The bacterial transposon Tn7 utilizes four Tn7-encoded proteins, TnsA, TnsB, TnsC and TnsD, to make insertions at a speci®c site termed attTn7. This target is selected by the binding of TnsD to attTn7 in a sequence-speci®c manner, followed by the binding of TnsC and activation of the transposase. We show that TnsD binding to attTn7 induces a distortion at the 5¢ end of the binding site and TnsC contacts the region of attTn7 distorted by TnsD. Previous work has shown that a target site containing triplex DNA, instead of TnsD±attTn7, can recruit TnsABC and effect sitespeci®c insertion of Tn7. We propose that the DNA distortion imposed by TnsD on attTn7, like the altered DNA structure via triplex formation, serves as a signal to recruit TnsC. We also show that TnsD primarily contacts the major groove of DNA, whereas TnsC is a minor groove binding protein. The footprint of the TnsC±TnsD±attTn7 nucleoprotein complex includes and extends beyond the Tn7 insertion site, where TnsC forms a platform to receive and activate the transposase to carry out recombination.

We have found that the bacterial transposon Tn7 can recognize and preferentially insert adjacent to triple-helical nucleic acid structures. Both synthetic intermolecular triplexes, formed through the pairing of a short triplex-forming oligonucleotide on a plasmid DNA, and naturally occurring mirror repeat sequences known to form intramolecular triplexes or H-form DNA are preferential targets for Tn7 insertion in vitro. This target site selectivity depends upon the recognition of the triplex region by a Tn7-encoded ATP-using protein, TnsC, which controls Tn7 target site selection: the interaction of TnsC with the triplex region results in recruitment and activation of the Tn7 transposase. Recognition of a nucleic acid structural motif provides both new information into the factors that influence Tn7's target site selection and broadens its targeting capabilities.

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