Site- and strand-specific nicking in vitro at oriT by the traY-traI endonuclease of plasmid R100.

We developed an in vitro system to reproduce a site- and strand-specific nicking at the oriT region of plasmid R100. The nicking reaction was dependent on the purified TraY protein and on the lysate, which was prepared from cells overproducing the TraI protein. This supports the idea that the protein products of two genes, traY and traI, constitute an endonuclease that introduces a specific nick in vivo in the oriT region of the conjugative plasmids related to R100. The products were the "complex" DNA molecules with a protein covalently linked with the 5'-end of the nick. The nick was introduced in the strand, which is supposed to be transferred to recipient cells during conjugation, and was located at the site 59 base pairs upstream of the TraY protein binding site, sbyA.