Streptococcus suis is a Gram-positive pathogen that has been identified as the cause of various diseases in pigs, other animals, and humans. We have previously shown that some strains of S. suis possessed a type II restriction-modification (R-M) system, whose genes were thought to be inserted into the genome between purH and purD from a foreign source by illegitimate recombination. In this study, we sequenced the purH-D locus of the S. suis genomes of 28 serotype reference strains. Four strains contained the R-M genes in the locus, as described before, whereas 11 strains possessed other genetic regions of seven classes. The mutually exclusive localization of the genetic regions with the atypical G+C contents indicated that these regions were also acquired by horizontal transfer. Alignment of the nucleotide sequences suggested that the foreign regions were integrated by illegitimate recombination via short stretches of nucleotide identity. The R-M genes were experimentally introduced into an S. suis strain that did not contain any foreign genes in the purH-D locus, using a suicide plasmid. Integration of the plasmid into the S. suis genome did not occur in the purH-D locus but occurred at various chromosomal loci where there were 2 to 10 bp of nucleotide identity between the chromosome and the plasmid. These results suggest that various foreign genes described here were incidentally integrated into the same locus of the S. suis genome.
(Molecular Bacteriology Section, Department of Infectious Diseases. TEL +81-29-838-7743)
Reference:
Sekizaki et al. (2005) J. Bacteriol. 187: 872-883
