Ballarino Monica, Pagano Francesca, Girardi Erika, Morlando Mariangela, Cacchiarelli Davide, Marchioni Marcella, Proudfoot Nicholas J, Bozzoni Irene
University of Rome La Sapienza, P.le A. Moro 5, 00185 Rome, Italy.
Mol Cell Biol. 2009 Oct;29(20):5632-8. doi: 10.1128/MCB.00664-09. Epub 2009 Aug 10.
The first step in microRNA (miRNA) biogenesis occurs in the nucleus and is mediated by the Microprocessor complex containing the RNase III-like enzyme Drosha and its cofactor DGCR8. Here we show that the 5'-->3' exonuclease Xrn2 associates with independently transcribed miRNAs and, in combination with Drosha processing, attenuates transcription in downstream regions. We suggest that, after Drosha cleavage, a torpedo-like mechanism acts on nascent long precursor miRNAs, whereby Xrn2 exonuclease degrades the RNA polymerase II-associated transcripts inducing its release from the template. While involved in primary transcript termination, this attenuation effect does not restrict clustered miRNA expression, which, in the majority of cases, is separated by short spacers. We also show that transcripts originating from a miRNA promoter are retained on the chromatin template and are more efficiently processed than those produced from mRNA or snRNA Pol II-dependent promoters. These data imply that coupling between transcription and processing promotes efficient expression of independently transcribed miRNAs.
微小RNA（miRNA）生物合成的第一步发生在细胞核中，由包含RNase III样酶Drosha及其辅因子DGCR8的微处理器复合物介导。我们在此表明，5'→3'核酸外切酶Xrn2与独立转录的miRNA结合，并与Drosha加工一起，减弱下游区域的转录。我们认为，在Drosha切割后，一种类似鱼雷的机制作用于新生的长前体miRNA，由此Xrn2核酸外切酶降解与RNA聚合酶II相关的转录本，诱导其从模板上释放。虽然参与初级转录本的终止，但这种衰减效应并不限制成簇miRNA的表达，在大多数情况下，成簇miRNA由短间隔序列分隔。我们还表明，源自miRNA启动子的转录本保留在染色质模板上，并且比由mRNA或snRNA Pol II依赖性启动子产生的转录本更有效地被加工。这些数据表明转录与加工之间的偶联促进了独立转录的miRNA的有效表达。
