In‐vivo‐modified gonococcal plasmid pJD1

Abstract
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 ·AvaII, R ·BamHI, R ·BglI, R ·Fnu4HI, R ·HaeII, R ·HaeIII, R ·HpaI, R ·HpaII, R ·KpnI, R ·MspI, R ·NaeI, R ·NarI, R ·NciI, R ·NgoI, R ·NgoII, and R ·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 ·MspI, but not by R ·HpaII as had been suggested by Walder et al. [J. Biol. Chem. (1983) 258, 1235–1241].