Bacteriophage T3 induces an enzyme activity which hydrolyzes S‐adenosylmethionine. This S‐adenosylmethionine hydrolase is interesting, not only because of its unique activity, but also because the protein has to overcome host restriction [F. W. Studier and N. R. Movva (1976) J. Virol. 19, 136–145]. S‐Adenosylmethionine hydrolase was purified to homogeneity using affinity chromatography on S‐adenosylhomocysteine‐Sepharose. The enzyme occurs in two forms, A and B. Form A consists of the viral peptide chain only; its native and subunit molecular weight is 17000. Form B contains, in addition, a host subunit with a molecular weight of 49000. The host subunit does not modify S‐adenosylmethionine cleavage in vitro and no apparent relationship to the host‐restriction system could be detected.
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