Abstract
On pre-infection of the host Escherichia coli B with u.v.-inactivated T3 or T7 phage able to express their early genes (like 0.3), B-specific modification of super-infecting, successfully multiplying viruses does not take place. The ocr gene function (gene 0.3) of T3 and T7 not only prevents host-specific DNA restriction but also modification, probably by inhibiting the same late step in the interaction between the restriction enzyme and DNA.