Distribution of the SsuDAT1I restriction-modification system among different serotypes of Streptococcus suis

The SsuDAT1I restriction-modification (R-M) system, which contains two meth yltransferases and two restriction endonucleases with recognition sequence 5 ' -GATC-3 ', was first found in a field isolate of Streptococcus suis ser otype 2. Isoschizomers of the R-M system were found in the same locus betwe en purH and purD in a field isolate of serotype 1/2 and the reference strai ns of serotypes 3, 7, 23, and 26 among 29 strains of different serotypes ex amined in this study. The R-M gene sequences in serotypes 1/2, 3, 7, and 23 were very similar to those of SsuDAT1I, whereas those in serotype 26 were less similar. These results indicate intraspecies recombination among them and genetic divergence through their evolution.