Conjugative DNA synthesis: R1162 and the question of rolling-circle replication.

Strand-replacement synthesis during conjugative mating has been characterized by introducing into donor cells R1162 plasmid DNA containing a base-pair mismatch. Conjugative synthesis in donors occurs in the absence of vegetative plasmid replication, but with a lag between rounds of transfer, and with most strands being initiated at the normal site within the replicative origin. These characteristics argue against the idea that multiple plasmid copies are generated for successive rounds of transfer by rolling-circle replication. However, the R1162 relaxase protein can process molecules containing multiple transfer origins in the manner expected for the conversion of single-strand multimers, generated by rolling-circle replication, to unit-length molecules. This capability appears to be the result of a secondary cleavage reaction carried out by the protein. The possibility is raised that the processing of molecules with more than one origin of transfer might be a repair mechanism directed against adventitious DNA synthesis during transfer.