Instituto de Biología Molecular y Celular de Plantas (UPV-CSIC), Valencia, Spain.
Semin Liver Dis. 2012 Aug;32(3):201-10. doi: 10.1055/s-0032-1323624. Epub 2012 Aug 29.
There is a subviral world, whose most prominent representatives are viroids. Despite being solely composed by a circular, highly structured RNA of 250 to 400 nucleotides without protein-coding ability (all viruses code for one or more proteins), viroids can infect and incite specific diseases in higher plants. The RNA of human hepatitis delta virus (HDV), the smallest genome of an animal virus, displays striking similarities with viroids: It is circular, folds into a rodlike secondary structure, and replicates through a rolling-circle mechanism catalyzed by host enzymes and cis-acting ribozymes. However, HDV RNA is larger (1700 nucleotides), encodes a protein in its antigenomic polarity (the ∂ antigen), and depends for transmission on hepatitis B virus. The presence of ribozymes in some viroids and in HDV RNA, along with their structural simplicity, makes them candidates for being molecular fossils of the RNA world that presumably preceded our extant world based on DNA and proteins.
存在一个亚病毒世界，其最突出的代表是类病毒。尽管类病毒仅由一条约 250 到 400 个核苷酸组成的、环状且高度结构的 RNA 组成，并无编码蛋白的能力（所有病毒都编码一个或多个蛋白），但类病毒可以感染高等植物并引发特定疾病。人类乙型肝炎病毒相关的δ病毒（HDV）是动物病毒中基因组最小的，其 RNA 与类病毒显示出惊人的相似性：它是环状的，折叠成棒状的二级结构，并通过宿主酶和顺式作用核酶催化的滚环机制进行复制。然而，HDV RNA 更大（约 1700 个核苷酸），在其反义基因组极性上编码一种蛋白（∂ 抗原），并依赖乙型肝炎病毒进行传播。一些类病毒和 HDV RNA 中存在核酶，以及它们的结构简单性，使它们成为 RNA 世界的分子化石的候选者，而我们现存的基于 DNA 和蛋白质的世界可能是从 RNA 世界进化而来的。
