An Unusual Ring Pattern in the Rosβ Lanthipeptide of the Two-Component Lantibiotic Roseocin

Two-component lantibiotics comprise two post-translationally modified peptides that synergistically exert antimicrobial activity. Most known two-component lantibiotics are made up of a structurally conserved α-lanthipeptide that binds to the peptidoglycan precursor lipid II and a β-lanthipeptide that interacts with the α-peptide-lipid II complex to form pores in the membranes of susceptible bacteria. A few two-component lantibiotics, including roseocin, do not appear to follow this general scheme and act by currently unresolved mechanisms. An important first step in studying these lanthipeptides is the determination of their chemical structure. Roseocin's β-peptide (Rosβ) is formed by the RosM lanthipeptide synthetase from the RosA1 precursor peptide. RosM carries out nine dehydrations of Ser and Thr residues to the corresponding dehydroamino acids followed by six Michael-type additions of the thiols of Cys residues to a subset of the dehydroamino acids forming six thioether cross-links. Sequence alignment with structurally characterized lanthipeptides does not allow prediction of the ring pattern of Rosβ. In this study, Rosβ was produced in <i>Escherichia coli</i> and its ring pattern was established by multidimensional NMR spectroscopy. The stereochemistry of the lanthionine and methyllanthionine residues was determined by Marfey's analysis with authentic standards. Rosβ is shown to have a unique ring pattern among previously characterized lanthipeptides.