Heterologous and in Vitro Reconstitution of Fuscanodin, a Lasso Peptide from Thermobifida fusca

Lasso peptides are a class of ribosomally derived natural products typified by their threaded rotaxane structure. The conversion of a linear precursor peptide into a lasso peptide structure requires two enzymatic activities: cleavage of the precursor via a cysteine protease and cyclization via isopeptide bond formation. In vitro studies of lasso peptide enzymology have been hampered by difficulties in obtaining pure, soluble enzymes. We reasoned that thermophilic bacteria would be a good source for well-behaved lasso peptide biosynthetic enzymes. The genome of the thermophilic actinobacterium Thermobifida fusca encodes for a lasso peptide with an unprecedented Trp residue at its N-terminus, a peptide we have named fuscanodin. Here we reconstitute fuscanodin biosynthesis in vitro with purified components, establishing a minimal fuscanodin synthetase. These experiments have allowed us to probe the kinetics of lasso peptide biosynthesis for the first time, and we report initial rates of fuscanodin biosynthesis. The fuscanodin biosynthetic enzymes are insensitive to substrate concentration and operate in a near single-turnover regime in vitro. While lasso peptides are often touted for their stability to both chaotropic and thermal challenges, fuscanodin is found to undergo a conformational change consistent with lasso peptide unthreading in organic solvents at room temperature.