Location of the relaxation complex nick site within the minimal origin of transfer region of RK2.

Transfer of plasmid DNA during bacterial conjugation begins at a specific site: the origin of transfer (oriT). The oriT region of the broad host range plasmid RK2 is located on a 250 bp fragment. Deletions involving either end of this region reduce transfer function, indicating that an extended sequence is required for optimal oriT activity. The single-strand nick induced by the RK2 DNA-protein relaxation complex is located adjacent to the 19 bp inverted repeat within the minimal oriT sequence. These results provide strong evidence that the plasmid relaxation event induced in vitro represents the nicking reaction that initiates DNA transfer at oriT during conjugation.