[
2006]
RNA interference (RNAi) describes a conserved biological response to double-stranded RNA (dsRNA) resulting in the degradation of homologous messenger RNA. In the last few years, this process of sequence-specific, post-transcriptional gene silencing has become a key technique for rapidly assessing gene function in species ranging from plants to mammals. Fire et al. provided the first insight into the RNAi mechanism by identifying dsRNA as the trigger of RNAi in Caenorhabditis elegans in 1998 [1]. However, a similar gene-silencing phenomenon was reported in earlier studies in both plants and Neurospora [2,3]. The basic RNAi response starts with long dsRNA being processed into small interfering RNAs (siRNAs) by a ribonuclease (RNase) III enzyme, Dicer. Next, the siRNA is incorporated into the RNA-induced silencing complex (RISC). For target RNA recognition to occur, the siRNA duplex must be unwound, allowing binding of one siRNA strand to the target mRNA. This is followed by RISC cleavage of the homologous mRNA. Recent work has shown that the RNAi machinery is also involved in antiviral responses, transposon silencing, development and heterochromatin formation [4].
[
1989]
Transposable elements have recently been described in several species: Caenorhabditis elegans, Caenorhabditis briggsae, Ascaris lumbricoides, and Panagrellus redivivus. Because of the intense interest in C. elegans as an experimental organism for developmental genetic studies and the availability of sophisticated genetics, most is know about transposons in this species. This review focuses principally on Tc1 (Tc=transposon) of C. elegans, the best understood element in nematodes. Other elements in C. elegans and also elements in other species of nematodes will be briefly surveyed. The interested reader should also see two recent related reviews. The genome of C. elegans is 8 x 10(7) base pairs (bp) in extent, the smallest known for any metazoan. There are six chromosomes per haploid set, and about 83% of C. elegans DNA behaves as single-copy sequence in renaturation experiments. The repeated sequences are of several types, including functional genes, inverted or "foldback" sequences, and short repeated sequences of a few hundred nucleotides. The global arrangment of these short repeats is of the "short-period-interspersion" or "Xenopus" pattern. Some of the repetitive sequences consist of transposable elements, and at least five distinct families have been identified in C. elegans, Tc1 through Tc5. The sequence of one Tc1 element has been determined and shows that Tc1 resembles bacterial insertion sequence elements with terminal inverted repeats and a central open reading frame. The complete sequences for any members of the other transposon families have not been determined, but the data suggest that Tc2, Tc3, and Tc5 are also insertion sequence-like in structure and that Tc4 is foldbacklike in structure. No "retrotransposon-like" elements have been identified in C. elegans, although such elements have been described in A.