Bacterial Transposable Elements and IS-Excision Enhancer (IEE)

Abstract

Insertion sequence (IS) elements are the simplest transposable elements (or mobile genetic elements) and are widely distributed in bacteria. The transposition and proliferation of IS elements induce not only insertional gene inactivation and modification of gene expression but also a wide range of genomic rearrangements, such as deletions, inversions, and duplications. IS-mediated bacterial genome diversification has been extensively studied in enterohemorrhagic Escherichia coli (EHEC). Excision of IS elements occurs frequently in O157, the major serogroup of EHEC isolated from humans, and IS-excision enhancer (IEE) promotes IS excision from the O157 genome in a transposase-dependent manner. IEE promotes the excision of IS elements belonging to several IS families, and various types of genomic deletions are also generated via IEE-promoted IS excision in O157. In addition, IEE has been found in specific lineages of enterotoxigenic E. coli (ETEC) strains isolated from swine, in which the iee genes are located on integrative elements that are similar to SpLE1 of EHEC O157. iee-positive ETEC lineages also contain multiple copies of IS elements at genomic locations that exhibit significant variations between strains, as observed in O157. These data and the phylogeny of IEE homologs found in a broad range of bacteria suggest that IEE has coevolved with IS elements and plays pivotal roles in bacterial genome evolution by inducing IS removal and genomic deletion.