The Evolutionarily Conserved AU-Dinucleotide at the 5’ End of U1 snRNA is Critical for the Biogenesis and Functionality of U1 snRNP

Splicing is initiated by a productive interaction of pre-mRNA and the U1 snRNP, which dictates the formation a short RNA duplex between the 5’ splice site of a pre-mRNA and the 5’ end of the U1 snRNA. A long-standing puzzle has been why the AU dincucleotide at the 5’-end of the U1 snRNA shows strict conservation, despite the absence of an apparent role in duplex formation. To explore this conundrum, we varied this AU dinucleotide into to all possible permutations and analyzed the resulting molecular, biochemical, and physiological consequences. This led to the findings that the AU dinucleotide governs the precision of transcription start site, the methylation status of the U1 snRNA 5’-cap, appropriate maturation of a functional U1 snRNP, and its subsequent utilization in the splicing pathway. Our data also provide an insight as to why the identity of the AU dinucleotide is strongly favored during evolution.剪接反应由前体mRNA（pre-mRNA）与U1小核糖核蛋白颗粒（U1 snRNP）的有效相互作用启动，该相互作用介导前体mRNA的5’剪接位点与U1小核RNA（U1 snRNA）5’端形成一段短RNA双链体。长期以来存在一个未解之谜：尽管U1 snRNA 5’端的AU二核苷酸在双链体形成过程中并无明确作用，却为何始终保持高度的序列保守性。为探究这一难题，我们将该AU二核苷酸替换为所有可能的二核苷酸组合，并分析了由此产生的分子、生化及生理效应。研究结果表明，AU二核苷酸可调控转录起始位点的精确性、U1 snRNA 5’帽的甲基化状态、功能性U1 snRNP的正常成熟，以及其后续在剪接通路中的功能利用。本研究数据还为解释AU二核苷酸的序列在进化过程中为何受到强烈偏好提供了新的见解。