Genomic loci of the Porphyromonas gingivalis insertion element IS1126
Dong H, Chen T, Dewhirst FE, Fleischmann RD, Fraser CM, Duncan MJ
The Porphyromonas gingivalis genome contains multiple copies of insertion element IS1126. When chromosomal DNA digests of different strains were probed with IS1126, between 25 and 35 hybridizing fragments per genome were detected, depending on the strain. Unrelated strains had very different restriction fragment length polymorphism (RFLP) patterns. When different laboratory copies of a specific strain were examined, the IS1126 RFLP patterns were very similar but small differences were observed, indicating that element-associated changes had occurred during laboratory passage. Within the next year, genome sequencing, assembly, and annotation for P. gingivalis W83 will be completed. Because repetitive elements complicate the assembly of randomly sequenced DNA fragments, we isolated and sequenced the flanking regions of IS1126 copies in strain W83. We also isolated and sequenced the flanking regions of IS1126 copies in strain ATCC 33277 in order to compare insertion sites in phylogenetically divergent strains. We identified 37 new sequences flanking IS1126 from strain ATCC 33277 and 30 from strain W83. The insertion element was found between genes except where it transposed into another insertion element. Examination of identifiable flanking genes or open reading frames indicated that the insertion sites were different in the two strains, except that both strains possess an insertion adjacent to the Lys-gingipain gene (J. P. Lewis and F. L. Macrina, Infect. Immun. 66:3035-3042, 1998). Most of the genes or sequences flanking IS1126 in ATCC 33277 were present in W83 but were contiguous and not insertion element associated. Thus, where genes were identified in both strains, their order was maintained, indicating that the two genomes are organized similarly, but the loci of IS1126 are different. In both strains, insertion element-associated duplicated target sites were lost from several copies of IS1126, providing evidence of homologous recombination between elements. Larger organizational differences between the genomes, such as deletions and inversions, may result from insertion element-mediated recombination events.