CHARACTERIZATION OF A MOSAIC ISS1 ELEMENT AND EVIDENCE FOR THE RECENTHORIZONTAL TRANSFER OF 2 DIFFERENT TYPES OF ISS1 BETWEEN STREPTOCOCCUS-THERMOPHILUS AND LACTOCOCCUS-LACTIS

A 12-kb region of the Streptococcus thermophilus CNRZ368 chromosome wa s found to contain two copies of IS981 (one complete and one truncated ) and three copies of ISS1 (two complete, ISS1SA and ISS1SC, and one t runcated, Delta ISS1/SB). Comparison of the nucleotide sequences of th ese ISS1 elements with those of previously identified iso-ISS1 element s from Lactococcus lactis and the Enterococcus genus indicated that th e ISS1 group is divided into three distinct subgroups which we have na med alpha, beta and gamma. Nucleotide sequences of elements belonging to the same subgroup share more than 97% identity whereas sequences of elements from different groups share only 75-85% identity. Sequence a nalysis of ISS1SA and Delta ISS1SB showed that they are members of the a group. We found that ISS1SC from S. thermophilus CNRZ368, an ISS1 f rom L. lactis IL964 and IS946 from L. lactis TEK1 resulted from recomb inations between alpha and beta elements. In addition, ISS1W from L. l actis Wg2 resulted from a recombination event between a gamma element and an ISS1 belonging to an unidentified subgroup. ISS1 sequences belo nging to the alpha and beta subgroups were found in both S. thermophil us and L. lactis and gamma sequences were found in both the Enterococc us genus and L. lactis. The quasi-identity of some ISS1 elements in S. thermophilus and L. lactis and the distribution of alpha and beta ele ments suggest that horizontal transfer of ISS1 elements recently took place from L. lactis to S. thermophilus, two lactic acid bacteria used in the manufacture of cheeses. Since the presence of IS981 in S. ther mophilus CNRZ368 also probably resulted from a horizontal transfer fro m L. lactis [Guedon et al. (1995) Mol. Microbiol. 16, 69-78], the 12-k b region bearing IS981 and ISS1 elements could be due to the integrati on of a lactococcal DNA fragment into the chromosome.