A new mobile genetic element inLactobacillus delbrueckii subsp.bulgaricus

A new IS element (ISL3) was discovered inLactobacillus delbrueckii subsp.bulgaricus during the characterization of the linkage relationships between the two genes important for milk fermentation,-galactosidase (lacZ) and the cell-wall associated protease (prtP). ISL3 is a 1494 by element, flanked by 38 by imperfect inverted repeats, and generates an 8 by target duplication upon insertion. It contains one open reading frame, encoding a potential polypeptide of 434 amino acids, which shows significant homology (34% identity) to the transposase of theLeuconostoc mesenteroides element IS1165. Molecular analysis of spontaneouslacZ mutants revealed some strains that had sustained deletions of 7 to 30 kb in size, centered on and eliminating the copy of ISL3 next tolacZ. Other deletion endpoints were identified as located immediately adjacent to ISL3. Furthermore, genetic translocations that had occurred via transposition of ISL3 were observed fortuitously in cultures screened for deletion mutants. ISL3 can be found in one to several copies in various strains ofL. delbrueckii. However, it was not present in other dairy lactic acid bacteria tested.