The translational apparatus very efficiently eliminates
errors that would cause a spontaneous shift in frames.
The probability of frameshifting can be increased dramatically
by either cis or trans-acting factors.
Programmed translational frameshift sites are cis-acting
sequences that greatly increase the frequency of such errors,
at least in part by causing a transient translational pause.
Pausing during programmed +1 frameshifts occurs because
of slow recognition of the codon following the last read
in the normal frame. Frameshifting can also be elevated
in strains carrying mutations in the homologous elongation
factors EF-Tu in bacteria, and EF-1α in the yeast Saccharomyces
cerevisiae. This phenotype implies that the factors
contribute to frame maintenance. Because EF-Tu/EF-1α
modulate the kinetics of decoding, it is possible that
the frameshift suppressor forms of the factors transiently
slow normal decoding, allowing spontaneous frameshifting
to occur more efficiently, resulting in phenotypic suppression.
We have used a set of frameshift reporter plasmids to test
the effect of suppressor forms of EF-1α on constructs
that differ widely in the efficiency with which they stimulate
+1 shifting. When these results were compared to the effect
of increased translational pausing, it was apparent that
the mutations affecting EF-1α do not simply prolong
the translational pause. Rather, they appear to generally
increase the likelihood of frame errors, apparently by
affecting the error correction mechanism of the ribosome.