Internal initiation in Saccharomyces cerevisiae mediated by an initiator tRNA/eIF2-independent internal ribosome entry site element

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Abstract
Internal initiation of translation can be mediated by specific internal ribosome entry site (IRES) elements that are located in certain mammalian and viral mRNA molecules. Thus far, these mammalian cellular and viral IRES elements have not been shown to function in the yeast Saccharomyces cerevisiae. We report here that a recently discovered IRES located in the genome of cricket paralysis virus can direct the efficient translation of a second URA3 cistron in dicistronic mRNAs in S. cerevisiae, thereby conferring uracil-independent growth. Curiously, the IRES functions poorly in wild-type yeast but functions efficiently either in the presence of constitutive expression of the eIF2 kinase GCN2 or in cells that have two initiator tRNAmet genes disrupted. Both of these conditions have been shown to lower the amounts of ternary eIF2-GTP/initiator tRNAmet complexes. Furthermore, tRNAmet-independent initiation was also observed in translation-competent extracts prepared from S. cerevisiae in the presence of edeine, a compound that has been shown to interfere with start codon recognition by ribosomal subunits carrying ternary complexes. Therefore, the cricket paralysis virus IRES is likely to recruit ribosomes by internal initiation in S. cerevisiae in the absence of eIF2 and initiator tRNAmet, by the same mechanism of factor-independent ribosome recruitment used in mammalian cells. These findings will allow the use of yeast genetics to determine the mechanism of internal ribosome entry.