PHYSICAL AND GENETIC-MAP OF THE LACTOCOCCUS-LACTIS SUBSP CREMORIS MG1363 CHROMOSOME - COMPARISON WITH THAT OF LACTOCOCCUS-LACTIS SUBSP LACTIS IL-1403 REVEALS A LARGE GENOME INVERSION

A physical and genetic map of the chromosome of the Lactococcus lactis subsp. cremoris reference strain MG1363 was established. The physical map was constructed for NotI, ApaI, and SmaI enzymes by using a strat egy that combines creation of new rare restriction sites by the random -integration vector pRL1 and ordering of restriction fragments by indi rect end-labeling experiments. The MG1363 chromosome appeared to be ci rcular and 2,560 kb long. Seventy-seven chromosomal markers were locat ed on the physical map by hybridization experiments. Integration via h omologous recombination of pRC1-derived plasmids allowed a more precis e location of some lactococcal genes and determination of their orient ation on the chromosome. The MG1363 chromosome contains six rRNA opero ns; five are clustered within 15% of the chromosome and transcribed in the same direction. Comparison of the L. lactis subsp. cremoris MG136 3 physical map with those of the two L. lactis subsp. lactis strains I L1403 and DL11 revealed a high degree of restriction polymorphism. At the genetic organization level, despite an overall conservation of gen e organization, strain MG1363 presents a large inversion of half of th e genome in the region containing the rRNA operons.