EFFECT OF FRAMESHIFT-INDUCING MUTANTS OF ELONGATION-FACTOR 1-ALPHA ONPROGRAMMED +1 FRAMESHIFTING IN YEAST

The translational apparatus very efficiently eliminates errors that wo uld cause a spontaneous shift ln frames. The probability of frameshift ing can be increased dramatically by either cis or trans-acting factor s. Programmed translational frameshift sites are cis-acting sequences that greatly increase the frequency of such errors, at least in part b y causing a transient translational pause. Pausing during programmed 1 frameshifts occurs because of slow recognition of the codon followin g the last read in the normal frame. Frameshifting can also be elevate d in strains carrying mutations in the homologous elongation factors E F-Tu in bacteria, and EF-1 alpha in the yeast Saccharomyces cerevisiae . This phenotype implies that the factors contribute to frame maintena nce, Because EF-Tu/EF-1 alpha modulate the kinetics of decoding, it is possible that the frameshift suppressor farms of the factors transien tly 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 f orms of EF-1 alpha on constructs that differ widely in the efficiency with which they stimulate +1 shifting. When these results were compare d to the effect of increased translational pausing, it was apparent th at the mutations affecting EF-1 alpha do not simply prolong the transl ational pause. Rather, they appear to generally Increase the likelihoo d of frame errors, apparently by affecting the error correction mechan ism of the ribosome.