Are horizontal transfers involved in the evolution of the Streptococcus thermophilus exopolysaccharide synthesis loci?

A 32.5 kb variable locus of the Streptococcus thermophilus CNRZ368 chromoso me, the eps locus, contains 25 ORF and seven insertion sequences (IS). The putative products of 17 ORF are related to proteins involved in the synthes is of polysaccharides in various bacteria. The two distal regions and a sma ll central region of the eps locus are constant and present in all or almos t all of the S. thermophilus strains tested. The other regions are variable and present in only some S. thermophilus strains tested, particularly in t he closely related strains CNRZ368 and A054. A 13.6 kb variable region of t he eps locus of S. thermophilus CNRZ368 contains two ORF that are almost id entical to epsL and orfY of the eps locus of Lactococcus lactis NIZOB40 and seven IS belonging to four different families, ISS1, IS981, 1S1193 and IS1 194. Five of these sequences were probably acquired by horizontal transfer from L. lactis (Bourgoin, F., et al., 1996. Gene 178, 15-23). Three probes of this 13.6 kb region hybridized with the DNA of several L, lactis strains tested. A specific probe for another sequence within the S, thermophilus e ps locus, epsF, hybridized with the DNA of one of the L. lactis strains tes ted. Sequence comparisons also suggest that five ORF of the eps locus have a mosaic structure and probably result from recombinations between sequence s that are 10 to 50% divergent. The chimeric structure of the eps locus sug gests a very complex evolution. This evolution probably involves both the a cquisition of the 13.6 kb region from L. lactis by horizontal transfer and exchanges within the S. thermophilus species. (C) 1999 Elsevier Science B.V . All rights reserved.