Sequences required for self-catalysed cleavage of the satellite RNA of tobacco ringspot virus.

The satellite RNA of tobacco ringspot virus (sTobRV) undergoes self-catalysed cleavage during replication. A plasmid for in vitro expression of sTobRV has been constructed and used to obtain a library of mutagenized sTobRV sequences. Screening of these mutants has allowed precise definition of the sequences required for (+) and (-) strand cleavage. The sequences and RNA structures associated with cleavage of each strand differ markedly. Cleavage of the (+) strand requires those sequences flanking the site for cleavage to form a 'hammerhead' domain, similar to those found in other satellite and viroid RNA. In contrast, cleavage of the (-) strand requires only a small region of 12 nucleotides (nt) at the site of cleavage, and a sequence of 55 nt positioned elsewhere in the molecule. Comparison with a closely related satellite suggests that a novel RNA structure may be involved in (-) strand cleavage.