SAMase gene of bacteriophage T3 is responsible for overcoming host restriction.

Deletion and point mutants of T3 have been isolated and used to show that the early region of T3 DNA is organized in the same way as that of T7 DNA. Homologous early RNAs and proteins of the two phages have been identified by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. Both phages have five early mRNA's, numbered 0.3, 0.7, 1,1.1 and 1.3 from left to right, although no T3 protein that corresponds to the 1.1 protein of T7 has yet been identified. In general, corresponding early RNAs and proteins of the two phages migrate differently on gels, indicating that they differ in molecular weight and/or conformation. In both T7 and T3, gene 0.3 is responsible for overcoming the DNA restriction system of the host, gene 0.7 specifies a protein kinase, gene 1 specifies a phage-specific RNA polymerase, and gene 1.3 specifies a polynucleotide ligase. The 0.3 protein of T3 is responsible for the S-adenosylmethionine cleaving activity (SAMase) induced after T3 (but not T7) infection. However, cleaving of S-adenosylmethionine does not appear to be the primary mechanism by which T3 overcomes host restriction, since at least one mutant of T3 has lost the SAMase activity without losing the ability to overcome host restriction.