PDF(Pubmed)
Abstract：
It is commonly held that there is a fundamental relationship between genome size and error rate, manifest as a notional \"error threshold\" that sets an upper limit on genome sizes. The genome sizes of RNA viruses, which have intrinsically high mutation rates due to a lack of mechanisms for error correction, must therefore be small to avoid accumulating an excessive number of deleterious mutations that will ultimately lead to population extinction. The proposed exceptions to this evolutionary rule are RNA viruses from the order Nidovirales (such as coronaviruses) that encode error-correcting exonucleases, enabling them to reach genome lengths greater than 40 kb. The recent discovery of large-genome flavi-like viruses (Flaviviridae), which comprise genomes up to 27 kb in length yet seemingly do not encode exonuclease domains, has led to the proposal that a proofreading mechanism is required to facilitate the expansion of nonsegmented RNA virus genomes above 30 kb. Herein, we describe a ~40 kb flavi-like virus identified in a Haliclona sponge metatranscriptome that does not encode a known exonuclease. Structural analysis revealed that this virus may have instead captured cellular domains associated with nucleic acid metabolism that have not been previously found in RNA viruses. Phylogenetic inference placed this virus as a divergent pesti-like lineage, such that we have provisionally termed it \"Maximus pesti-like virus.\" This virus represents an instance of a flavi-like virus achieving a genome size comparable to that of the Nidovirales and demonstrates that RNA viruses have evolved multiple solutions to overcome the error threshold.
摘要：
通常认为基因组大小和错误率之间存在基本关系，显示为概念上的“错误阈值”，它设置了基因组大小的上限。RNA病毒的基因组大小,由于缺乏纠错机制，它们具有固有的高突变率，因此必须很小，以避免积累过多的有害突变，最终导致种群灭绝。该进化规则的提议例外是来自Nidovirales顺序的RNA病毒（例如冠状病毒），其编码纠错核酸外切酶，使它们能够达到大于40kb的基因组长度。最近发现的大基因组黄病毒（黄病毒科），它们包含长度高达27kb的基因组，但似乎不编码外切核酸酶结构域，提出了这样的建议，即需要一种校对机制来促进30kb以上的非分段RNA病毒基因组的扩展。在这里，我们描述了在Haliclona海绵转移基因组中鉴定出的约40kb的类黄酮病毒，该病毒不编码已知的外切核酸酶。结构分析显示，该病毒可能已经捕获了与核酸代谢相关的细胞结构域，这些结构域以前在RNA病毒中没有发现过。系统发育推断将这种病毒作为一个不同的pesti-like谱系，所以我们暂时称之为“马克西姆斯pesti样病毒”。“这种病毒代表了一种类黄酮病毒的实例，其基因组大小与Nidovirales相当，并表明RNA病毒已经进化出多种解决方案来克服错误阈值。
