THMF)] binds to the periplasmic LsrB protein (21). LsrB is the recognition component of an ABC transporter for AI-2. LsrACD are the membrane-bound components of the ABC transporter for AI-2. AI-2 is generated by the enzyme LuxS, and the YdgG (TqsA) protein has been implicated in AI-2 export from the cytoplasm (14).AI-2 is a known chemoattractant for Escherichia coli (4, 10), but the receptor(s) involved in AI-2 sensing has not been identified. This work was initiated to characterize the proteins involved in AI-2 recognition by E. coli strain CV1, which is equivalent to the standard wild-type chemotaxis strain RP437. The strains used in this study are shown in Table 1.The microplug (Plug) assay (9), a modified plug-in-pond assay, provides a qualitative but highly visual representation of chemotaxis. Cells containing the green fluorescent protein (GFP)-encoding plasmid pCM18 were grown overnight at 32°C in tryptone broth (TB) (20) co...