Translation. Cinderella factors have a ball

Translation by ribosome shunting on adenovirus and hsp70 mRNAs facilitated by complementarity to 18S rRNA

Translation initiation on eukaryotic mRNAs involves 40S ribosome association with mRNA caps (m7GpppN), mediated by initiation factor eIF4F. 40S eukaryotic ribosomes and initiation factors undergo 5′ scanning to the initiation codon, with no known role for complementarity between eukaryotic 18S rRNA and the 5′ noncoding region of mRNAs. We demonstrate that the 5′ noncoding region of human adenovirus late mRNAs, known as the tripartite leader, utilizes a striking complementarity to 18S rRNA to facilitate a novel form of translation initiation referred to as ribosome shunting, in which 40S ribosomes bind the cap and bypass large segments of the mRNA to reach the initiation codon. Related elements are also shown to promote ribosome shunting in adenovirus IVa2 intermediate phase mRNA during virus infection and in human heat shock protein 70 (hsp70) mRNA for selective translation during heat shock. The importance of mRNA complementarity to 18S rRNA suggests that ribosome shunting may involve either specific RNA structural features or a prokaryotic-like interaction between mRNA and rRNA.

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).