Lsr-mediated transport and processing of Al-2 in Salmonella typhimurium

The LuxS-dependent autoinducer Al-2 is proposed to function in interspecies cell-cell communication in bacteria. In Salmonella typhimurium, Al-2 is produced and released during exponential growth and is subsequently imported into the bacteria via the Lsr (luxS regulated) ATP binding cassette (ABC) transporter. Al-2 induces transcription of the lsrACDBFGE operon, the first four genes of which encode the Lsr transport apparatus. In this report, we identify and characterize LsrK, a new protein that is required for the regulation of the lsr operon and the Al-2 uptake process. LsrK is a kinase that phosphorylates Al-2 upon entry into the cell. Our data indicate that phosphorylatlon of Al-2 results in its sequestration in the cytoplasm. We suggest that phospho-Al-2 is the inducer responsible for inactivation of LsrR, the represser of the lsr operon. We also show that two previously uncharacterized members of the lsr operon, LsrF and LsrG, are necessary for the further processing of phospho-Al-2. Transport and processing of Al-2 could be required for removing the quorum-sensing signal, conveying the signal to an internal detector and/or scavenging boron.