Characterisation of plasmids coding for the restriction endonuclease EcoRI.

The properties of two plasmids coding for the CcoRI restriction and modification enzymes are described. Both plasmids are non auto-transferring (NTP) but can be mobilised by transfer factors. Strains carrying NTP13 produce colicin E1 and the EcoRI enzymes. This plasmid has a molecular weight of 6 X 10(6) daltons and is present as approximately 12 copies per chromosome. The second plasmid, NTP14, was detected after mobilisation of the EcoRI plasmid with the R factor RI-19. NTP14 codes for ampicillin resistance, synthesis of the EcoRI enzymes and colicin E1. The molecular weight of NTP14 is 10.7 X 10(6) daltons and there are about 14 copies per chromosome. DNA-DNA reassociation experiments were performed to determine the interrelationships of NTP13, NTP14, ColE1 and the R factor R1-19. NTP13 and NTP14 continue to replicate when cellular protein synthesis is inhibited by the addition of chloramphenicol.