HOST SPECIFICITY OF DNA PRODUCED BY Escherichia coli , XI. In vitro MODIFICATION OF PHAGE FD REPLICATIVE FORM

An enzymatic activity, having the properties expected of a B-specific host-controlled modification enzyme, has been purified from an extract of Escherichia coli strain B. This activity renders the unmodified replicative form of phage fd resistant to B-specific restriction and is only present in strains carrying intact genes for type B modification. In phosphate buffer, the enzyme acts optimally at pH 6 and is dependent upon a single cofactor, S-adenosylmethionine.