A multiple mutant of bacteriophage T4, which had been proved to contain cytosine-substituted DNA, was grown in a suppressor-containing or non-containing (
sup°) strain of
Escherichia coli B or K12, and the progeny phage obtained was grown on test strains carrying various restriction systems. As a result, the growth of this phage was found to be subject to the
rK,
rB, and
rP1 restriction systems. Two T4 strains, named UNF-r1 and UNF-r
+1, sensitive to the
rK,
rB, and
rP1 restriction systems, were newly derived by the genetic cross and mutagenesis. The former had the
unf-39 mutation instead of the
alc-8 mutation, the latter had the
rII
+ region and the
unf-39 mutation, besides the
amE51 (gene 56; dCTPase),
denA (endonuclease II), and
denB (endonuclease IV) mutations. It was demonstrated that UNF-r
+1 DNA possessed a
SalI recognition site in the
rII-
denB region which was deleted in UNF-r1 DNA.
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