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Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Retroviruses

Retroviruses are single-stranded RNA animal viruses that employ a double-stranded DNA intermediate for replication. The RNA is copied into DNA by the enzyme reverse transcriptase. The life cycle of a typical retrovirus is shown in Figure 13-18. Some retroviruses, such as mouse mammary tumor virus (MMTV) and Rous sarcoma virus (RSV), are responsible for the induction of cancerous tumors. When integrated into host chromosomes as double-stranded DNA, these retroviruses are termed proviruses. Proviruses, like the mu phage in bacteria, can be considered transposable elements, because they can, in effect, transpose from one location to another.

Retroviruses have structural features in common with some transposable elements from bacteria and other organisms. In particular, the ends of the proviruses have long terminal repeats (LTRs) reminiscent of the sequences of the Ty1 elements in yeast and the long terminal repeats of the copia-like elements in Drosophila. In addition, integration results in the duplication of a short target sequence in the host chromosome.

On the other hand, similarities between retroviruses and Ty1 elements in yeast and copia-like elements in Drosophila suggest that the Ty1 and copia-like elements might also be integrated forms of retrovirus-like elements (see Figure 13-19). This was nicely confirmed in 1985 by Jef Boeke and Gerald Fink and their co-workers in the experiments described in the next section.

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