The reactivity of an unusual amidase may explain colibactin's DNA cross-linking activity

Certain commensal and pathogenic bacteria produce colibactin, a small molecule genotoxin that causes interstrand cross-links in host cell DNA. Though colibactin has been found to alkylate DNA, the molecular basis for cross-link formation is unclear. Here, we report that the colibactin biosynthetic enzyme ClbL is an amide bond-forming enzyme that links aminoketone and β-keto thioester substrates in vitro and in vivo. The substrate specificity of ClbL strongly supports a role for this enzyme in terminating the colibactin NRPS-PKS assembly line. This transformation would incorporate two electrophilic cyclopropane warheads into the final natural product scaffold. Overall, this work provides a biosynthetic explanation for colibactin9s DNA cross-linking activity and paves the way for further study of its chemical structure.