Conjugal mobilization of plasmid DNA: termination frequency at the origin of transfer of plasmid R1162.

Conjugal transfer of plasmid R1162 is initiated and terminated at a 38-bp origin of transfer (oriT). Plasmids containing two directly repeated copies of oriT were used to determine the actual frequency of termination at this site. This frequency was calculated both for oriTnic, a mutated origin that cannot act as the initiation site of transfer, and for an unmutated oriT where transfer had been initiated. In both cases, the termination frequency decreased as the distance between the initiation and termination sites became greater and was significantly less than one for plasmids the size of R1162. A substantial proportion of recipient cells received more than one plasmid copy during transfer. Our results indicate that termination is inefficient but that this is partly compensated for by the transmission of multiple plasmid copies.