Small RNAs are found in eukaryotic organisms with several abundance of activities on the genic expression regulation by mRNA degradation or gene silencing. Argonauts proteins guide these mature small RNAs to their targets (Ghildiyal e Zamore, 2009). The Piwi sub-family of Argonauts are more specific bound to the piRNAs(Mani e Juliano, 2013). The role of this gene regulation complex is important by the fact that are related to biologic process of cell renovation and differentiation of stem-cells (Gangaraju e Lin, 2009), animal development(Stefani e Slack, 2008), germline cell development(Saxe e Lin, 2011) and some types of human cancer (Esteller, 2011).
piRNAs are sequences of small RNAs that have approximate 26 to 31 nucleotides(Aravin et al., 2006). Their functions are related to mRNA expression of transposable elements by the degradation, possibly inhibition of translate process and chromatin alteration (Watanabe e Lin, 2014). This surveillance system play a role of control and degradation can be observed in the presence and in your activities on cancer cells (Moyano e Stefani, 2015). In additional, have some functions related to the silencing by methylation of transposable elements, deadenylathion of Drosophylas transcripts (Rouget et. Al, 2010), De Novo methylation of DNA (Aravin et al., 2008), and Rasgrf1 imprinting of rats (Watanabe et al., 2011).
These piRNAs are originated from single strand transcripts of non-coding sequences. These transcripts contain a great concentration of truncated transposable elements sequences or transcripts that are from 3’UTR protein coding genes. By the way, the major part of these transcripts are originated from places where occur often precursors of piRNAs, these place are called clusters (review by Yamanaka et al., 2014), which are related to the transposable elements repertory what is recognized and silenced. The clusters transcripts are selected and processed randomly inside the cell to a great quantity of piRNAs. Instead of the fact that they sequences are different in which species, even in individuals from the same species, some patterns are found, like the presence of a Uridine on the 5’ terminal or the commonly presence of an Adenosine in the position 10 of the nucleotide sequence (Siomi et al. 2011). The specific localization of the clusters of the mammalian genome appears to be conserved too (Roovers et al., 2015).
There are two models that describe the production of piRNAs. In the first, the transcript of on cluster of piRNA is broken generating a 5’ terminal. This transcript can be broken in virtually every position. There are just one preference, that this 5 terminal be a Uridine. So, after this transcript has bound to the specific Piwi protein, a second sequence break happen, generating the 3’ terminal of this RNA. (Aravin et al., 2007).
The second model of piRNA arsenal construction is the called ping-pong amplification cycle. Starts with the formation of an enormous quantity of piRNAs from the sense and anti-sense strand of the cluster, just like the first model. In the moment which the Piwi-piRNA finds their specific target, this complex cut the target transcript 10 nucleotides by the 5’ terminal of the piRNA. This process not only inactive the target mRNA but create a new fragment with the 5’ terminal that will bound to the AGO3 argonaut protein. This new complex can be bound to complementary elements to this sequence, like a cluster transcript. The result of this second transcript cut are various fragments identical to the original piRNAs. Allowing a amplification cycle that produce a lot of piRNAs from a low source of initial sequences(Aravin et al,. 2007).
The formation of this Piwi-piRNA complex occur in cytoplasm, and so is imported to the nucleus to play his roles of transposons repression and guiding histones to this loci. The repression state of these transposons requires a continuous activity of the Piwi-piRNA complex, by the fact that the reduced Piwi activity faster cause unrepression of the transposable elements (reviewed by Yamanaka et al., 2014).
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