Two single-strand DNA initiation signals located in the oriV region of plasmid RSF1010.

Two single-strand initiation signals (ssi) are found in the oriV region of broad-host-range plasmid RSF1010, using a plaque assay system with a mutant M13 phage which lacks the greater part of the complementary DNA strand origin (oric). These two signals, designated ssiA and ssiB, have RSF1010-specific properties, because they require one or more RSF1010-specific factors provided in trans. The functional activity of ssiA is higher than that of ssiB. The two signals are located on separate DNA strands, so that the DNA chain elongations initiated from them in the opposite directions may pass each other. It is conceivable that these signals, ssiA and ssiB, direct DNA priming functions at the initiation stage in vegetative DNA replication of RSF1010.