SEQUENCE SIMILARITIES BETWEEN THE GENES ENCODING THE S.NGOI AND HAEIIRESTRICTION MODIFICATION SYSTEMS/

The DNA sequence encoding the S.Ngol restriction/ modification (R/M) s ystem was identified from a gene bank made from Neisseria gonorrhoeae strain WR302 by identifying recombinant plasmids that induced the repo rter system in a methylase detection strain AP1200-9 (Piekarowicz et a l., 1991) and were resistant to digestion with Ngol. The DNA sequence was determined from one of these (pUCP30). M.Ngol is a protein of 315 aa with a predicted MW of 35 296 Da and R.Ngol is a protein of 350 aa with a predicted MW of 40 650 Da. The termination codon of M.Ngol over lapped the start codon of R.Ngol. The same strategy was used to clone the R/M system encoding Haell from Haemophilus aegyptius strain ATCC 1 1116. The DNA sequence from one clone representing this class (pAP704) was determined. Haell methylase is a protein of 318 aa with a predict ed MW of 35 669 Da and R.Haell contains 352 aa with a predicted MW of 40 800 Da. aa alignments between the two methylases indicated that the y were 74.3% identical and 79% similar. DNA sequence alignments reveal ed 68% identity. An aa alignment between the two restriction enzymes i ndicated that they were 60% identical and 68% similar. DNA sequence al ignments revealed 61% identity. The DNA sequences flanking these two s ystems were identified and used to determine the genomic organization of the two systems. The S.Ngol genes were found between two genes, one with high homology to GTP binding proteins of unknown function and on e with homology to genes involved in tRNA synthetase synthesis. The Ha ell R/M genes were located between two genes, mucF and mucE. The DNA s equence of the Haell R/M system was compared to the genomic DNA sequen ce of H. influenzae Rd. Although the DNA sequences flanking the Haell system were > 99% identical to contiguous DNA fragments found in the g enome of H. influenzae Rd, no homology was seen with the DNA sequences encoding the Haell R/M system, indicating that it is not found in thi s strain. Given the vast difference in the GC content of S.Ngol and Ha ell, their apparent insertion into polycistronic operons, and their di fference in codon usage when compared to the species from which they w ere isolated, the data suggest that these R/M systems originated in an organism other than Neisseria or Haemophilus.