FIG. 1.

(A) Lsr ABC transporter system (LuxS-regulated genes) from S. enterica serovar Typhimurium and EHEC serotype O157:H7. All of the lsr genes from Salmonella are present in EHEC, with the exception of lsrE. The lsrACDBFGE genes are transcribed in an operon, and divergently transcribed from this operon are lsrR, encoding the SorC-like transcription factor that represses expression of the LsrABC transporter, and lsrK, whose product phosphorylates AI-2 upon its entry into the bacterial cell (79, 80). (B) LsrB exhibits homology with the ribose-binding periplasmic protein RbsB, which together with RbsC, RbsA, and RbsK is responsible for transport of ribose into the cell. (C) LsrB is the receptor for AI-2 in Salmonella and E. coli (47). Upon binding to LsrB, AI-2 is transported through the LsrR ABC transporter, which closely resembles the ribose ABC transporter. Once inside the cell AI-2 is phosphorylated by LsrK. Phospho-AI-2 is thought to interact with LsrR to relieve the repression of the lsr operon.