Representative structures of the class II lasso peptides microcin J25 (Protein Data Bank code 1Q71) (27) (a), streptomonomicin (Protein Data Bank code 2MW3) (21) (b), and astexin-3 (Protein Data Bank code 2M8F) (8) (c) are shown. The ring is shown in yellow, the tail in blue, the ring-forming Asp/Glu in red, and the plug side chains in green. d, biosynthetic gene clusters of the three shown lasso peptides (8, 18, 21). e, scheme of the suggested mechanism of lasso peptide biosynthesis (14).