Structural similarities between viroids and transposable genetic elements.

Abstract

The primary structures of the tomato planta macho and tomato apical stunt viroids were determined, and probable secondary structures are proposed. Both virods can assume the rodlike conformation with extensive base-pairing characteristic of all known viroids. Sequence homologies between the 2 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 similarties, 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.