Transposons and Insertion Sequences

Bacteria have two classes of transposons. Insertion sequences (simple transposons) contain only the sequences required for transposition and the genes for proteins (transposases) that promote the process. Complex transposons contain one or more genes in addition to those needed for transposition. These extra genes might, for example, confer resistance to antibiotics and thus enhance the survival chances of the host cell. The spread of antibiotic-resistance elements among disease-causing bacterial populations that is rendering some antibiotics ineffectual (pp. 925-926) is mediated in part by transposition. 

Transposition of DNA, which is discussed in Chapter 27, Section D,4, may seem to be a rare and relatively unimportant event in our body cells. However, transposon DNA accounts for 35% or more of the human genome740 and apparently plays a major role in evolution. Like other transposons, the DNA sequences known as retrotransposons also move about within DNA. However, they use an indirect mechanism that involves synthesis of mRNA and reverse transcription.740 741 The reverse transcribed complementary DNA may be inserted back into the genome at new locations. The necessary chemical reactions parallel those involved in the replication of retroviruses (Fig. 28-23, 28-24). Retrotransposens, truncated retrotransposons, and related sequences constitute as much as 16% of the human genome.741... 

Feschotte, C. (2004) Merlin, a new superfamily of DNA transposons identified in diverse animal genomes and related to bacterial IS1016 insertion sequences. Molecular Biology and Evolution 21, 1769-1780. 

Bennett, P.M. Genome plasticity insertion sequence elements, transposons and integrons, and DNA rearrangement. Methods Mol Biol 266 (2004) 71-113. 

Figure 1. Map of the transposon Tn5. The insertion sequences IS50L and IS50R flank the central region which contains the genes for resistance to kanamycin (Km ), bleomycin (Bler), and streptomycin (Sm ). The P refers to the position of the promoters for expression of antibiotic resistance, transposase and transpoase inhibitor (on IS50R). The restriction sites are marked X=XhoI, B=BgIII, and H=HindIII.

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. 

Both DNA transposons and LINE retrotransposons have been shown to occasionally carry unrelated flanking sequences when they insert into new sites by the mechanisms diagrammed In Figure 10-18. These mechanisms likely also contributed to exon shuffling during the evolution of contemporary genes. 

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. 

It was obvious that what was needed was detailed knowledge, at the nucleotide level, of the sequences involved in the excision of petite genomes. The simplest interpretation of the sequence data was that excision was clue to a crossing-over process, and that the primary event in the spontaneous petite mutation was very similar to the excision of the lambda prophage from the E.coli chromosome, or to (he dissociation of a bacterial transposon from its host plasmid in this case, the GC clusters and sequences in the AT spacers play the same role as the insertion sequences delimiting a bacterial transposon. 

These studies have shown that a drug resistance translocation sequence (the transposon Tn5) can be transferred from Escherichia coli to R. leguminosamm and inserted into a known plasmid. When this genetically marked plasmid was transferred to/ , trifolii, R. phaseoli, or Cicer rhizobia the recipient was capable of nodulating peas. 

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