Altered tRNA dynamics during translocation on slippery mRNA as determinant of spontaneous ribosome frameshifting

Slippery sequences in mRNA can cause the ribosome to change its reading frame. Using smFRET, Poulis et al. show how reversible fluctuations of peptidyl-tRNA slow down translocation, alter ribosome dynamics, and favor spontaneous ribosome frameshifting. When reading consecutive mRNA codons, ribosomes move by exactly one triplet at a time to synthesize a correct protein. Some mRNA tracks, called slippery sequences, are prone to ribosomal frameshifting, because the same tRNA can read both 0- and -1-frame codon. Using smFRET we show that during EF-G-catalyzed translocation on slippery sequences a fraction of ribosomes spontaneously switches from rapid, accurate translation to a slow, frameshifting-prone translocation mode where the movements of peptidyl- and deacylated tRNA become uncoupled. While deacylated tRNA translocates rapidly, pept-tRNA continues to fluctuate between chimeric and posttranslocation states, which slows down the re-locking of the small ribosomal subunit head domain. After rapid release of deacylated tRNA, pept-tRNA gains unconstrained access to the -1-frame triplet, resulting in slippage followed by recruitment of the -1-frame aa-tRNA into the A site. Our data show how altered choreography of tRNA and ribosome movements reduces the translation fidelity of ribosomes translocating in a slow mode.

Automated summary

Slippery sequences in mRNA can cause the ribosome to change its reading frame, affecting the translation process. Research has shown that during translocation on slippery sequences, the ribosome switches from a fast and accurate translation mode to a slow and frameshifting-prone mode, reducing translation fidelity.