ISL3 family

Except for ISCx1 for which only a partial sequence is available, and IS1069 which is significantly larger, members range in size from 1186 bp (ISSg1) to 1553 bp (IS1165). They carry IRs of between 15 and 39 bp which are closely related (Fig.) and generate a direct target repeat of 8 bp. One long open reading frame is present which gives potential proteins of roughly 400-440 aa showing good alignment particularly in the C-terminal half. IS1096 harbours two orfs. The upstream orf exhibits similarities to the ISL3 family Tpases. The second, tnpR, is related to orfs from Agrobacterium rhizogenes and Rhizobium sp. plasmids. In IS1167 the single reading frame appears to be distributed between two consecutive orfs with a potential for translational coupling suggested by overlapping initiation and termination codons (ATGA). For analysis purposes, we have considered only one fused frame. The elements fall into three deeply branching groups with about 35%, 30% and 25-60% identities respectively (Fig.). Small sequences (130-340 bp) related to the IS1167 IRs have been detected in Streptococcus sanguis, S. pneumoniae, and S. agalactiae (8), (533). No obvious target sequence specificity has yet been observed although there is some suggestion that there may be a preference for AT rich regions.

Transposition of most of these elements has been demonstrated but no analysis of their transposition mechanism has yet appeared. Some evidence has been obtained suggesting that IS1411 from P. putida forms a circular species with abutted IRs separated by 5 bp (232). In one case, IS31831, derivatives of which have been used in mutagenesis, the majority of insertions were found to be simple and only rare clones carried accompanying vector DNA (492). DNA species with a size expected for an excised transposon have also been observed (493).

Figure . The ISL3 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