Korch C, Hagblom P
Eur J Biochem. 1986 Dec 15;161(3):519-24. doi: 10.1111/j.1432-1033.1986.tb10473.x.
The 4207-bp cryptic plasmid (pJD1) of Neisseria gonorrhoeae has 5-methylcytosine bases present at several positions in the DNA sequence. Fortuitously, these modified bases lie in the recognition sequences of many restriction enzymes. This feature makes the cryptic plasmid a model system for assaying the effect of these modified cytosines on the activities of the following restriction endonucleases and their isoschizomers: R X AvaII, R X BamHI, R X BglI, R X Fnu4HI, R X HaeII, R X HaeIII, R X HhaI, R X HpaII, R X KpnI, R X MspI, R X NaeI, R X NarI, R X NciI, R X NgoI, R X NgoII, and R X Sau96I. Of particular interest was the finding that methylation of one of the external cytosines of the palindrome 5'-CCGG-3' prevented its cleavage by R X MspI, but not by R X HpaII as had been suggested by Walder et al. [J. Biol. Chem. (1983) 258, 1235-1241].
淋病奈瑟菌的4207碱基对隐蔽质粒（pJD1）在DNA序列的多个位置存在5 - 甲基胞嘧啶碱基。幸运的是，这些修饰碱基位于许多限制酶的识别序列中。这一特性使该隐蔽质粒成为一个模型系统，用于分析这些修饰的胞嘧啶对以下限制内切酶及其同裂酶活性的影响：RX AvaII、RX BamHI、RX BglI、RX Fnu4HI、RX HaeII、RX HaeIII、RX HhaI、RX HpaII、RX KpnI、RX MspI、RX NaeI、RX NarI、RX NciI、RX NgoI、RX NgoII和RX Sau96I。特别令人感兴趣的是，发现回文序列5'-CCGG-3'外部胞嘧啶之一的甲基化阻止了RX MspI对其的切割，但正如Walder等人所指出的[《生物化学杂志》（1983年）258卷，1235 - 1241页]，并未阻止RX HpaII的切割。
