Internal Initiation of Translation of mRNA in the Methylotrophic Yeast Hansenula polymorpha

Besides regular cap-dependent translation of mRNA, eukaryotes exploit internal initiation of translation driven by internal ribosome entry sites (IRESs). It is supposed that internal initiation provides translation of cellular mRNAs under stress conditions where the cap-dependent initiation is reduced. A number of IRESs have been characterized in mammalian mRNAs, but only a few examples are known in lower eukaryotes, particularly in yeasts. Here we identified two IRESs in the thermotolerant methylotrophic yeast Hansenula polymorpha DL-1. These sites are located in 5'-untranslated regions of genes HPODL_02249 and HPODL_04025 encoding a hypothetical membrane protein and actin-binding protein, respectively. In Saccharomyces cerevisiae cells, both IRESs drive expression of a second gene of a bicistronic mRNA, as well as translation of hairpin-containing monocistronic mRNA. The possibility of spurious splicing or presence of a cryptic promoter in the IRES sequences was ruled out, indicating that expression of a second gene of a bicistronic mRNA was IRES-dependent. We evaluated IRES activity of both elements and found that under normal physiological conditions its contribution to the overall translation of the respective mRNAs in yeast cells is about 0.3-0.4%. Therefore, these results suggest that the IRES-dependent translation initiation mechanism exists in Hansenula polymorpha.