Quorum-Sensing Regulation of the Production of Blp Bacteriocins in Streptococcus thermophilus

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Abstract
The blp gene cluster identified in the genome sequences of Streptococcus thermophilus ( blp St ) LMG18311, CNRZ1066, and LMD-9 displays all the characteristics of a class II bacteriocin locus. In the present study, we showed that the blp St locus is only fully functional in strain LMD-9 and regulates the production of antimicrobial peptides that inhibit strains LMG18311 and CNRZ1066. The blp St cluster of LMD-9 contains 23 genes that are transcriptionally organized in six operons: blpABC St (peptide transporter genes and pheromone gene); blpRH St (two-component regulatory system genes); blpD St - orf1, blpU St - orf3 , and blpE-F St (bacteriocin precursors and immunity genes); and blpG-X St (unknown function). All the operons, except the regulatory unit blpRH St , were shown to be coregulated at the transcriptional level by a quorum-sensing mechanism involving the mature S. thermophilus pheromone BlpC* (BlpC* St ), which was extracellularly detected as two active forms (30 and 19 amino acids). These operons are differentially transcribed depending on growth phase and pheromone concentration. They all contain a motif with two imperfect direct repeats in their mapped promoter regions that could serve as binding sites of the response regulator BlpR St . Through the construction of deletion mutants, the blp St locus of strain LMD-9 was shown to encode all the essential functions associated with bacteriocin production, quorum-sensing regulation, and immunity.