Vitamin B3 Triggers Biosynthesis of Secondary Metabolite Dormancy Signals in Streptococcus suis

Human-associated streptococci have not been viewed as productive sources of natural products. Against expectation, bioinformatic searches recently revealed a large collection of diverse biosynthetic gene clusters coding for ribosomally synthesized and post-translationally modified peptides (RiPPs) in streptococcal genomes. The most abundant of these, the tqq gene cluster, is specific to Streptococcus suis, a burdensome agricultural pathogen and zoonotic agent. Herein, we used high-throughput elicitor screening to identify both small molecule elicitors and products of the tqq cluster. We show that the B-3 vitamin niacin effectively elicits the tqq cluster leading to the biosynthesis of a family of RiPP natural products, which we termed threoglucins and characterized structurally. The defining feature of threoglucins is an aliphatic ether bond giving rise to a substituted 1,3-oxazinane heterocycle in the peptide backbone. Isolation of 22 congeners of threoglucins facilitated structure activity relationship studies, demonstrating the requirement for the oxazinane substructure and a Trp-Tyr C-terminal dyad for biological activity, namely antibiotic persistence and allolysis at low and high doses, respectively. Potential therapeutic applications of threoglucins are discussed.