During bacterial conjugation, an enzyme called relaxase introduces a singlestrand break (nick) into the plasmid DNA at the spot (oriT) where transfer begins. The atomic structure of relaxase has been determined by X-ray crystallography, and the active part of the enzyme has been identified. It contains a pair of tyrosine amino acids that form covalent bonds with DNA during the DNA breakage and transfer process. Small molecules called phosphonates inhibit the relaxase, thereby blocking conjugation. Surprisingly, the phosphonates selectively kill plasmid-containing cells. Because relaxase is not required for cell survival, the phosphonate probably creates a toxic event rather than simply removing an essential cellular component.153 One possibility is that a phosphonate-relaxase-DNA complex forms that triggers a cascade of lethal reactive oxygen species, as is seen with quinolones that trap gyrase on DNA.154 Work on this type of process may eventually lead to the control of horizontal transfer.