truncatedprotein is unable to catalyse transposition and is actually a repressor of the transposase. The
mRNA which gives rise to this defective protein retains intron 3 of the transposase gene, leading to prema-
ture translational termination of the transposase open reading frame within the intron. In germ cells, intron
3 is efficiently removed from the mRNA, active transposase is translated and the result is transposition of
the P elements.
6.8.3.2.2 Eukaryotic Class I Transposable Elements. Eukaryotic Class I transposable elements fall
into three major categories, namely retroviruses, retrotransposonsand dispersed pseudogenes^64 (Figure
6.47). Retroviruses and retrotransposons are closely linked evolutionarily and structurally. The key distinction
between the two is that retroviruses are capable in principle of producing an extracellular infectious virus. In
practice, there are very many defective retroviruses, some of which have been called retrotransposons.
246 Chapter 6
Figure 6.45 Deletion or inversion of DNA surrounding adjacent transposable element insertions
Figure 6.44 Cointegrate structures are intermediates in replicative transposition