Polymorphism of eps loci involved in exopolysaccharide synthesis of Streptococcus thermophilus

Overview & Synopsis

The nucleotide sequence of two ORFs and five copies of three insertion sequences (IS) types belonging to the eps locus of Streptococcus thermophilus CNRZ368 involved in exopolysaccharide synthesis are almost identical to ISs and ORFs sequences from Lactococcus lactis. Furthermore, sequence comparison 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. thermophilus 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 presence or absence of a hybridization signal with the different probes tested. A phylogenetic analysis indicated that the ropy NST2280 strain and the non-ropy IP6757 strain are very closely related strains but possess different eps loci. The eps locus of IP6757 could result from sequence replacement in an IP6757 ancestor. Sequence comparison of different eps loci of S. thermophilus suggests that the high polymorphism of these loci largely results from sequence replacement following horizontal transfers.