中文摘要：

在大多数生物体中,核遗传密码是通用的。真核生物核基因组中已发现的非标准遗传密码的使用非常少。大多数非标准遗传密码通常是将1种或者2种终止密码子重新分配为有义密码子,至少保留1种密码子作为翻译终止信号。然而,近期有研究发现,在2种纤毛虫中,所有3种终止密码子既可编码氨基酸,又可作为翻译终止信号;此外,基于转录组的分析表明,在游仆虫的读码框内终止密码子处存在广泛的编程性核糖体移码现象。这些发现提示,终止密码子具有多种解读方式,其翻译终止过程可能依赖某些未知的调控元件。本文基于近期发现的纤毛虫中终止密码子模糊使用的现象,重点讨论了这些生物区分有义＂终止＂密码子和真正终止密码子的分子机制。对于这些生物体中终止密码子使用的特殊性及翻译终止的研究,将有助于深入理解真核生物中的翻译终止及基因表达调控的分子机制。

英文摘要：

The nuclear genetic code is universal in the vast majority of living organisms. Rare noncanonical genetic codes have been described in eukaryotic nuclear genomes. Most non-canonical genetic codes reassign one or two stop codons as sense ones,and have at least one codon encoding the translation terminal signal. However,a recent study revealed that in two ciliates all three stop codons encode amino acids as well as signal for translation termination. Furthermore,a transcriptome-based analysis revealed that Euplotes exhibit widespread programmed ribosomal frameshifting at in-frame stop codons. These findings imply that there is more than one way to read stop codons and that translation termination depends on molecular factors that we do not entirely understand. In this review,we examine evidence for ambiguous stop codon usage in ciliates, with a focus on the possible molecular mechanisms thatdifferentiate true stop codons from ＂sense＂stop codons. The results obtained by studying these amazing phenomena will provide insights into translation termination and gene regulation in all eukaryotes.