Comparison of the operons coding for the Lsr-like receptors in the luxS -negative α-proteobacteria R. sphaeroides 2.4.1 and S. meliloti 1021 with the lsr cluster of AI-2 producers S. typhimurium and E. coli K12. The lsr-like operons are located on the highly variable chromosome II of R. sphaeroides 2.4.1 (accession number NC_007494) and on the pSymB megaplasmid of S. meliloti 1021 (NC_003078) respectively. The transporter core is composed of a periplasmic AI-2 binding protein (LsrB), two hydrophobic proteins which for an homodimeric transmembrane channel (LsrC and LsrD) and a hydrophilic ATP-binding protein (LsrA). The expression of the lsr operon is controlled by repressor protein LsrR and AI-2 kinase LsrK which is responsible for the production of phospho-AI-2, the lsr operon inducer. In S. typhimurium (NC_003197), the LsrF and LsrG proteins are involved in modifying phospho-AI-2 . For comparison ribose and xylose ABC transporters of E. coli K12 (NC_000913) are shown below. Proteins with the same function are coded with the same color. As point of reference, locus tags for selected genes are marked inside the arrows.