instance is replicative. Some transposons, including IS1, can use both mechanisms, indicating that these
two processes are mechanistically linked. The way in which this is achieved is shown in Figure 6.43.
The Shapiro transposition intermediate(Figure 6.43) is as important to models of transposition as the
Holliday junction is to homologous recombination mechanisms. It is important to note that replicative
transposition results in the donor and recipient strands being joined to form a structure called a cointe-
grate. This can be seen during transposition from one plasmid to another and necessitates a homologous
recombination step to separate the two duplexes (Figure 6.44). This latter step is catalysed by a transposon-
encoded resolvaseenzyme. The resolvase stimulates recombination at a specific site (called res) inside the
transposon by endonucleolytic cleavage in a similar manner to that used by integrase (Section 6.8.2).
Furthermore, the sequence ofresis similar to that of the core sequence used by
resGATAATTTATAATAT
attGCTTTTTTATACTAA
A final point to note from Figure 6.43 is that during non-replicative transposition the donor chromo-
some is left broken. In prokaryotes this often results in the loss of the chromosome. In addition to simple
transposition, transposons can catalyse rearrangements of the DNA surrounding them. Deletion or inversion
of flanking DNA is often seen. Such rearrangements are abortive by-products of the normal transposition
process (Figure 6.45).
6.8.3.2 Eukaryotic Transposons. There exists a wide variety of different types of transposable elem-
ent in eukaryotes.^63 In common with their prokaryote relatives, most eukaryotic transposable elements
possess small inverted terminal repeats and generate small direct repeats of the target site during integra-
tion. Perhaps all eukaryotes contain at least one type and the variety of size and structure is bewildering.
However, two major classes are apparent. Class I transposable elementsall use RNA intermediates for
transposition and Class II elementsall use only DNA transposition intermediates. Class II elements are
evolutionarily related to IS elements and will be considered first.
6.8.3.2.1 Eukaryotic Class II Transposable Elements: The P Element. The fruit fly
D. melanogasteris host to a wide variety of transposable elements, which together comprise at least 10%
244 Chapter 6
Figure 6.42 Structures of prokaryotic transposable elements