Internal ribosome entry sites (IRESs): reality and use

IRESs are known to recruit ribosomes directly, without a previous scanning of untranslated region of mRNA by the ribosomes. IRESs have been found in a number of viral and cellular mRNAs. Experimentally, IRESs are commonly use d to direct the expression of the second cistrons of bicistronic mRNAs. The mechanism of action of IRESs is not fully understood and a certain number of laboratories were not successful in using them in a reliable manner. Thr ee observations done in our laboratory suggested that IRESs might not work as functionally as it was generally believed. Stem loops added before IRESs inhibited mRNA translation. When added into bicistronic mRNAs, IRESs initi ated translation of the second cistrons efficiently only when the intercist ronic region contained about 80 nucleotides, and they did not work any more effectively with intercistronic regions containing at least 300-400 nucleo tides. Conversely, IRESs inserted at any position into the coding region of a cistron interrupted its translation and initiated translation of the fol lowing cistron. The first two data are hardly compatible with the idea that IRESs are able to recruit ribosomes without using the classical scanning m echanism. IRESs are highly structured and cannot be scanned by the 40S ribo somal subunit. We suggest that IRESs are short-circuited and are essentiall y potent stimulators favoring translation in particular physiological situa tions.