Inhibition of the type I restriction-modification enzymes EcoB and EcoK by the gene 0.3 protein of bacteriophage T7.

The gene 0.3 protein of bacteriophage T7 is a potent inhibitor of the restriction-modification enzymes EcoB and EcoK, both in vivo and in vitro. We have analyzed the ability of purified 0.3 protein to inhibit different steps in the reactions of EcoB and EcoK with DNA. Most of our experiments were done with EcoK, but selected tests with EcoB indicate that the two enzymes are affected by 0.3 protein in the same way. Purified 0.3 protein binds tightly to free enzyme, apparently to one of the small subunits, and prevents it from binding to DNA. If EcoK is allowed to form specific recognition complexes with unmodified DNA before 0.3 protein is added, relatively low levels of 0.3 protein prevent the nuclease activity that would otherwise appear upon addition of ATP, but considerably higher levels are needed to prevent formation of filter-binding complexes or ATPase activity. This, together with other results, suggests that the binding site for 0.3 protein is protected in recognition complexes and in the early stages of the ATP-stimulated reactions, but that it becomes accessible again before cleavage of the DNA, perhaps after the translocation step. If added after the nuclease reaction is substantially over, 0.3 protein has little effect on ATPase activity, and indeed, the subunit having the binding site for 0.3 protein apparently dissociates from the enzyme-DNA complex. The methylase activity of EcoK on hemi-methylated recognition sites is strongly inhibited by 0.3 protein added at any stage of the reaction.