A NATURAL LARGE CHROMOSOMAL INVERSION IN LACTOCOCCUS-LACTIS IS MEDIATED BY HOMOLOGOUS RECOMBINATION BETWEEN 2 INSERTION SEQUENCES

Comparative analysis of chromosomal macrorestriction polymorphism of t he two closely related Lactococcus lactis subsp. cremoris strains MG13 63 and NCDO763 revealed the presence of a large inversion covering hal f of the genome. To determine what kind of genetic element could be im plicated in this rearrangement, the two inversion junctions of MG1363 and NCDO763 chromosomes were cloned and characterized. Nucleotide sequ ence analysis showed the presence of one copy of the lactococcal IS905 element in each junction. Each copy of this element contained the sam e nucleotide mutation that inactivates the putative transposase. Compa rison of the sequences surrounding the insertion sequence demonstrated that the large inversion arose from a single-step homologous recombin ation event between the two defective copies of the IS905 element. The large inversion presumably conferred no selective disadvantage on str ain NCDO763 because this rearrangement did not alter the oriC-terC sym metry of the chromosome and the local genetic environment.