The LuxS-dependent autoinducer Al-2 controls the expression of an ABC transporter that functions in Al-2 uptake in Salmonella typhimurium

In a process called quorum sensing, bacteria communicate with one another using secreted chemical signalling molecules termed autoinducers. A novel autoinducer called Al-2, originally discovered in the quorum-sensing bacterium Vibrio harveyi, is made by many species of Gram-negative and Grampositive bacteria. In every case, production of Al-2 is dependent on the LuxS autoinducer synthase. The genes regulated by Al-2 in most of these luxS-containing species of bacteria are not known. Here, we describe the identification and characterization of Al-2-regulated genes in Salmonella typhimurium. We find that LuxS and Al-2 regulate the expression of a previously unidentified operon encoding an ATP binding cassette (ABC)-type transporter. We have named this operon the Isr (luxS regulated) operon. The Lsr transporter has homology to the ribose transporter of Esctoerichia coli and S. typhimurium. A gene encoding a DNA-binding protein that is located adjacent to the Lsr transporter structural operon is required to link Al-2 detection to operon expression. This gene, which we have named IsrR, encodes a protein that represses Isr operon expression in the absence of Al-2. Mutations in the Isr operon render S. typhimurium unable to eliminate Al-2 from the extracellular environment, suggesting that the role of the Lsr apparatus is to transport Al-2 into the cells. It is intriguing that an operon regulated by Al-2 encodes functions resembling the ribose transporter, given recent findings that Al-2 is derived from the ribosyl moiety of S-ribosylhomocysteine.