Nucleotide sequence of the recognition site for the restriction-modification enzyme of Escherichia coli B.

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Abstract
The nucleotide sequence of the recognition site for the restriction-modification enzyme of E. coli B (SB site) was determined. The recognition site is a 15-nucleotide sequence consisting of the trimer 5''TGA3'', followed by an 8-nucleotide domain of variable sequence, which in turn is followed by the tetramer 5''TGCT3''. The sequence has no 2-fold rotational symmetry. Single base changes in the constant nucleotide domains result in the loss of sensitivity to both restriction and modification. These data are also consistent with modification occurring by methylation of 2 adenine residues/SB site; one on the adenine of the trimer 5''TGA3'' and the other on the complementary strand on the adenine complementary to the 1st thymine of the tetramer 5''TGCT3''. All 9 independently isolated spontaneous mutants at the SB1 site of bacteriophage f1 are caused by a G-to-T transversion. Mutations at the SB2 site are caused by various single base changes.