Active protection by bacteriophages T3 and T7 against E. coli B- and K-specific restriction of their DNA.

The bacteriophages T3 and T7 are not modified and restricted by E. coli strains with different host specificity (E. coli B, K, O) in vivo. The phages code for a gene product with the ability to overcome classical restriction (ocr): ocr- mutants are subject to modification and restriction via DNA methylation vs cleavage. The T3 genome possesses recognition sites for the restriction endonuclease R.EcoB which, unless the DNA is B-specifically modified, trigger 5-7 DNA cleavages. The ocr gene function of T3 and T7 is located within the gene 0.3 region of these phages and is not identical with the sam (SAMase) function of T3. The mechanism of ocr protection remains unclear, while it is certain that this protection by the gene 0.3 protein is exerted in the infected cell and not through "over-all" modification in the preceding growth cycle of the phage.