piRNAs (PIWI-interacting RNAs)
Siomi, Mikiko C. Department of Molecular Biology, Keio University School of Medicine, Tokyo, Japan.
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piRNAs are a subset of small ribonucleic acids that are expressed primarily in germline cells. Their length ranges from 23 to 31 nucleotides, and they show extreme sequence diversity. The origins of piRNAs include transposons (transposable elements), other classes of intergenic repetitive sequences, and noncoding, nonrepetitive genomic elements, called piRNA clusters. piRNAs function in RNA silencing, that is, the pathways that negatively control selected target genes. Of those, the best known are transposons. piRNAs also, rarely, target protein-coding genes. piRNAs alone are not able to silence genes. Rather, they associate with protein partners—PIWI (P element–induced wimpy testes) proteins—to perform their functions. Hence, this is the origin of their name, PIWI-interacting RNAs (piRNAs). PIWI proteins are a subgroup of the Argonaute (AGO) proteins, which are the key factors in RNA silencing. PIWI proteins are expressed primarily in germline cells, as are piRNAs. Presumably, for a given animal to maintain its reproduction system, multiple PIWI proteins are expressed that show peptide sequence similarity. However, piRNAs that bind to each PIWI member have unique characteristics. These observations have led to the proposal of two models for piRNA production: the amplification and the nonamplification loop pathways. Loss of PIWI proteins causes overexpression of transposons and failure of germline development, indicating that piRNAs, together with PIWI proteins, are necessary for germline survival and for maintaining the lineage of a species.
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