The pyridine core of the thiocillins has long been postulated to arise from a late-stage tail-to-tail condensation of two dehydroalanines. Genetic disruption of tc/M, a proposed "Diels-Alderase", allowed isolation of acyclic precursors to this pyridine ring. The isolated products possess the full cohort of post-translational modifications that are normally displayed by the thiocillins, including dehydrobutyrines, thiazoles, C-terminal decarboxylation, and the two previously unconfirmed dehydroalanines. Additionally, leader peptides have undergone extensive N-terminal degradation and the remaining leader peptide residues have been N-succinylated. These results identify TcIM and its homologues in other thiazolyl peptide producing strains as the enzymes responsible for the trans-annular heteroannulation at core of this class of molecules.