The origin of greater-than-unit-length plasmids generated during bacterial conjugation

In Gram-negative bacteria, the general mechanism of conjugal plasmid transfer, which is probably similar for many different groups of plasmids, involves the transfer of a single plasmid DNA strand with 5' to 3' polarity. Transfer is initiated by nicking of the duplex DNA at a particular site, i.e. the origin of transfer (oriT). We constructed plasmids containing two directly repeated copies of oriT, derived from the broad-host-range plasmid R1162 and flanking the lac operator. The number of lacO copies in the plasmid after transfer could be determined from the colour of transconjugant colonies on medium containing X-Gal. When the oriTs were mutated to prevent initiation and termination of transfer at the same oriT, almost all of the transconjugant cells contained greater-than-unit-length plasmids with two copies of lacO and three copies of oriT. We show that these molecules were generated by an intramolecular, conjugation-dependent mechanism unlikely to depend solely on a pre-existing population of circular or linear multimers in donor cells. We propose that the greater-than-unit-length molecules were instead generated by a rolling-circle mechanism of DNA replication.