This family is somewhat disparate. Like members of the IS91 family, members of this family show important differences with most of the other IS families.Members ( Fig) vary between 1136 bp and 1558 bp, with most clustered in the 1450 bp size range. All family members have no or very small (<7 bp) IRs and do not generally create direct target repeats (IS492 might generate 5 bp repeats, (22)). Little overall similarity can be detected between the ends. A single long, relatively well conserved, reading frame is present and shows some clusters of conservation within the N- and C-terminal portions. These elements share a highly conserved tetrad motif with reverse transcriptase (197) whose significance is at present unknown. None of the signatures typical of rolling circle replication functions found in the Tpases of IS91-related elements, are detectable in the predicted protein. The inclusion of two additional elements, the Streptomyces coelicolor minicircle, IS117 (194) and a Moraxella bovis (and Moraxella lacunata) DNA inversion system determining pilus synthesis (Piv) is based on significant similarities in their putative recombinase proteins (205). The Tpases of several members have been identified in various sequenced bacterial genomes although the ends of most of these elements have not been defined. This family of recombinases is not related to the site-specific recombinases of the l integrase or resolvase/invertase families. Preliminary modelling studies raise the interesting possibility that this site-specific invertase/transposase is a phosphoryltransferase, similar to the DDE Tpases, rather than an enzyme whose activity necessitates the formation of phosphotyrosine or phosphoserine enzyme substrate intermediates (474).
Analysis of IS1383 (275) suggested that the family is divided into two distinct subgroups and that these should be classified as separate families. Alignment of the potential Tpases of presently available members defines two clearly distinguishable groups, IS1111 and IS110. The IS1111 group includes IS1111, IS1328, IS1492, IS1533, IS5780 and IS43211 (Fig), while the IS110 group contains the remaining members. The two groups have not been separated here into individual families since they are clearly linked by BLAST analysis.
Little information is available concerning the mechanism of transposition of these elements. However, the Streptomyces coelicolor IS117 was initially demonstrated in a circular form and integrates at a frequency two orders of magnitude higher than when cloned as a "linear" copy (195). In recent studies with IS492 (363) and IS900 (J. McFadden, pers. comm.) DNA fragments carrying abutted IS ends have been detected by PCR analysis in vivo and the structures confirmed by nucleotide sequencing. Their appearance is dependent on an intact Tpase gene and their nucleotide sequence is consistent with the formation of a circular form of the element. In the case of IS492 the size of the species carrying the abutted ends is consistent with a circular form of the element and its sequence shows that the ends are separated by 5 bp matching one of the DR copies present in the original insertion. The nucleotide sequence of 5 IS492 insertion sites showed that they were remarkably similar with a conserved 5 bp flanking sequence: 5'CTTGT3'. This could be extended on one side to 5'CTTGTTA3'. IS492 circle formation also occurs in E. coli. Using this system it was shown that excision of the element is Tpase-dependent with reconstitution of the donor molecule. Excision required between 5 and 10bp of DNA flanking the insertion. Moreover, the junction of IS492, like several other ISs, appears to form a strong promoter in E. coli (363). For both IS117 (195) and IS900 (J. McFadden, pers. comm.) the target sequences exhibit some similarities to the circle junction suggesting that insertion may occur by a site-specific recombination.
Finally, although little data is available concerning enzymatic activities of the putative Tpases of this family of elements, IS900 Tpase has been detected by immunological methods in the Mycobacterium paratuberculosis host (473) and IS492 Tpase has recently been purified and appears to exhibit DNA cleavage activity specific for the ends of the element (Perkins-Balding and Glasgow, pers. comm.).
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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