Ravetch J V, Horiuchi K, Zinder N D
Proc Natl Acad Sci U S A. 1978 May;75(5):2266-70. doi: 10.1073/pnas.75.5.2266.
The nucleotide sequence of the recognition site for the restriction-modification enzyme of Escherichia coli B (SB site) has been 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 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. Our data are also consistent with modification occurring by methylation of two adenine residues per SB site: one on the adenine of the trimer 5'TGA3' and the other on the complementary strand on the adenine complementary to the first thymine of the tetramer 5'TGCT3'. All nine 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.
已确定大肠杆菌B（SB位点）限制修饰酶识别位点的核苷酸序列。识别位点是一个15个核苷酸的序列，由三聚体5'TGA3'组成，后面跟着一个可变序列的8个核苷酸结构域，该结构域又接着四聚体5'TGCT3'。该序列没有2倍旋转对称性。恒定核苷酸结构域中的单碱基变化导致对限制和修饰的敏感性丧失。我们的数据也与每个SB位点通过两个腺嘌呤残基甲基化进行修饰一致：一个在三聚体5'TGA3'的腺嘌呤上，另一个在与四聚体5'TGCT3'的第一个胸腺嘧啶互补的腺嘌呤的互补链上。噬菌体f1的SB1位点的所有九个独立分离的自发突变体都是由G到T的颠换引起的。SB2位点的突变是由各种单碱基变化引起的。
