Recognition site of Escherichia coli B restriction enzyme on ϕXsB1 and simian virus 40 DNAs: An interrupted sequence

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Abstract
Methyl groups placed on ϕXsB1 replicative form DNA by the Escherichia coli B modification enzyme are located in the overlap between fragments Mbo II-3 and Alu I-2, a 61-base-pair DNA segment. Mutations that led to loss of susceptibility to restriction by E. coli B occurred within this segment at three positions spanning 14 nucleotides. A sequence difference between ϕXsB1 and ϕXam3cs70, a ϕX174 strain not restricted by E. coli B, occurs at one of these positions. The site on simian virus 40 DNA methylated by the modification enzyme is located in the 115-base-pair overlap between fragments Hae III-I and Alu I-G. The sequences of these segments of ϕXsB1 and simian virus 40 DNA and two regions of phage f1 DNA recognized by the E. coli B restriction enzyme [Ravetch, J. V., Horiuchi, K. & Zinder, N. D. (1978) Proc. Natl. Acad. Sci. USA 75, 2266-2270] contain a homology of nine bases in the configuration: 5′-T-G-A... 8N... T-G-C-T... 9N... T-N-N-T-3′. The sequence 5′-T-G-A... 8N... T-G-C-T-3′ may constitute the restriction enzyme recognition site since it does not occur in ϕXam3cs70 DNA and occurs only once in simian virus 40 DNA, and since all observed mutations leading to loss of the site occur at one of the bases specified by this sequence. Analysis of the sequence of ϕXam3cs70 showed that if no other residues are recognized, all seven of these bases are essential for recognition and the interval between the two groups of specified bases must be precisely eight.