Xnb

Reverse transcriptase

Target DNA

Transposed mobile elements

▲ FIGURE 10-8 Classification of mobile elements into two major classes. (a) Eukaryotic DNA transposons (orange) move via a DNA intermediate, which is excised from the donor site. (b) Retrotransposons (green) are first transcribed into an RNA molecule, which then is reverse-transcribed into double-stranded DNA. In both cases, the double-stranded DNA intermediate is integrated into the target-site DNA to complete movement. Thus DNA transposons move by a cut-and-paste mechanism, whereas retrotransposons move by a copy-and-paste mechanism.

ate are called retrotransposons because their movement is analogous to the infectious process of retroviruses. Indeed, retroviruses can be thought of as retrotransposons that evolved genes encoding viral coats, thus allowing them to transpose between cells. Retrotransposons can be further classified on the basis of their specific mechanism of transposition. We describe the structure and movement of the major types of mobile elements and then consider their likely role in evolution.

Mobile Elements That Move as DNA Are Present in Prokaryotes and Eukaryotes

Most mobile elements in bacteria transpose directly as DNA. In contrast, most mobile elements in eukaryotes are retro-transposons, but eukaryotic DNA transposons also occur. Indeed, the original mobile elements discovered by Barbara McClintock are DNA transposons.

Bacterial Insertion Sequences The first molecular understanding of mobile elements came from the study of certain E. coli mutations caused by the spontaneous insertion of a DNA sequence, =1-2 kb long, into the middle of a gene. These inserted stretches of DNA are called insertion sequences, or IS elements. So far, more than 20 different IS elements have been found in E. coli and other bacteria.

Transposition of an IS element is a very rare event, occurring in only one in 105-107 cells per generation, depending on the IS element. Many transpositions inactivate essential genes, killing the host cell and the IS elements it carries. Therefore, higher rates of transposition would probably result in too great a mutation rate for the host cell to survive. However, since IS elements transpose more or less randomly, some transposed sequences enter nonessential regions of the genome (e.g., regions between genes), allowing the cell to survive. At a very low rate of transposition, most host cells survive and therefore propagate the symbiotic IS element. IS elements also can insert into plasmids or lysogenic viruses, and thus be transferred to other cells. When this happens, IS elements can transpose into the chromosomes of virgin cells.

The general structure of IS elements is diagrammed in Figure 10-9. An inverted repeat, usually containing =50 base pairs, invariably is present at each end of an insertion sequence. In an inverted repeat the 5' n 3'sequence on one strand is repeated on the other strand, as:

GCTC 3'

Between the inverted repeats is a region that encodes trans-posase, an enzyme required for transposition of the IS ele-

IS element («1-2 kb)
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