PIWI-interacting RNA (piRNA) biogenesis

In germ cells of humans and mice, precursors of PIWI-interacting RNAs (piRNAs) are transcribed from a few hundred sequence clusters, as well as individual transposons, intergenic regions, and genes in the genome. These longer transcripts are processed to yield piRNAs of 26-30 nucleotides independently of DICER, the enzyme responsible for microRNAs (miRNAs) and small interfering RNAs (siRNAs) (reviewed in Girard and Hannon 2008, Siomi et al. 2011, Ishizu et al. 2012, Pillai and Chuma 2012, Bortvin 2013, Chuma and Nakano 2013, Sato and Siomi 2013). The initial step in processing long transcripts to piRNAs is cleavage by PLD6 (MitoPLD), which generates the mature 5' end. The cleavage products of PLD6 are bound by either PIWIL1 (HIWI, MIWI) or PIWIL2 (HILI, MILI) in complexes with several other proteins. The 3' end is trimmed by an unknown exonuclease to generate the mature piRNA. PIWIL1:piRNA complexes appear to be involved in post-transcriptional silencing in the cytosol while PIWIL2:piRNA complexes generate further piRNAs from transposon transcripts and other transcripts in the cytosol. Cleavage products from PIWIL2:piRNA may be loaded into either PIWIL2 or PIWIL4 (HIWI2, MIWI2). Loading into PIWIL2 forms a step in a cytosolic amplification loop called the "ping-pong cycle" which yields further PIWIL2:piRNA complexes from cleaved precursor RNAs. Loading into PIWIL4 yields a complex also containing TDRD9 that translocates to the nucleus and directs DNA methylation of cognate loci, causing transcriptional silencing during spermatogenesis. Transcriptional silencing by piRNAs is necessary to limit transposition of endogenous transposons such as L1 elements in the genome.

在人类和鼠类的生殖细胞中，PIWI结合RNA（piRNA）的前体主要转录自数百个序列簇，以及基因组中的个别转座子、基因间区域和基因。这些较长的转录本经过加工，生成26至30个核苷酸长的piRNA，此过程独立于DICER酶，而DICER酶负责生成miRNA（小干扰RNA）和siRNA（微小RNA）。关于此过程的综述可见于Girard和Hannon（2008年）、Siomi等人（2011年）、Ishizu等人（2012年）、Pillai和Chuma（2012年）、Bortvin（2013年）、Chuma和Nakano（2013年）、Sato和Siomi（2013年）的研究。将长转录本加工为piRNA的初始步骤是PLD6（MitoPLD）的切割，生成成熟的5'端。PLD6的切割产物被PIWIL1（HIWI、MIWI）或PIWIL2（HILI、MILI）与多种蛋白形成的复合物所结合。3'端由一种未知的核酸外切酶进行修剪，以生成成熟的piRNA。PIWIL1:piRNA复合物似乎参与细胞质中的转录后沉默，而PIWIL2:piRNA复合物则从转座子转录本和其他细胞质中的转录本生成更多的piRNA。PIWIL2:piRNA的切割产物可能被装载到PIWIL2或PIWIL4（HIWI2、MIWI2）中。装载到PIWIL2中形成细胞质扩增循环中的步骤，称为“乒乓循环”，从而从切割的先驱RNA中产生更多的PIWIL2:piRNA复合物。装载到PIWIL4中产生一个也包含TDRD9的复合物，该复合物转移到细胞核中，并指导同源位点的DNA甲基化，导致在精子发生过程中的转录沉默。piRNA介导的转录沉默对于限制基因组中内源性转座子（如L1元件）的转座是必要的。