Mapping of the nick site on conjugative plasmid pVT745 by interrupted mating.

Conjugal transfer of plasmid DNA initiates and terminates at a specific non-coding site called the origin of transfer (oriT). Previous analysis of conjugative plasmid pVT745 from Aggregatibacter actinomycetemcomitans suggested that oriT was located adjacent to the operon responsible for initiation of ssDNA transfer. The location of oriT was confirmed by assaying both subclones of the region as well as a pVT745 deletion derivative for mobilization. The precise DNA nick site (nic) and polarity of DNA transfer were identified by use of interrupted mating assays, a technique originally used for the mapping of bacterial chromosomes. Nucleotide sequence analysis revealed that the pVT745-specific nick region was similar to the consensus nick sequence of the IncP family albeit the actual cleavage site differed. Functionality of nic was confirmed by point mutations.