Abstract
The N-sulfonated monocyclic β-lactam ring characteristic of the monobactams confers resistance to zinc metallo-β-lactamases and affords the most effective class to combat carbapenem-resistant enterobacteria (CRE). Here we report unprecedented nonribosomal peptide synthetase activities, wherein an assembled tripeptide is N-sulfonated in trans before direct synthesis of the β-lactam ring in a noncanonical, cysteine-containing thioesterase domain. This means of azetidinone synthesis is distinct from the three others known in nature.