Torsional restraint: a new twist on frameshifting pseudoknots

mRNA pseudoknots have a stimulatory function in programmed −1 ribosomal frameshifting (−1 PRF). Though we previously presented a model for how mRNA pseudoknots might activate the mechanism for −1 PRF, it did not address the question of the role that they may play in positioning the mRNA relative to the ribosome in this process [E. P. Plant, K. L. M. Jacobs, J. W. Harger, A. Meskauskas, J. L. Jacobs, J. L. Baxter, A. N. Petrov and J. D. Dinman (2003) RNA, 9, 168–174]. A separate ‘torsional restraint’ model suggests that mRNA pseudoknots act to increase the fraction of ribosomes directed to pause with the upstream heptameric slippery site positioned at the ribosome's A- and P-decoding sites [J. D. Dinman (1995) Yeast, 11, 1115–1127]. Here, experiments using a series of ‘pseudo-pseudoknots’ having different degrees of rotational freedom were used to test this model. The results of this study support the mechanistic hypothesis that −1 ribosomal frameshifting is enhanced by torsional resistance of the mRNA pseudoknot.