Iterative acylation on mature lasso peptides by widespread acetyltransferases.

The biosynthesis of ribosomally synthesized and posttranslationally modified peptides (RiPPs) leverages iterative catalysis to enhance structural and biological diversity. Traditionally, iterative enzymes install posttranslational modifications on linear peptides, rather than mature RiPPs with intricate three-dimensional structures, which require complex changes in substrate binding. Here we present a prolific class of GCN5-related N-acetyltransferases (GNATs) that iteratively and consecutively acylate two Lys residues within the loop and ring motifs of lasso peptides, diverging from the typical iterative modification of linear peptides. Utilizing high-resolution cryogenic-electron microscopy and enzymatic reconstitution, we define the lasso peptide-binding pocket of IatT and pinpoint key residues that distinguish its two distinct acetylation steps. Structure-based engineering of IatT's acetyl-recognition site expands the cavity to accommodate longer-chain acyl groups, enabling the creation of lipolasso peptides, a class of ribosomal lipopeptide. This engineering strategy can be applied to any RiPP biosynthetic gene cluster encoding GNAT, facilitating the efficient diversification of ribosomal lipopeptides.