rasiRNA pathway controls antisense expression of Drosophila telomeric retrotransposons in the nucleus

Abstract

Telomeres in Drosophila are maintained by the specialized telomeric retrotransposons HeT-A, TART and TAHRE. Sense transcripts of telomeric retroelements were shown to be the targets of a specialized RNA-interference mechanism, a repeat-associated short interfering (rasi)RNA-mediated system. Antisense rasiRNAs play a key role in this mechanism, highlighting the importance of antisense expression in retrotransposon silencing. Previously, bidirectional transcription was reported for the telomeric element TART. Here, we show that HeT-A is also bidirectionally transcribed, and HeT-A antisense transcription in ovaries is regulated by a promoter localized within its 3′ untranslated region. A remarkable feature of noncoding HeT-A antisense transcripts is the presence of multiple introns. We demonstrate that sense and antisense HeT-A-specific rasiRNAs are present in the same tissue, indicating that transcripts of both directions may be considered as natural targets of the rasiRNA pathway. We found that the expression of antisense transcripts of telomeric elements is regulated by the RNA silencing machinery, suggesting rasiRNA-mediated interplay between sense and antisense transcripts in the cell. Finally, this regulation occurs in the nucleus since disruption of the rasiRNA pathway leads to an accumulation of TART and HeT-A transcripts in germ cell nuclei.

Vasa promotes Drosophila germline stem cell differentiation by activating mei-P26 translation by directly interacting with a (U)-rich motif in its 3′ UTR

“…Piwi, a key component of rasiRNA and piwi-interacting RNA (piRNA) pathways, has been implicated in stem cell self-renewal (Cox et al 2000). Vas is associated with Piwi, Aubergine, and other components of the rasiRNA pathway (Megosh et al 2006;Vagin et al 2006), and has itself been linked to retrotransposon silencing (Shpiz et al 2009). Therefore, Vas appears to be involved in both the AGO1 pathway, through its regulation of Mei-P26, and the rasiRNA pathway (Vagin et al 2006), potentially making it a key regulator of processes mediated by small RNAs in GSCs and early-stage committed cells.…”

Control of Telomere Length in Drosophila

“…An unusual feature of the telomeric retroelements is that the promoters of HeT-A and TAHRE are localized in the 3' UTR and drive transcription of a downstream element (Danilevskaya et al, 1997;Shpiz et al, 2007). An antisense promoter was detected in close proximity to the HeT-A promoter, which drives sense expression (Shpiz et al, 2009). It appears as if the common promoter drives bidirectional expression of HeT-A.…”

“…The TART element was shown to also be transcribed bidirectionally from internal sense and antisense promoters that are localized within non-terminal direct repeats in the TART 5' and 3' regions (Danilevskaya et al, 1999;Maxwell et al, 2006). An unusual feature of non-coding HeT-A and TART antisense transcripts is that they are spliced (Maxwell et al, 2006;Shpiz et al, 2009). The role of antisense transcripts in the RNA silencing of the telomeric retrotransposons will be discussed below.…”

“…In spite of their vital genomic function, Drosophila telomeric retroelements as well as other parasitic transposons were shown to be targets of the piRNA-mediated silencing pathway (Vagin et al, 2004;Savitsky et al, 2006;Shpiz et al, 2007;Shpiz et al, 2009). Mutations in the spn-E, aub, piwi, squ and zuc genes and vasa locus result in accumulation of telomeric element transcripts in ovaries (Vagin et al 2004;Savitsky et al 2006;Pane et al 2007;Shpiz et al 2007;Shpiz et al 2009), but the same mutations do not affect telomeric element expression in Drosophila testes (A.K., unpublished data).…”

Telomere Maintenance in Organisms without Telomerase

“…Thus, silencing orchestrated by TAS arrays is not sufficiently strong to regulate transcription of the terminal retrotransposons. Second, it was noted that HeT-A and TART transcript levels are under the control of an RNA interference pathway (Savitsky et al, 2006;Shpiz et al, 2009). Transposition frequency and terminal array length, however, did not increase with increasing retrotransposon transcript levels (Capkova Frydrychova et al, 2008).…”