Department of Cell Biology, University of Medicine and Dentistry of New Jersey, Stratford, NJ 08084, USA.
Nucleic Acids Res. 2011 Mar;39(5):1811-22. doi: 10.1093/nar/gkq1050. Epub 2010 Oct 29.
Ribosome biogenesis requires multiple nuclease activities to process pre-rRNA transcripts into mature rRNA species and eliminate defective products of transcription and processing. We find that in mammalian cells, the 5' exonuclease Xrn2 plays a major role in both maturation of rRNA and degradation of a variety of discarded pre-rRNA species. Precursors of 5.8S and 28S rRNAs containing 5' extensions accumulate in mouse cells after siRNA-mediated knockdown of Xrn2, indicating similarity in the 5'-end maturation mechanisms between mammals and yeast. Strikingly, degradation of many aberrant pre-rRNA species, attributed mainly to 3' exonucleases in yeast studies, occurs 5' to 3' in mammalian cells and is mediated by Xrn2. Furthermore, depletion of Xrn2 reveals pre-rRNAs derived by cleavage events that deviate from the main processing pathway. We propose that probing of pre-rRNA maturation intermediates by exonucleases serves the dual function of generating mature rRNAs and suppressing suboptimal processing paths during ribosome assembly.
核糖体生物发生需要多种核酸酶活性来将 pre-rRNA 转录物加工成成熟的 rRNA 种类,并消除转录和加工过程中的有缺陷产物。我们发现,在哺乳动物细胞中,5'外切核酸酶 Xrn2 在 rRNA 的成熟和多种废弃的 pre-rRNA 种类的降解中都发挥着主要作用。用 siRNA 介导的 Xrn2 敲低后,小鼠细胞中含有 5'延伸的 5.8S 和 28S rRNA 前体积累,表明哺乳动物和酵母之间在 5'-端成熟机制上具有相似性。引人注目的是,许多异常的 pre-rRNA 种类的降解主要归因于酵母研究中的 3'外切核酸酶,但在哺乳动物细胞中是从 5'到 3'进行的,并且由 Xrn2 介导。此外,Xrn2 的耗竭揭示了源自偏离主要加工途径的切割事件的 pre-rRNA。我们提出,核酸外切酶对 pre-rRNA 成熟中间产物的探测具有双重功能,即在核糖体组装过程中既能产生成熟的 rRNA,又能抑制次优的加工途径。