The majority of messenger RNAs (mRNAs) are translated by a cap-dependent mechanism of initiation. However, about 5% to 10% of mRNAs use an alternative mechanism of initiating protein synthesis, utilizing an internal ribosome entry site (IRES) to recruit the ribosomes to the message. IRESs are found in mRNAs that encode proteins involved in cell growth, proliferation, apoptosis, hypoxia, and angiogenesis. Because many cellular mRNAs that are involved in tumorigenesis contain IRESs, we anticipate that IRES-mediated translation is a good target for the development of anti-cancer therapies that will be broadly applicable to several types of cancer. Since the mechanism is not understood we are interested in how IRESs recruit ribosomes. Several studies have suggested that the cricket paralysis virus (CrPV) intergenic region IRES (IGR-IRES) and cellular IRESs share some mechanisms for binding ribosomes suggesting that the IGR-IRES is a good model IRES. Because translation is highly conserved between yeast and mammalian cells we have developed a genetic system in yeast to answer the following questions: 1) Which IGR-IRES structures or sequences are important for IRES function? 2) What components of the ribosome are required for IGR-IRES mediated translation? 3) How do these contacts facilitate ribosome binding? 4) What is the contribution of rRNA modifications for IRES activity? 5) How do these findings apply to cellular IRESs in mammalian cells? This system provides us with genetics, biochemistry, and will allow us to manipulate components of the translational machinery in ways that are not possible in mammalian cells. We chose to use an innovative genetic approach to understand IRES-mediated translation because the mechanism of initiation by cellular IRESs has been elusive despite many years of effort by multiple laboratories.
Some viral and cellular mRNAs utilize an alternative mechanism of translation initiation that recruits ribosomes internally to the message using a highly structured internal ribosome entry site (IRES). IRES-mediated translation of cellular mRNAs is involved in regulating cancer, cell death, cell growth, and angiogenesis, as well as translation of viral RNAs. We are using yeast genetics to understand the mechanism of IRES-mediated translation initiation.
