of its total genome. One of the most interesting and best-studied elements in Drosophilais the P element.
This transposable element is structurally simple and very similar to IS elements. It has a single transposase
gene, this time interrupted by three introns (Figure 6.46). It transposes by a ‘cut and paste’ mechanism
effectively identical to that displayed by IS elements.
The P element is only transposed in the germ cells of the female fly and only if the female lacks intact
P elements but her mate has them. No other combination of mating produces movement of the transposable
element. Interestingly, P element RNA is found in all flies containing these elements and is made in all their
tissues. If this RNA encodes the transposase, what prevents transposition in all tissues of flies containing
P elements? It turns out that the protein synthesised in somatic tissue (the entire body minus the germ cells)
of the fly only contains sequence information from the first three exons of the P element (Figure 6.46). This
Genes and Genomes 245
Figure 6.43 The Shapiro intermediate is central to the mechanism of illegitimate recombination. Either replicative
or non-replicative transposition may result from this structure