Disparate end structures of leading and lagging telomeres in Saccharomyces cerevisiae dictate the nature of end replication problem

The end replication problem refers to the incomplete replication of parental DNA at telomeres, a process whose molecular depiction is hampered by the complex nature of telomere ends. Here we recapitulate this process using a synthetic de novo telomere in Saccharomyces cerevisiae and delineate distinct molecular fates of telomere ends in vivo. We show that the lagging strand telomere carries a ~10 nucleotides 3' overhang, while the leading strand telomere has a Yku-protected blunt end, a feature that is prevalent on native telomeres. Additionally, RNase H2 is primarily responsible for removing the terminal RNA primer. Consistently, the absence of RNase H2 activity results in the retention of the RNA primer on the lagging strand telomere, attenuating telomere erosion and delaying senescence in telomerase-null cells. These findings highlight incongruent end structures on yeast telomeres and clarify that the primary culprit behind end replication problem is the incompletely replicated lagging strand telomere.

端粒末端复制问题（end replication problem）指的是亲本DNA在端粒（telomeres）处复制不完全的现象，由于端粒末端结构复杂，该过程的分子机制解析长期受阻。本研究通过在酿酒酵母（Saccharomyces cerevisiae）中引入人工从头合成的端粒，复现了这一过程，并阐明了体内端粒末端的不同分子结局。我们发现，后随链端粒带有长度约10个核苷酸的3'单链悬突，而前导链端粒则具有受Yku蛋白保护的平末端，这一特征在天然端粒中普遍存在。此外，核糖核酸酶H2（RNase H2）主要负责切除末端RNA引物。与之相符的是，核糖核酸酶H2活性缺失会导致RNA引物在后随链端粒上滞留，从而减轻端粒侵蚀并延缓端粒酶缺陷细胞的衰老进程。本研究结果揭示了酿酒酵母端粒末端结构的异质性，并阐明了端粒末端复制问题的核心诱因是复制不完全的后随链端粒。