The optimal use of IRES (internal ribosome entry site) in expression vectors

In higher eucaryotes, natural bicistronic mRNA have been rarely found so far. The second cistron of constructed bicistronic mRNAs is generally considered as not translated unless special sequences named internal ribosome entry site (IRES) are added between the two cistrons. These sequences are believed to recruit ribosomes independently of a cap structure. In the present report, a new IRES found in the HTLV-1 genome is described. A systematic study revealed that this IRES, but also the poliovirus (polio) and the encephalomyocarditis virus (EMCV) IRES work optimally when they are added about 100 nucleotides after the termination codon of the first cistron. Unexpectedly, these IRES became totally inefficient when added after 300-500 nucleotide spacers. This result and others are not compatible with the admitted mechanism of IRES action. The IRES appear to be rather potent translation stimulators. Their effects are particularly emphasized in cells in which the normal mechanism of translation initiation is inhibited. For these reasons, we suggest to call IRES rescue translation stimulators (RTS).