Many viruses regulate translation of polycistronic mRNA using a -1 ribosomal frameshift induced by an RNA pseudoknot. A pseudoknot has two stems that form a quasi-continuous helix and two connecting loops. A 1.6 Å crystal structure of the beet western yellow virus (BWYV) pseudoknot reveals rotation and a bend at the junction of the two stems. A loop base is inserted in the major groove of one stem with quadruple-base interactions. The second loop forms a new minor-groove triplex motif with the other stem, involving 2'-OH and triple-base interactions, as well as sodium ion coordination. Overall, the number of hydrogen bonds stabilizing the tertiary interactions exceeds the number involved in Watson–Crick base pairs. This structure will aid mechanistic analyses of ribosomal frameshifting.
Minor groove RNA triplex in the crystal structure of a ribosomal frameshifting viral pseudoknot