How translational accuracy influences reading frame maintenance

Most missense errors have little effect on protein function, since they onl y exchange one amino acid for another. However, processivity errors, frames hifting or premature termination result in a synthesis of an incomplete pep tide. There may be a connection between missense and processivity errors, s ince processivity errors now appear to result from a second error occurring after recruitment of an errant aminoacyl-tRNA, either spontaneous dissocia tion causing premature termination or translational frameshifting, This is dearest in programmed translational frameshifting where the mRNA programs e rrant reading by a near-cognate tRNA; this error promotes a second frameshi fting error (a dual-error model of frameshifting). The same mechanism can e xplain frameshifting by suppressor tRNAs, even those with expanded anticodo n loops. The previous model that suppressor tRNAs induce quadruplet translo cation now appears incorrect for most, and perhaps for all of them. We sugg est that the 'spontaneous' tRNA-induced frameshifting and 'programmed' mRNA -induced frameshifting use the same mechanism, although the frequency of fr ameshifting is very different. This new model of frameshifting suggests tha t 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 in herent feature of the ribosome.