Non-mRNA 3' end formation: how the other half lives.

The release of nascent RNA from transcribing RNA polymerase complexes is required for all further functions carried out by RNA molecules. The elements and processing machinery involved in 3′ end formation therefore represent key determinants in the biogenesis and accumulation of cellular RNA. While these factors have been well-characterized for messenger RNA, recent work has elucidated analogous pathways for the 3′ end formation of other important cellular RNA. Here, we discuss four specific cases of non-mRNA 3′ end formation—metazoan small nuclear RNA, Saccharomyces cerevisiae small nuclear RNA, Schizosaccharomyces pombe telomerase RNA, and the mammalian MALAT1 large noncoding RNA—as models of alternative mechanisms to generate RNA 3′ ends. Comparison of these disparate processing pathways reveals an emerging theme of evolutionary ingenuity. In some instances, evidence for the creation of a dedicated processing complex exists; while in others, components are utilized from the existing RNA processing machinery and modified to custom fit the unique needs of the RNA substrate. Regardless of the details of how non-mRNA 3′ ends are formed, the lengths to which biological systems will go to release nascent transcripts from their DNA templates are fundamental for cell survival.

从转录的RNA聚合酶复合物中释放新生RNA是RNA分子执行所有后续功能所必需的。因此，参与3′端形成的元件和加工机制是细胞RNA生物发生和积累的关键决定因素。虽然这些因素对于信使RNA已经有了很好的特征描述，但最近的研究已经阐明了其他重要细胞RNA的3′端形成的类似途径。在这里，我们讨论非mRNA 3′端形成的四个具体案例——后生动物小核RNA、酿酒酵母小核RNA、粟酒裂殖酵母端粒酶RNA和哺乳动物MALAT1长链非编码RNA——作为产生RNA 3′端的替代机制的模型。对这些不同加工途径的比较揭示了一个进化巧妙性的新兴主题。在某些情况下，存在形成一种专用加工复合物的证据；而在其他情况下，利用现有的RNA加工机制的成分并对其进行修改以适应RNA底物的独特需求。不管非mRNA 3′端是如何形成的细节如何，生物系统为了从其DNA模板中释放新生转录本所采取的措施对于细胞生存是至关重要的。