CHROMOSOME MAPPING OF SOME LABORATORY LACTOCOCCUS-LACTIS STRAINS

A combined physical and genetic map of the chromosome of Lactococcus l actis subsp lactis IL 1403 was established. The restriction map was co nstructed for the NotI, ApaI and SmaI enzymes. The restriction fragmen ts were ordered by using the random-integrative plasmid pRL1 and by in direct end-labeling experiments. The IL 1403 chromosome was found to b e circular and 2 420 kb in size. Twenty-seven chromosomal markers were mapped on the chromosome by hybridization experiments with gene probe s of Lactococcus lactis and of various other bacteria. Integration of pRC1-derived plasmids via homologous recombination allowed a more prec ise location of some lactococcal genes as well as the determination of their orientation on the chromosome. Recurrent sequences such as IS e lements and rRNA gene (rm) clusters were also mapped. At least 7 copie s of the IS1076 sequences were present and were located in 50% of the chromosome. In contrast, no copy of ISS1RS could be detected. Six ribo somal operons were found on the IL 1403 chromosome; 5 were located wit hin 16% of the chromosome and transcribed in the same direction. A com parison of the physical maps of L lactis subsp lactis IL 1403 and DL11 strains showed that these 2 strains are closely related and that the variable regions are located mainly near the rm gene clusters. In cont rast, despite major restriction pattern dissimilarities between L lact is IL 1403 and MG 1363, the overall genetic organization of the genome seems to be conserved between these 2 strains.