RESUMEN
Ty3 elements of S. cerevisiae contain two overlapping coding regions, GAG3 and POL3, which are functional homologues of retroviral gag and pol genes, respectively. Pol3 is translated as a Gag3-Pol3 fusion protein dependent on a +1 programmed frameshift at a site with the overlap between the two genes. We show that the Ty3 frameshift frequency varies up to 10-fold in S. cerevisiae cells depending on carbon source. Frameshift efficiency is significantly lower in cells growing on glucose as carbon source than in cells growing on poor alternative carbon sources (glycerol/lactate or galactose). Our results indicate that Ty3 programmed ribosomal frameshift efficiency in response to glucose signalling requires two protein kinases: Snf1p and cAMP-dependent protein kinase A (PKA). Increased frameshifting on alternative carbon sources also appears to require cytoplasmic localization of Snf1p, mediated by the Sip2p protein. In addition to the two required protein kinases, our results implicate that Stm1p, a ribosome-associated protein involved in nutrient sensing, is essential for the carbon source-dependent regulation of Ty3 frameshifting. These data indicate that Ty3 programmed ribosomal frameshift is not a constitutive process but that it is regulated in response to the glucose-signalling pathway.