Deoxyribonucleic acid modifications and restriction endonuclease production in Neisseria gonorrhoeae

Modification of gonococcal deoxyribonucleic acid (DNA) was investigated, and the relationship with endonuclease production was explored. Both chromosomal and plasmid DNA from different gonococcal strains, irrespective of their plasmid content, was poorly cleaved by the restriction endonucleases HaeII, HaeIII, SacII, and BamHI. The fragment pattern of the Tn3 segment present on the 7.2-kilobase gonococcal resistance plasmid, when compared to its known DNA sequence, allowed us to conclude that the HaeIII and BamHI resistance was due to modification of these sites. A comparison of the fragment pattern of the resistance plasmid, when isolated from Escherichia coli or Neisseria gonorrhoeae, revealed that the resistance of HaeII must also be due to modification of its recognition sequence. Isoschizomers of HaeII and HaeIII can be found in isolates of N. gonorrhoeae (NgoI and NgoII, respectively). A new restriction endonuclease in gonococci, NgoIII, with a specificity similar to SacII, is reported here. High-pressure liquid chromatography of gonococcal DNA showed the presence of 5-methylcytosine. It is suggested that the methylation of cytosine residues in the HaeII (NgoI), HaeIII (NgoII), and SacII (NgoIII) recognition sites is the basis for the resistance of gonococcal DNA to cleavage by these enzymes. This methylation may be part of a host restriction modification system. In two out of five gonococcal strains the sequence -GATC- was modified. One strain unable to modify this sequence was a spontaneous mutant of a strain carrying such a modifying function.