Polymorphism of eps loci involved in exopolysaccharide synthesis of Streptococcus thermophilus

The nucleotide sequence of two ORFs and five copies of three insertion sequ ences (IS) types belonging to the eps locus of Streptococcus thermophilus C NRZ368 involved in exopolysaccharide synthesis are almost identical to ISs and ORFs sequences from Lactococcus lactis. Furthermore, sequence compariso n of eps loci of three S. thermophilus strains and hybridization of probes isolated from the S. thermophilus CNRZ368 eps locus with DNA of 17 S. therm ophilus strains revealed a very high polymorphism. A small constant region is detected in all the strains whereas a large region is extremely variable . The 17 tested strains could be arranged in six groups according to the pr esence or absence of a hybridization signal with the different probes teste d. A phylogenetic analysis indicated that the ropy NST2280 strain and the n on-ropy IP6757 strain are very closely related strains but possess differen t eps loci. The eps locus of IP6757 could result from sequence replacement in an IP6757 ancestor. Sequence comparison of different eps loci of S. ther mophilus suggests that the high polymorphism of these loci largely results from sequence replacement following horizontal transfers.