Daròs José-Antonio, Elena Santiago F, Flores Ricardo
Instituto de Biología Molecular y Celular de Plantas (CSIC-UPV), Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain.
EMBO Rep. 2006 Jun;7(6):593-8. doi: 10.1038/sj.embor.7400706.
Viroids are structurally, functionally and evolutionarily different from viruses. Despite their small, non-protein-encoding, single-stranded circular RNA genome, viroids can infect higher plants and cause certain diseases. Members of the two viroid families, Pospiviroidae and Avsunviroidae, have evolved to usurp the transcriptional machinery of their host nuclei and chloroplasts, respectively, in which replication proceeds through a rolling-circle mechanism involving RNA polymerization, cleavage and ligation. Remarkably, viroids subvert certain DNA-dependent RNA polymerases to transcribe RNA templates, and, in the family Avsunviroidae, post-transcriptional cleavage is catalysed by hammerhead ribozymes. Viroids are models for studying RNA evolution and for analysing RNA transport in plants, because they can move intracellularly, intercellularly through plasmodesmata and to distal parts of the plant through the vascular system. Viroids elicit RNA-silencing phenomena, which might mediate some of their biological properties, including pathogenesis. As some viroids behave as catalytic RNAs, they are regarded as remnants of the RNA world.
类病毒在结构、功能和进化上与病毒不同。尽管它们的基因组很小，是非蛋白质编码的单链环状RNA，但类病毒能感染高等植物并引发某些疾病。两个类病毒科，马铃薯纺锤块茎类病毒科和鳄梨日斑类病毒科的成员，已分别进化到篡夺其宿主细胞核和叶绿体的转录机制，在其中复制通过涉及RNA聚合、切割和连接的滚环机制进行。值得注意的是，类病毒颠覆某些依赖DNA的RNA聚合酶来转录RNA模板，并且在鳄梨日斑类病毒科中，转录后切割由锤头状核酶催化。类病毒是研究RNA进化和分析植物中RNA转运的模型，因为它们可以在细胞内移动，通过胞间连丝在细胞间移动，并通过维管系统到达植物的远端部分。类病毒引发RNA沉默现象，这可能介导它们的一些生物学特性，包括发病机制。由于一些类病毒表现为催化性RNA，它们被视为RNA世界的残余物。
