RNA silencing in Drosophila

Abstract

Knowledge of the role of RNA in affecting gene expression has expanded in the past several years. Small RNAs serve as homology guides to target messenger RNAs for destruction at the post-transcriptional level in the experimental technique known as RNA interference and in the silencing of some transgenes. These small RNAs are also involved in sequence-specific targeting of chromatin modifications for transcriptional silencing of transgenes, transposable elements, heterochromatin and some cases of Polycomb-mediated gene silencing. RNA silencing processes in Drosophila are described.

On the origin and functions of RNA-mediated silencing: from protists to man

“…Indeed, Dcl1 only includes the RNaseIII motifs and a C-terminal DSRM. In the incomplete genome of Phytophthora sojae the only recognizable Dicer-like sequence consists of a poorly conserved helicase domain, the DUF283 motif, and the two RNaseIII Lee et al (1993) and Wienholds and Plasterk (2005) Anopheles gambiae Hoa et al (2003) and Keene et al (2004) Drosophila melanogaster Aravin et al (2003) and Lagos-Quintana et al (2001) Kennerdell and Carthew (1998) and Piccin et al (2001) Direct b Pal-Bhadra et al (2004) and Kavi et al (2005) Gesellchen and Boutros (2004) and Nakahara et al…”

“…For instance, a Piwi-like protein, Twi1, is required for the conjugation-induced accumulation of small RNAs involved in programmed DNA elimination in T. thermophila (Lee and Collins 2006;Liu et al 2004b). The D. melanogaster Piwi-like genes Aubergine and Piwi have been implicated in both transcriptional, partly associated with histone H3 lysine 9 (H3K9) methylation, and post-transcriptional gene silencing (Aravin et al 2004;Kavi et al 2005;PalBhadra et al 2004). Members of the Argonaute-like class of proteins have been identified as key components of the RISC Rivas et al 2005;Zamore and Haley 2005).…”

“…As discussed above, (degradative) dsRNA-induced RNAi appears to be widespread (Table 2) and likely one of the ancestral functions of the RNAi machinery. However, the sources of long dsRNA are quite variable resulting, in different species, in the silencing of diverse sequences from genomic parasites and repetitive DNA to specific genes (Baulcombe 2004;Chicas et al 2004;Kavi et al 2005;Matzke and Birchler 2005;Nakayashiki 2005;Yoshikawa et al 2005;Zamore and Haley 2005).…”

“…RNAi-dependent transcriptional silencing has been demonstrated to entail histone modifications, such as H3K9 methylation, in eukaryotes belonging to at least three different supergroups (Table 2). Moreover, siRNA-triggered transcriptional repression occurs in organisms that lack cytosine DNA methylation such as S. pombe and C. elegans (Grishok et al 2005;Martienssen et al 2005;Ponger and Li 2005;Robert et al 2005;Volpe et al 2002) and in organisms with very limited DNA methylation such as T. brucei and D. melanogaster (Kavi et al 2005;Pal-Bhadra et al 2004;Ponger and Li 2005;Shi et al 2004;Ullu et al 2004). In contrast, RNA-directed DNA methylation has, thus far, only been demonstrated in plants and mammals (Kawasaki and Taira 2004;Matzke and Birchler 2005;Morris et al 2004) and the role of DNA methylation in this type of gene silencing is somewhat debatable in mammals (Ting et al 2005;Weinberg et al 2006).…”

“…Interestingly, the mechanism(s) of RNAi-mediated (hetero)chromatin formation also appears to have diverged in present day eukaryotes since, for instance, an RdRP has been implicated in this process in S. pombe Verdel et al 2004;Volpe et al 2002) but it occurs in the absence of RdRPs in D. melanogaster and vertebrates (Fukagawa et al 2004;Kanellopoulou et al 2005;Kavi et al 2005;Pal-Bhadra et al 2004;Ting et al 2005;Weinberg et al 2006). Adding to this complexity, cytosine DNA methylation and histone modifications seem to be interconnected in self-reinforcing feedback loops in higher eukaryotes (Fuks 2005), although the role (if any) of the RNAi machinery in this cycle is not clear.…”