14 Drosophila Telomeres

Abstract

THE DNA ENDS IN DROSOPHILA
Despite the fact that they were first defined in Drosophila, fruit fly telomeres are dramatically different from those of other organisms, at least regarding replication functions. This species lacks telomerase; instead, peculiar transposable elements seem to act as buffers against telomere loss during DNA replication. The Drosophila telomeres contain arrays of two non-LTR (long terminal repeat) retrotransposon elements called HeT-A and TART (Fig. 1) (Rubin 1978; Young et al. 1983; Levis et al. 1993; for reviews, see Mason and Biessmann 1995; Pardue 1995; Pardue and DeBaryshe 2003). Like many other non-LTR retroelements, TART encodes the Gag and Pol proteins, whereas HeT-A encodes only the Gag protein; both have been placed in the jockey clade (Eickbush 1994; Malik et al. 1999) (for a description of their structure see Fig. 2). The elements in these arrays are in a head-to-tail arrangement with their 5′ ends always oriented toward the terminus. Many of the elements in these telomeric arrays are truncated to varying degrees at their 5′ ends. The probable mechanism of telomere elongation is the addition of HeT-A and TART copies generated from RNA templates by the activity of a reverse transcriptase (Fig. 3) (Mason and Biessmann 1995; Pardue et al. 1996). Their targeting seems to be independent of specific terminal DNA sequences (Traverse and Pardue 1988; Biessmann et al. 1990, 1992; Sheen and Levis 1994). HeT-A is transcribed in the normal 5′ to 3′ direction from a promoter located at the 3′ UTR (untranslated region) of...