The poly(A) signal, together with auxiliary elements, directs cleavage of a pre-mRNA and thus determines the 3′ end of the mature transcript. In many species, including humans, the poly(A) signal is an AAUAAA hexamer, but we recently found that the deeply branching eukaryote Giardia lamblia uses a distinct hexamer (AGURAA) and lacks any known auxiliary elements. Our discovery prompted us to explore the evolutionary dynamics of poly(A) signals and auxiliary elements in the eukaryotic kingdom. We use direct RNA sequencing to determine poly(A) signals for four protists within the Metamonada clade (which also contains G. lamblia) and two outgroup protists. These experiments reveal that the AAUAAA hexamer serves as the poly(A) signal in at least four different eukaryotic clades, indicating that it is likely the ancestral signal, whereas the unusual Giardia version is derived. We find that the use and relative strengths of auxiliary elements are also plastic; in fact, within Metamonada, species like G. lamblia make use of a previously unrecognized auxiliary element where nucleotides flanking the poly(A) signal itself specify genuine cleavage sites. Thus, despite the fundamental nature of pre-mRNA cleavage for the expression of all protein-coding genes, the motifs controlling this process are dynamic on evolutionary timescales, providing motivation for future biochemical and structural studies as well as new therapeutic angles to target eukaryotic pathogens.

中文翻译：

---

原生生物多聚腺苷酸化信号的进化动力学及其识别策略


poly（A） 信号与辅助元件一起指导前 mRNA 的切割，从而确定成熟转录本的 3' 端。在包括人类在内的许多物种中，poly（A） 信号是 AAUAAA 六聚体，但我们最近发现，深分支的真核生物 Giardia lamblia 使用不同的六聚体 （AGURAA），并且缺乏任何已知的辅助元件。我们的发现促使我们探索真核生物王国中 poly（A） 信号和辅助元件的进化动力学。我们使用直接 RNA 测序来确定 Metamonada 分支（也包含 G. lamblia）内四个原生生物和两个外群原生生物的 poly（A） 信号。这些实验表明，AAUAAA 六聚体在至少四个不同的真核生物分支中充当 poly（A） 信号，表明它可能是祖先信号，而衍生出不寻常的贾第鞭毛虫版本。我们发现辅助元素的用途和相对强度也是可塑性的;事实上，在 Metamonada 中，像 G. lamblia 这样的物种利用了以前未被识别的辅助元件，其中 poly（A） 信号侧翼的核苷酸本身指定了真正的切割位点。因此，尽管前体 mRNA 切割对所有蛋白质编码基因的表达具有基本性质，但控制这一过程的基序在进化时间尺度上是动态的，为未来的生化和结构研究提供了动力，并为靶向真核病原体提供了新的治疗角度。