NATURAL VARIATION OF THE NGOII RESTRICTION-MODIFICATION SYSTEM OF NEISSERIA-GONORRHOEAE

The NgoII restriction-modification (R-M) system of Neisseria gonorrhoe ae recognizes the sequence 5'-GGCC-3'. This system is encoded by two s eparate genes, dcmB for the methyltransferase (MTase) and dcrB for the restriction endonuclease (ENase). Three strains that vary in their Ng oII phenotype were examined. Strain Pgh3-2 produced detectable levels of both enzymes, strain F62 lacked detectable levels of the dcrB gene product, and strain WR302 failed to produce either gene product. Strai ns that lacked either enzyme activity still possessed the genes that e ncode them. Transcriptional fusions of dcrB in strains F62 and Pgh3-2 indicate that this gene is transcribed at nearly identical levels in e ach strain. The DNA encoding the NgoII R-M system was cloned from the three strains, and the nucleotide sequence was determined. The dcrB ge nes of WR302 and F62 possess the same frameshift mutation (base positi on 1435) which would result in a truncated protein. The WR302 dcmB was found to have a point mutation that changed Arg288 (a residue that is conserved in all prokaryotic and phage cytosine MTases sequenced to d ate) to Trp.