How translational accuracy influences reading frame maintenance.

Most missense errors have little effect on protein function, since they only exchange one amino acid for another. However, processivity errors, frameshifting or premature termination result in a synthesis of an incomplete peptide. There may be a connection between missense and processivity errors, since processivity errors now appear to result from a second error occurring after recruitment of an errant aminoacyl-tRNA, either spontaneous dissociation causing premature termination or translational frameshifting. This is clearest in programmed translational frameshifting where the mRNA programs errant reading by a near-cognate tRNA; this error promotes a second frameshifting error (a dual-error model of frameshifting). The same mechanism can explain frameshifting by suppressor tRNAs, even those with expanded anticodon loops. The previous model that suppressor tRNAs induce quadruplet translocation now appears incorrect for most, and perhaps for all of them. We suggest that the 'spontaneous' tRNA-induced frameshifting and 'programmed' mRNA-induced frameshifting use the same mechanism, although the frequency of frameshifting is very different. This new model of frameshifting suggests that the tRNA is not acting as the yardstick to measure out the length of the translocation step. Rather, the translocation of 3 nucleotides may be an inherent feature of the ribosome.