Dicer recognizes the 5' end of RNA for efficient and accurate processing

摘要

在植物和动物的基因沉默中发挥显著作用的微rnRNA(mIRNA)长度一般约为22个核苷酸。它们rn是因一个更长的前体被两个核糖核酸酶依次解rn理而生成的，这两个核糖核酸酶其中之一是rnDicer。过去人们认为，Dicer利用一个“尺子”rn机制(即从dsRNA的3‘端来测量其长度)来进行rn解理，这样将会把功能性22一核苷酸RNA释放rn出来。在这项研究中，Narry Kim及其同事发rn现，Dicer含有一个与该前体的磷酸化的5’端进rn行结合的“凹穴”，而且这种解理事实上是通rn过计算距这一端的距离来测量的。Dicer的5‘rn“凹穴”的突变在活体中影响miRNA的处理，rn从而显示了这种计算机制的重要性。%A hallmark of RNA silencing is a class of approximately 22-nucleotide RNAs that are processed from double-stranded RNA precursors by Dicer. Accurate processing by Dicer is crucial for the functionality of microRNAs (miRNAs). The current model posits that Dicer selects cleavage sites by measuring a set distance from the 3' overhang of the double-stranded RNA terminus. Here we report that human Dicer anchors not only the 3' end but also the 5' end, with the cleavage site determined mainly by the distance (~22 nucleotides) from the 5' end (5' counting rule). This cleavage requires a 5' -terminal phosphate group. Further, we identify a novel basic motif (5' pocket) in human Dicer that recognizes the 5'-phosphorylated end. The 5' counting rule and the 5' anchoring residues are conserved in Drosophila Dicer-1, but not in Giardia Dicer. Mutations in the 5' pocket reduce processing efficiency and alter cleavage sites in vitro. Consistently, miRNA biogenesis is perturbed in vivo when Dicer-null embryonic stem cells are replenished with the 5'-pocket mutant. Thus, 5'-end recognition by Dicer is important for precise and effective biogenesis of miRNAs. Insights from this study should also afford practical benefits to the design of small hairpin RNAs.