This is my seminar at  2006/11/28

21 July 2006:Vol. 313. no. 5785, pp. 320 - 324
A Distinct Small RNA Pathway Silences Selfish Genetic Elements in the Germline
Vasily V. Vagin, Alla Sigova, Chengjian Li, Hervé Seitz, Vladimir Gvozdev, Phillip D. Zamore

       RNA silencing mechanism in eukaryotic cells, such as small interfering RNAs (siRNAs) and microRNAs (miRNAs), which is thought to defend against viruses, regulate gene expression, and protect the genome against selfish genetic elements like retrotransposons and repetitive sequences. Recently, a third type of small RNAs, repeat-associated interfering RNAs (rasiRNAs) have been identified, and be distinct from miRNAs and siRNAs by their longer length, 24 to 29 nt. However, the mechanism of rasiRNAs is not clear. The authors indicated that the rasiRNAs are generated mainly from antisense strand, whereas siRNAs are derived from both sense and antisense strand of their double-strand RNA precursors. In flies, Dicer-1 generates miRNAs and Dicer-2 generates siRNAs, but rasiRNAs have been proposed to be diced from long dsRNA by Dicer-1 or Dicer-2 independent pathway. The simplest interpretation of these data is that rasiRNAs physically associate with Piwi subfamily (Piwi and Aub). Unlike siRNA and miRNA, rasiRNAs lake the 2’, 3’ hydroxyl termini but contain one modified and one unmodified hydroxyl group. These data suggest the rasiRNAs follow a different way to protect fly germline through a silencing mechanism distinct from both miRNA and siRNA pathways.

The common knowledge about RNA interferance (RNAi) are siRNA and miRNA pathway.  siRNAs are come from forign dsRNA and miRNA  are come from endogenous hearpin. After diceing, unwinding, and bind with RISC, these small RNAs either perform mRNA degrdation or transcriptional repression by accumulation at 3'UTR.

The central component s of siRNAs are Dcr-2, R2D2, and Ago2, and the centeral component of miRNAs are Dcr-1, Loqs, and Ago1. 
Ago1, Ago2 belongs to Argonaute subfamily Argonaute family. Recently , a goup of small RNAs were identified by their longer size (24-29nt)(siRNAs/miRNAS are 21-24nt).

These type of siRNAs was named as piRNAs (Piwi-interacting RNAs)  in rat and rasiRNAs (repeat-associated) in Drosophila .
They were found to interact with Piwi subfamily Argonaute family.
By microarray data, northern blotting, western blottting, and terminal structure analysis. The authors indicated that rasiRNAs is a different way to protect fly germline through a silencing mechanism distinct from both miRNA and siRNA pathways.

What is the protein involved in rasiRNAs cleavage?
Is the rasiRNAs or piRNAs pathway  specific occur in germline?
What genes are regulated by rasiRNAs pathway ? what is it for?

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