ALTERNATIVE TERTIARY STRUCTURE ATTENUATES SELF-CLEAVAGE OF THE RIBOZYME IN THE SATELLITE RNA OF BARLEY YELLOW DWARF VIRUS

A self-cleaving satellite RNA associated with barley yellow dwarf virus (sBYDV) contains a sequence predicted to form a secondary structure similar to catalytic RNA molecules (ribozymes) of the 'hammerhead' class (Miller et al., 1991, Virology 183, 711 - 720). However, this RNA differs from other naturally occurring hammerheads both in its very slow cleavage rate, and in some aspects of its structure. One striking structural difference is that an additional helix is predicted that may be part of an unusual pseudoknot containing three stacked helices. Nucleotide substitutions that prevent formation of the additional helix and favor the hammerhead increased the self-cleavage rate up to 400-fold. Compensatory substitutions, predicted to restore the additional helix, reduced the self-cleavage rate by an extent proportional to the calculated stability of the helix. Partial digestion of the RNA with structure-sensitive nucleases supported the existence of the proposed alternative structure in the wildtype sequence, and formation of the hammerhead in the rapidly-cleaving mutants. This tertiary interaction may serve as a molecular switch that controls the rate of self-cleavage and possibly other functions of the satellite RNA.