Selection of plasmid molecules for conjugative transfer and replacement strand synthesis in the donor

Plasmid selection and strand replacement synthesis in donor cells during conjugative transfer was examined by a procedure involving electroporation of test plasmid DNA, containing a base pair mismatch, into donor cells prior to mating. Multiple copies of the plasmid were transferred from a donor cell that allowed vegetative replication of the plasmid. Under conditions non-permissive for vegetative replication, there were further rounds of transfer after a lag period. Strand replacement in the donor did not depend solely on the initiation mechanism for vegetative replication, indicating a conjugation-specific mechanism was also available. The lag period between first and second rounds of transfer argues against the transfer of multiple copies into recipients by the spooling of copies generated on a master molecule by rolling-circle replication.