Restriction of bacteriophage T3 and T7 ocr+ strains by the type II restriction endonuclease EcoRV.

When E. coli cells carrying the plasmid pLG13 (coding for the newly discovered type II restriction endonuclease EcoRV) are infected with phage T3 or T7, only T7 is able to replicate normally. T3 wild-type as well as its ocr- mutants are subject to DNA restriction in vivo and in vitro. The EcoRV enzyme cuts T3 DNA at 5 sites. T7 and its ocr- mutants have no EcoRV sites in their DNA. In contrast to the anti-restriction activity of the T3 and T7 ocr+ gene function against type I and III restriction enzymes, the ocr+ protein is unable to inactivate the type II restriction endonuclease EcoRV.