Transposons inverted repeats

Transposons are mobile DNA elements (sizes 2.5-23 kbp) that move from one place to another in the chromosome or onto extrachromosomal genetic elements within the same cell. They are flanked by inverted repeats at then-ends and encode among other proteins a transposase that is needed for the transposition process. Resistance genes in the transposon are often parts of integrons. These are structures that cany an integrase responsible for the insertion of the resistance gene cassettes into the integron. 

Following the discovery of the IS elements it was found that transposable elements named transposons could transfer resistance to antibiotics between bacteria. All of these transposable elements have inverted repeat sequences at the ends. For example, IS1 contains the following sequence at both ends but with opposite orientation as if in a... 

A second Drosophila transposon called mariner630 typifies the mariner / Tel transposon superfamily, which also contains members from nematodes,631 other invertebrates, fishes,632 amphibia,633 and possibly human beings.634 These transposons encode a transposase containing a D, D, D or D, D, E motif630 but no other proteins. They contain short 30-bp terminal inverted repeats and become inserted into host TA sequences.631 Movement of some repetitive sequences of the LINE635 and SINE636 families within the human genome may be assisted by mariner transposons.637... 

Several bacterial transposons, referred to as insertion elements (ISelements), consist only of a gene that codes for a transposition enzyme (i.e., transposase), flanked by short DNA segments called inverted repeats (Figure 18.16). (Inverted repeats are short palindromes.) More complicated bacterial transposable elements, called composite transposons, contain additional genes, several of which may code for antibiotic resistance. Because transposons can jump between bacterial chromosomes, plasmids, and viral genomes, transpositions are now believed to play an important role in the spread of antibiotic resistance among bacteria. 

Class I elements (Figure 25.35) encode a transposase but not a resolvase, and are of two types. The simplest is called an insertion sequence (IS), which consists simply of a gene for transposase, flanked by two short inverted repeat sequences of about 15 to 25 base pairs. A less simple structure, called a composite transposon, consists of a protein-encoding gene, such as a gene conferring antibiotic resistance, flanked by two insertion sequences, or IS-like elements. These elements may be in either identical or inverted orientations. 

Raz and colleagues used Tc3 from C. elegans to stably introduce a reporter construct containing GFP into zebrafish embryos by coinjechon of the Tc3 transposase mRNA together with the reporter flanked by inverted repeats (46). In one line, they could show transposon-mediated integrahon, expression of the reporter construct and germline transmission. 

Composite iransposons contain a central region flanked by two identical (or nearly identical) IS-like sequencea These IS-like sequences have either the same or an inverted orientation, and are themselves flanked by inverted repeats. It therefore appears that composite transposons arose from the combination of a stretch of gene-containing DNA (the central region) with two independent insertion sequencer... 

Since McClintock s early work on mobile elements in corn, transposons have been identified in other eukaryotes. For Instance, approximately half of all the spontaneous mutations observed in Drosophila are due to the Insertion of mobile elements. Although most of the mobile elements in Drosophila function as retrotransposons, at least one—the P element—functions as a DNA transposon, moving by a cut-and-paste mechanism similar to that used by bacterial insertion sequences. Current methods for constructing transgenic Drosophila depend on engineered, high-level expression of the P-element transposase and use of the P-element Inverted terminal repeats as targets for transposition. 

Cni Z, Geurts AM, Liu G, Kaufman CD, Hackett PB (2002) Structure-function analysis of the inverted terminal repeats of the Sleeping Beauty transposon. J Mol Biol 318 1221-1235. 

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