RasiRNA

Repeat associated small interfering RNA (rasiRNA) is a class of small RNA that is involved in the RNA interference (RNAi) pathway. RasiRNA are in fact Piwi-interacting RNAs (piRNAs), which are small RNA molecules that interact with Piwi proteins. Piwi proteins are a clade of the Argonaute family of proteins. In the germline, RasiRNA is involved in establishing and maintaining heterochromatin structure, controlling transcripts that emerge from repeat sequences, and silencing transposons and retrotransposons.

There are at least three Argonaute subfamilies that have been found in eukaryotes. Unlike the Ago subfamily which is present in animals, plants, and fission yeast, the Piwi subfamily has only been found in animals. RasiRNA has been observed in Drosophila and some unicellular eukaryotes but its presence in mammals has not been determined, unlike piRNA which has been observed in many species of invertebrates and vertebrates including mammals; however, since proteins which associate with rasiRNA are found in both vertebrates and invertebrates, it is possible that active rasiRNA exist and have yet to be observed in other animals. RasiRNAs have been observed in Schizosaccharomyces pombe, a species of yeast, as well in some plants, neither of which have been observed to contain the Piwi Argonaute protein subfamily. It has been observed that both rasiRNA and piRNA are maternally linked, but more specifically it is the Piwi protein subfamily that is maternally linked and therefore leads to the observation that rasiRNA and piRNA are maternally linked.

RasiRNA is distinct from other RNAi pathways such as microRNA (miRNA) and small interfering RNA (siRNA) as well as from piRNA. Unlike miRNA and siRNA which function through the Ago Argonaute protein subfamily, RasiRNA function through the Piwi Argonaute protein subfamily. RasiRNA is also distinct in its size. Contrary to miRNAs which are 21-23 nucleotides in length, siRNAs which are 20-25 nucleotides in length, and piRNAs which are 24-31 nucleotides in length, rasiRNAs are 24-29 nucleotides in length depending on the organism of origin. Unlike siRNA which are derived from both the sense and antisense strand, rasiRNA are derived from the antisense. Interestingly, while miRNA requires Dicer-1 for its production, and siRNA requires Dicer-2, rasiRNA does not require either; however, in some plants there are Dicer-like (Dcl) proteins that have been identified where Dcl1 produces 24 nucleotide miRNA and siRNA while Dcl2 produces 24 nucleotide rasiRNA. This research shows that not only is rasiRNA production distinct from miRNA and siRNA, but that rasiRNA may be found in plants while Piwi proteins are not.

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