The control of the transcription of a short gene encoding a cyclic peptide in Streptococcus thermophilus: a new quorum sensing system?

INRA, Unité de Biochimie Bactérienne, UR477, F-78350 Jouy en Josas, France; INRA, PAPSS, F-78350 Jouy en Josas, France

^* To whom correspondence should be addressed. Email: veronique.monnet{at}jouy.inra.fr.

	Abstract

Gram-positive bacteria secrete a variety of peptides that are often subjected to post-translational modifications and which are either antimicrobials or pheromones involved in bacterial communication. Our objective was to identify peptides secreted by Streptococcus thermophilus, a non-pathogen bacterium, widely used in dairy technology in association with other bacteria and to understand their potential role in cell-cell communication. Using reverse-phase liquid chromatography, mass spectrometry and Edman sequencing, we analyzed the culture supernatants of three S. thermophilus strains (CNRZ1066, LMG18311 and LMD-9), grown in a medium containing no peptides. We identified several peptides in the culture supernatants, some of them being found with the three strains while others were specific for the LMD-9 strain. We focused our study on a new modified peptide secreted by S. thermophilus LMD-9 and designated Pep1357C. This peptide contains nine amino acids and lost 2 Da in a post-translational modification, most probably a dehydrogenation, leading to a linkage between the Lys2 and Trp6 residues. Production of Pep1357C and transcription of its encoding gene depend both on medium composition and growth phase. Furthermore, we demonstrated that transcription of the gene coding for Pep1357C is drastically decreased in mutants inactivated for the synthesis of a short hydrophobic peptide, a transcriptional regulator or the oligopeptide transport system. Taken together, our results lead us to deduce that the transcription of the Pep1357C encoding gene is controlled by a new quorum-sensing system.