SALMONELLA-TYPHIMURIUM ENCODES AN SDIA HOMOLOG, A PUTATIVE QUORUM SENSOR OF THE LUXR FAMILY, THAT REGULATES GENES ON THE VIRULENCE PLASMID

Quorum sensing is a phenomenon in which bacteria sense and respond to their own population density by releasing and sensing pheromones, In g ram-negative bacteria, quorum sensing is often performed by the LuxR f amily of transcriptional regulators, which affect phenotypes as divers e as conjugation, bioluminescence, and virulence gene expression, The gene encoding one LuxR family member, named sdiA (suppressor of cell d ivision inhibition), is present in the Escherichia coli genome, In thi s report, we have cloned the Salmonella typhimurium homolog of SdiA an d performed a systematic screen for sdiA-regulated genes, A 4.4-kb fra gment encoding the S. typhimurium sdiA gene was sequenced and found to encode the 3' end of YecC (homologous to amino acid transporters of t he ABC family), all of SdiA and SirA (Salmonella invasion regulator), and the 5' end of UvrC. This gene organization is conserved between E, coli and S, typhimurium. We determined that the S. typhimurium sdiA g ene was able to weakly complement the E, coli sdiA gene for activation of ftsQAZ at promoter 2 and for suppression of filamentation caused b y an ftsZ(Ts) allele, To better understand the function of sdiA in S, typhimurium, we screened 10,000 random lacZY transcriptional fusions ( MudJ transposon mutations) for regulation by sdiA, Ten positively regu lated fusions were isolated, Seven of the fusions were within an appar ent operon containing ORF8, ORF9, rck (resistance to complement killin g), and ORF11 of the S, typhimurium virulence plasmid, The three ORFs have now been named srgA, srgB, and srgC (for sdiA-regulated gene), re spectively, The DNA sequence adjacent to the remaining three fusions s hared no similarity with previously described genes.