The downstream RNA stimulator is usually a hairpintype RNA pseudoknot in which nucleotides from a hairpin loop form base pairs with a single-stranded region outside the hairpin. It leads to a quasi-continuous RNA double-helical structure, with a topology featuring two helical stems of the base-pairing region (stems 1 and 2) connected by two single-stranded loops (loops 1 and 2) (16) . It is thought that the resistance of a stimulator against deformation by the ribosome helicase activity can cause the marching ribosome to pause. However, RNA hairpin not capable of stimulating À1 PRF also causes the ribosome to pause (13, 14, 17) . As unwinding of stem 1 by ribosome will require rotation of the rest of the pesudoknot, torsional restraint hypothesis suggests that pseudoknot topology can restrain loop rotation during *To whom correspondence should be addressed. Tel: +886 4 22840468 (ext. 218); Fax: +886 4 22853487; Email: kychang@dragon.nchu.edu.tw the unwinding of stem 1, and thus interferes with this process (18) . Interestingly, only a subset of RNA pseudoknots can stimulate À1 PRF and non-pseudoknot RNA elements have been characterized to induce À1 PRF (19) (20) (21) , implicating the existence of uncharacterized determinants. In addition, the involvement of specific interactions between pseudoknot stimulator and ribosomal components during the unwinding process cannot be ruled out (22, 23) .