The members of this family are class II transposons (30). This represents a highly homogeneous group in terms of their transposition enzymes and terminal IRs. The latter are generally 38 bp long, start with the sequence GGGG and terminate internally with TAAG. Their Tpases also carry a DDE motif (524). Many are complicated in structure and include multiple antibiotic resistance genes carried by another type of transposable element, the integron (391). Although a complete survey is out of the scope of this chapter, they are included here because several are small and might qualify as insertion sequences (e.g. Tn1000, also called gd, and IS101: see 357; IS1071: 119a, 347; and ISXc4/ISXc5: 292).
Tn3 family members transpose using a replicative mechanism. This involves formation of cointegrates promoted by the transposase, TnpA, which are then resolved by site-specific recombination between the two directly repeated transposon copies. Recombination is mediated by the product of a second gene encoding a site-specific recombinase of the resolvase (TnpR) or phage integrase (TnpI) family. Some of these elements lack phenotypically detectable traits while others do not appear to carry a site-specific recombinase. Tn1000 carries both tnpA and tnpR together with a cryptic gene whose function is unknown and Tn5401 carries tnpA and tnpI together with a putative poison/antidote system (see 171). Tn4430 from B. thuringiensis represents a simpler organization and carries only tnpA and tnpI (312). IS101, from plasmid pSC101, carries IRs but no functional Tpase gene and IS1071, from Alcaligenes carries the IRs together with a functional Tpase gene. These represent the simplest type of Tn3 family member.
The resolution reaction has been studied in exquisite detail for Tn3 and Tn1000. In vitro resolution systems have also been developed for the Tn4430 (TnpI) system (183). However, for members not bearing the site-specific recombination module, homologous recombination can substitute for the highly efficient site-specific recombination reaction as is the case for members of the IS6 family.
The transposition reaction is less well characterised. Tpase binding to the terminal repeats has been described for Tn3 (213, 307) and Tn1000 (508, 511) and Tpase-induced cleavage of the ends has been observed. In the case of Tn3, this is stimulated by the presence of acyl carrier protein (ACP, (308a). Members of this family exhibit transposition immunity. An in vitro transposition system has also been developed (212, 241).
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.
Last modification : December 19 2001