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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Transposition through an RNA Intermediate

Figure 13-20 diagrams the experimental design used by Boeke and Fink and their colleagues to alter a yeast Ty element, cloned on a plasmid. First, a promoter was inserted near the end of an element that could be activated by the addition of galactose to the medium. The use of a galactose-sensitive promoter allows the manipulation of the expression of Ty RNA. In the presence of galactose, more transcription of Ty RNA occurs. Second, an intron from another yeast gene was introduced into the coding region of the Ty transposon.

The addition of galactose greatly increases the frequency of transposition of the altered Ty element. This increased frequency suggests the involvement of RNA, because galactose-stimulated transcription begins at the galactose-sensitive promoter and continues through the element (Figure 13-20). The key experimental result, however, is the fate of the transposed Ty DNA. When the researchers examined the Ty DNA resulting from transpositions, they found that the intron had been removed. Because introns are excised only during RNA processing (see Chapter 3), the transposed Ty DNA must have been copied from an RNA intermediate transcribed from the original Ty element and then processed by RNA-RNA splicing. The DNA copy of the spliced mRNA is then integrated into the yeast chromosome.

Transposable elements that utilize reverse transcriptase to transpose through an RNA intermediate are termed retrotransposons. They are widespread among eukaryotes and are generally divided into two classes. Viral retrotransposons, as exemplified by yeast Ty and Drosophila copia elements, have properties similar to retroviruses (see Figure 13-19), including LTRs (long terminal repeats), as shown in Figure 13-21. Nonviral retrotransposons, which are the most frequently encountered transposable elements in mammals, include the LINES (long interspersed elements) and SINES (short interspersed elements) discussed in Chapter 12.

© 1999 by W. H. Freeman and Company. All rights reserved.