Evidence for Lateral Transfer of the Suilysin Gene Region of Streptococcus suis

Suilysin is a cholesterol-binding cytolysin encoded by sly in Streptococcus suis. DNA sequence determination of the sly locus in a strain lacking sly revealed the presence of another gene, designated orf102, in the place of sly. No transposable element or long-repeat sequence was found in the close vicinity. Except for six strains whose corresponding loci have been rearranged, all of the remaining 62 strains examined had either sly or orf102 at the same locus and their flanking regions were conserved. The genetic organizations having either sly or orf102 were found in the strains whose 16S rRNA sequences were identical. These results suggest that S. suis acquired sly or orf102 from a foreign source and that these genes subsequently spread among S. suis strains by homologous recombination.