Davies C, Haseloff J, Symons R H
Department of Biochemistry, University of Adelaide, South Australia.
Virology. 1990 Jul;177(1):216-24. doi: 10.1016/0042-6822(90)90475-7.
Both the genomic and viroid-like satellite RNAs (virusoids) from four subterranean clover mottle virus isolates described by Francki et al. were analyzed in detail. Restriction endonuclease mapping of cDNAs prepared from the genomic RNAs from all isolates showed that these RNAs are closely related if not identical. The two virusoids, which can occur together in the same isolate or individually, were sequenced and shown to be able to form highly base-paired viroid-like secondary structures. The left-hand portions of these structures are almost entirely homologous but the right-hand portions show little similarity. The plus, but not the minus, virusoid RNAs contain sequences that can form the hammerhead self-cleavage structure of certain other self-cleaving viroid, virusoid, and satellite RNAs. Plus, but not minus, RNA transcripts from cDNA clones self-cleaved essentially to completion at the predicted site during transcription in vitro. Northern blot analysis of infected leaf tissue extracts revealed the presence of an oligomeric series of plus RNAs (of monomer size and greater) but minus RNAs were present only as high molecular weight species of heterogeneous size. These findings are in agreement with the lack of minus RNA self-cleavage in vitro. Hence, these virusoid RNAs appear to replicate by a rolling-circle mechanism in which only the plus RNAs self-cleave to form monomeric RNAs.
对Francki等人描述的4种地下三叶草斑驳病毒分离株的基因组RNA和类病毒卫星RNA（病毒卫星）进行了详细分析。对从所有分离株的基因组RNA制备的cDNA进行限制性内切酶图谱分析表明，这些RNA即使不完全相同也密切相关。两种病毒卫星可以在同一分离株中同时出现或单独出现，对其进行了测序，并显示能够形成高度碱基配对的类病毒二级结构。这些结构的左侧部分几乎完全同源，但右侧部分几乎没有相似性。正链病毒卫星RNA（而非负链）含有能够形成某些其他自我切割类病毒、病毒卫星和卫星RNA的锤头状自我切割结构的序列。来自cDNA克隆的正链RNA转录本（而非负链）在体外转录过程中基本上在预测位点自我切割至完全。对感染叶片组织提取物的Northern印迹分析显示存在一系列寡聚正链RNA（单体大小及更大），但负链RNA仅以大小不均一的高分子量形式存在。这些发现与负链RNA在体外缺乏自我切割一致。因此，这些病毒卫星RNA似乎通过滚环机制进行复制，其中只有正链RNA自我切割形成单体RNA。
