Discovery and Biosynthesis of Streptosactin, a Sactipeptide with an Alternative Topology Encoded by Commensal Bacteria in the Human Microbiome

Abstract

Mammalian microbiomes encode thousands of biosynthetic gene clusters (BGCs) and represent a new frontier in natural product research. We recently found an abundance of quorum sensing-regulated BGCs in mammalian microbiome streptococci that code for ribosomally synthesized and post-translationally modified peptides (RiPPs) and contain one or more radical S-adenosylmethionine (RaS) enzymes, a versatile superfamily known to catalyze some of the most unusual reactions in biology. In the current work, we target a widespread group of streptococcal RiPP BGCs and elucidate both the reaction carried out by its encoded RaS enzyme and identify its peptide natural product, which we name streptosactin. Streptosactin is the first sactipeptide identified from Streptococcus spp.; it contains two sequential four amino acid sactionine macrocycles, an unusual topology for this compound family. Bioactivity assays reveal potent but narrow-spectrum activity against the producing strain and its closest relatives that carry the same BGC, suggesting streptosactin may be a long-suspected fratricidal agent of Streptococcus thermophilus. Our results highlight mammalian streptococci as a rich source of unusual enzymatic chemistries and bioactive natural products.

Quorum Sensing in Streptococcus mutans Regulates Production of Tryglysin, a Novel RaS-RiPP Antimicrobial Compound

“…11). A final alternative that needs to be investigated is whether tryglysins also represent fratricidal agents (34,63). The options above are not mutually exclusive, and it is possible that tryglysins serve multiple functions.…”

Current Advancements in Sactipeptide Natural Products

“…Mammalian microbiomes encode thousands of biosynthetic gene clusters (BGCs). Among them, some encode ribosomally synthesized and post-translationally modified peptides (RiPPs) which are regulated by quorum sensing (Bushin et al, 2020 ) ( Figure 6A ). Bushin et al mined out streptosactin in mammalian microbiome streptococci, which is the newestly characterized member in the growing list of sactipeptides to date.…”

“…GggB was heterologously expressed and reconstituted under anaerobic conditions, subsequently in vitro reaction with precursor peptide GggA prepared by solid-phase peptide synthesis. GggB exhibites the same appearance that initial generation of the substrate Cα radical is a conservative process catalyzed by sactisynthase, but its real mechanism remains to be disclosed to date (Bushin et al, 2020 ).…”

“…Bushin and coworkers produced putative mature product with heterologous co-expression of the precursor peptide and modification enzyme genes in Escherichia coli and compared its chromatographic and HRMS/MS properties with the natural streptosactin isolated from the fermentation broth. Comparing with authentic standard, it indicated that streptosactin is a 14mer peptide with two S-Cα bonds between Cys15 and Ser28, Cys31 and Gly34 (Bushin et al, 2020 ). Despite recent structural studies of sactipeptides, the exact mechanism of thioether cross-links (sactionine) formation remains to be uncovered.…”

“…FIGURE 6 | rSAM enzyme-modified RiPPs in streptococci(Bushin et al, 2020). (A) Biosynthesis gene cluster of streptosactin encoded by a precursor peptide (black), a rSAM enzyme (red), a protease (green), and regulators (brown).…”