DnaG-dependent priming signals can substitute for the two essential DNA initiation signals in oriV of the broad host-range plasmid RSF1010.

Broad host-range plasmid RSF1010 contains in the oriV region two DNA initiation signals, ssiA(RSF1010) and ssiB(RSF1010), which are essential for plasmid replication. Each of ssiA and ssiB could be substituted functionally by either of the two G4-type (DnaG-dependent) priming signals, the oric of bacteriophage G4 and an ssi signal from plasmid pSY343 (an R1 plasmid derivative). Functions of the chimeric oriVs of RSF1010 thus constructed were dependent on the RSF1010-specific replication proteins, RepA, RepB' and RepC. When both of ssiA and ssiB were replaced by the G4-type ssi signals, functions of the chimeric oriVs were no longer dependent on RepB' (RSF1010-specific DNA primase). The replication activities of the chimeric oriVs of RSF1010 were not influenced markedly by the type of heterologous priming signals they contained. It is conceivable that DNA replication of RSF1010 does not need the priming mechanism for lagging strand synthesis and proceeds by the strand displacement mechanism.