The cps locus of Streptococcus suis serotype 2: genetic determinant for the synthesis of sialic acid.

The capsule of S. suis serotype 2 is composed of glucose, galatose, N-acetylglucosamine, rhamnose and sialic acid. Recently, we described a major part of the cps2 locus of S. suis serotype 2. Based on sequence homology genes encoding potential glucosyl-, galactosyl-, N-acetylglucosaminyl- and rhamnosyltransferase activities could be identified. However, we did not find genes involved in the synthesis of sialic acid. Here, we describe the cloning and characterization of a remaining part of the cps2 locus. Based on the establish sequence 11 potential genes, designated orf2L, orf2M, orf2N, cps2O to cps2T, orf2U and orf2V were identified. A gene homologous to genes involved in the polymerization of the repeating oligosaccharide unit (cps2O) as well as genes involved in the synthesis of sialic acid (cps2P to cps2T) were identified. Moreover, hybridizing experiments showed that the genes involved in the sialic acid synthesis are present in S. suis serotype 1, 2, 14, 27 and 1/2. The orf2M and orf2N regions showed similarity to proteins involved in the polysaccharide biosynthesis of other Gram-positive bacteria. However, these regions seemed to be truncated or were non-functional as the result of frame-shift or point mutations. At its 3;-end the cps2 locus contained two insertional elements (orf2U and orf2V), both of which seemed to be non-functional.