Location and nucleotide sequence of the transfer origin of the broad host range plasmid RK2.

The origin of plasmid DNA transfer, oriT, has been localized on RK2, a conjugative drug-resistance plasmid of the IncP group with a very broad host range in gram-negative bacteria. The transfer origin is contained in a 760-base-pair Hae II restriction fragment that maps in the same region as the single-strand nick made by the RK2 relaxation complex. The functional oriT was subcloned as a 112-base-pair Hpa II fragment, and the DNA sequence of this region was determined. The dominant structural feature of the oriT sequence is a 19-base-pair inverted repeat, with 15 of the 19 bases able to form pairs in a hairpin structure. This inverted repeat may be the recognition site for the relaxation complex proteins, which nick the plasmid DNA molecule and initiate the transfer process.