ISAs1 family

Members of this family have lengths of 1207 (ISPg1) to 1326 bp (IS1358) and generally carry terminal IRs of between 14 and 22 bp (Fig)(although several of the IRs remain to be defined). A single orf of between 294 and 376 amino acids occupies almost the entire length and shows between 26 and 50% identity. There are several conserved D and E residues but no clear C1 domain can be detected. The putative Tpase of IS1358 has been visualised using a phage T7 promoter-driven derivative (458) and that of ISAs1 has been detected in Escherichia coli minicells (175).

The family also includes the "H-repeats" which form part of several so-called Rhs (Rearrangement hotspot) elements (see (198), (530)). Escherichia coli K-12 contains 5 large repetitions of this type (RhsA to RhsE, scattered around the chromosome with lengths of between 3.7 and 9.6 kb). They represent nearly 1 % of the chromosome and provide homology for RecA-dependent rearrangements. These elements are present in many but not all wild-type isolates of E. coli. The most prominent Rhs component is a giant core orf whose features are suggestive of a cell wall surface ligand-binding protein. Each Rhs element also contains another repeated sequence, the H-rpt element (Hinc-repeat) displaying features of typical insertion sequences, although no transposition activity has yet been detected. For the sake of clarity, the H-rpt DNA sequences B (RhsB), C1to C3 (RhsC), E (RhsE), and min.5 (531) as well as H-rptF (530) have been renamed ISEc1 to ISEc7, respectively (Table 1)(310). It is interesting to note that other members of the family (IS1358, ISAs1 and ISSe1 ) have been found associated with cell surface component genes in their respective hosts. Little is known about the transposition properties of this family of elements. However, recent experiments with the Vibrio cholera element IS1358 have demonstrated that insertion generates 10 bp DRs and that, in Escherichia coli, it undergoes simple insertion into a target plasmid, pOX38 (125).

Figure . The ISAs1 family. A) Dendrogram based on transposase alignments. B) Terminal inverted repeats.

Mahillon J.  and  Chandler M.  (1998) Microbiology and Molecular Biology Reviews.  62 : 725-774
Chandler, M. and Mahillon, J.(2002) Insertion Sequences Revisited Mobile DNA II Edited by N.L., Craig et al. ASM Press 305-366
with permission of American Society of  Microbiology the 10-26-01.

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Last modification : December 19 2001