Proteins in this family all perform a KxxxW cyclisation of a peptide via a radical SAM mechanism.
Ibrahim M, Guillot A, Wessner F, Algaron F, Besset C, Courtin P, Gardan R, Monnet V
Control of the transcription of a short gene encoding a cyclic peptide in Streptococcus thermophilus: a new quorum-sensing system?
▸ Abstract
Gram-positive bacteria secrete a variety of peptides that are often subjected to posttranslational modifications and that are either antimicrobials or pheromones involved in bacterial communication. Our objective was to identify peptides secreted by Streptococcus thermophilus, a nonpathogenic bacterium widely used in dairy technology in association with other bacteria, and to understand their potential roles 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 found with the three strains while others were specific to 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 9 amino acids and lost 2 Da in a posttranslational 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 on both the medium composition and the 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 led us to deduce that the transcription of the Pep1357C-encoding gene is controlled by a new quorum-sensing system.
Structure and biosynthesis of a macrocyclic peptide containing an unprecedented lysine-to-tryptophan crosslink
▸ Abstract
Streptococcal bacteria use peptide signals as a means of intraspecies communication. These peptides can contain unusual post-translational modifications, providing opportunities for expanding our understanding of nature's chemical and biosynthetic repertoires. Here, we have combined tools from natural products discovery and mechanistic enzymology to elucidate the structure and biosynthesis of streptide, a streptococcal macrocyclic peptide. We show that streptide bears an unprecedented post-translational modification involving a covalent linkage between two unactivated carbons within the side chains of lysine and tryptophan. The biosynthesis of streptide was addressed by genetic and biochemical studies. The former implicated a new SPASM-domain-containing radical SAM enzyme StrB, while the latter revealed that StrB contains two [4Fe-4S] clusters and installs the unusual lysine-to-tryptophan crosslink in a single step. By intramolecularly stitching together the side chains of lysine and tryptophan, StrB provides a new route for biosynthesizing macrocyclic peptides.
