TqqB Is a Sactisynthase RiPP Maturase with Minimal Leader Sequence Dependence

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a growing family of peptide natural products with diverse structures and functions where the genetically encoded peptide undergoes one or more modifications by maturases that are encoded by the same biosynthetic gene cluster. RiPP maturases carry out modifications that are generally challenging, making understanding the function and specificity of these enzymes of interest for the development of biotechnology applications. TqqB from Streptococcus suis has been suggested to form a C–O-linked 6-membered ring in the 37-residue TqqA peptide between Thr30 and the α-carbon of Gln31 in the biosynthesis of threoglucin peptides. Herein, we show that contrary to the published structure, TqqB forms a C–S-linked macrocycle between Cys25 and the α-carbon of Gln31. The resulting peptide is inert to modification with iodoacetamide. Consistent with the assignment, TqqB also cross-links Thr30Ala variant of TqqA, which lacks a side chain hydroxyl, forming a Cys25–Gln31 cross-link. Using a Gln31Ala TqqA that had been isotopically enriched with deuterium at Ala 31, we show that the α-carbon deuterium at position 31 is lost upon cross-link formation. NMR analysis of 13C-labeled Gln31Ala variant revealed a 13C chemical shift for the attachment site that is consistent with a thioether and not an ether. Finally, TqqB can form the analogous cross-link in a TqqA peptide spanning residues 22–37, revealing significant tolerance for truncated substrates.