The radical SAM enzyme AlbA catalyzes thioether bond formation in subtilosin A

The unusual crosslinks between cysteine residues and the peptide backbone in the antibiotic peptide subtilosin A are formed by a new member of the radical SAM enzyme superfamily that contains two functionally linked [4Fe-4S] clusters. Subtilosin A is a 35-residue, ribosomally synthesized bacteriocin encoded by the sbo - alb operon of Bacillus subtilis . It is composed of a head-to-tail circular peptide backbone that is additionally restrained by three unusual thioether bonds between three cysteines and the α-carbon of one threonine and two phenylalanines, respectively. In this study, we demonstrate that these bonds are synthesized by the radical S -adenosylmethionine enzyme AlbA, which is encoded by the sbo - alb operon and comprises two [4Fe-4S] clusters. One [4Fe-4S] cluster is coordinated by the prototypical CXXXCXXC motif and is responsible for the observed S -adenosylmethionine cleavage reaction, whereas the second [4Fe-4S] cluster is required for the generation of all three thioether linkages. On the basis of the obtained results, we propose a new radical mechanism for thioether bond formation. In addition, we show that AlbA-directed substrate transformation is leader-peptide dependent, suggesting that thioether bond formation is the first step during subtilosin A maturation.