SAMase gene of bacteriophage T3 is responsible for overcoming host restriction

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Abstract
Deletion and point mutants of [phage] T3 were isolated and used to show that the early region of T3 DNA is organized in the same way as that of [phage] T7 DNA. Homologous early RNAs and proteins of the 2 phages were identified by sodium dodecy sulfate polyacrylamide gel electrophoresis. Both phages had 5 early mRNAs, 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 was identified. In general, corresponding early RNAs and proteins of the 2 phages migrate differently on gels, indicating that they differ in MW and/or conformation. In T7 and T3, gene 0.3 is responsible for overcoming the DNA restriction system of the host [Escherichia coli], 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. Cleaving of S-adenosylmethionine is probably not the primary mechanism by which T3 overcomes host restriction, since at least 1 mutant of T3 lost the SAMase activity without losing the ability to overcome host restriction.

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