Kiefer M C, Owens R A, Diener T O
Proc Natl Acad Sci U S A. 1983 Oct;80(20):6234-8. doi: 10.1073/pnas.80.20.6234.
The primary structures of the tomato planta macho and tomato apical stunt viroids have been determined, and probable secondary structures are proposed. Both viroids can assume the rodlike conformation with extensive base-pairing characteristic of all known viroids. Sequence homologies between the two viroids (75%) and with members of the potato spindle tuber viroid group (73-83%) indicate that they both belong to this group. Comparative sequence analysis of all members of the group reveals striking similarities with the ends of transposable genetic elements. These similarities, the presence of inverted repeats often ending with the dinucleotides U-G and C-A, and flanking imperfect direct repeats suggest that viroids may have originated from transposable elements or retroviral proviruses by deletion of interior portions of the viral (or element) DNA.
番茄雄性不育和番茄顶端矮化类病毒的一级结构已被确定，并提出了可能的二级结构。两种类病毒都能呈现出所有已知类病毒特有的具有广泛碱基配对的棒状构象。这两种类病毒之间的序列同源性（75%）以及与马铃薯纺锤块茎类病毒组成员的序列同源性（73 - 83%）表明它们都属于该组。对该组所有成员的比较序列分析揭示了与可移动遗传元件末端的惊人相似性。这些相似性、通常以二核苷酸U - G和C - A结尾的反向重复序列的存在以及侧翼不完美的直接重复序列表明，类病毒可能是通过删除病毒（或元件）DNA的内部部分而从可移动元件或逆转录病毒前病毒起源的。
