Starved Saccharomyces cerevisiae cells have the capacity to support internal initiation of translation

Internal initiation of translation, whereby ribosomes are directed to inter nal AUG codon independently of the 5' end of the mRNA, has been observed ra rely in higher eucaryotes and has not been demonstrated in living yeast. We report here that starved yeast cells are capable of initiating translation of a dicistronic message internally. The studied element that functions as an internal ribosome entry site (IRES) is hardly functional or not functio nal at all in logarithmically growing cells, Moreover, during the logarithm ic growth phase, this element seems to inhibit translation reinitiation whe n placed as an intercistronic spacer or to inhibit translation when placed in the 5'-untranslated region of a monocistronic message. Inhibition of tra nslation is likely due to the putative strong secondary structure of the IR ES that interferes with the cap-dependent scanning process. When cells exit the logarithmic growth phase, or when artificially starved for carbon sour ce, translation of the IRES-containing messages is substantially induced. O ur findings imply that the capacity to translate internally is a characteri stic of starved rather than vegetatively growing yeast cells.