typically 4–12 nucleotide pairs, but constant for a given transposable element, is duplicated as a result of
the insertion. The transposable element itself moves as a unit. Thus, further copies of the same sequence
are found in other genomic locations. Also important to most transposition processes are small inverted
repeats at each end of the transposable element in addition to at least one gene inside the transposable
element which encodes an enzyme (usually called a transposase) that catalyses the transposition process.
The inverted repeats constitute the recognition site for the transposase.
Transposition depends on endonucleolytic cleavage by the transposase enzyme at both the ends of the
transposable element and at the target site (Figure 6.41).^61 The target site duplication arises as a result of
staggered nicking of the target DNA by the transposase. For a given transposable element the target sequence
may be effectively random but some elements have preferred DNA motifs for insertion within chromo-
somal regions.
6.8.3. 1Prokaryotic Transposable Elements. There are several major classes of transposable element
known in prokaryotes.^62 The simplest are termed insertion sequence elements (IS elements; Figure 6.42).
These contain just a single transposase gene and terminal repeats. More complicated prokaryotic transposable
elements comprise a central region flanked by long repeats, which are either direct or inverted with respect
to each other. The long repeats are IS elements or derivatives thereof. These more complex elements are called
transposons. An example of a transposon with direct repeats is Tn9 and one with inverted repeats is Tn10
(Figure 6.42). Finally, large transposons without internal repeats, for example Tn3, are also known.
The central regions of transposons contain antibiotic resistance genes and it is the transposition of these
mobile DNAs that causes the rapid dissemination of antibiotic resistance between different strains and
species of bacteria. They are therefore of considerable medical importance. It is presumed that complex
transposons evolved from the fortuitous juxtaposition of two IS elements. In the laboratory the IS elements
which comprise a complex transposon can sometimes be mobilised independently. Such events, however,
are much less common than transposition of the entire genetic element.
6.8.3.1.1 The Transposition Mechanism. Transposition in bacteria can result in two fundamentally
different outcomes.^61 In the first case, the transposable element simply leaves one site on the one chromo-
some and enters another (‘cut and paste’). In the second case, the transposon is preserved at its original
location and a new element appears at a distant site (‘copy and paste’). Thus, transposition in the latter
Genes and Genomes 243
Figure 6.41 Transposable elements create small duplications of target sequence when they insert into the chromosome