Eukaryotic ribosomal protein RPS25 interacts with the conserved loop region in a dicistroviral intergenic internal ribosome entry site

The intergenic region-internal ribosome entry site (IGR-IRES) of dicistroviruses binds to 40S ribosomal subunits in the absence of eukaryotic initiation factors (eIFs). Although the conserved loop sequences in dicistroviral IGR-IRES elements are protected from chemical modifications in the presence of the 40S subunit, molecular components in the 40S subunit, which interacts with the loop sequences in the IRES, have not been identified. Here, a chemical crosslinking study using 4-thiouridine-labeled IGR-IRES revealed interactions of the IGR-IRES with several 40S proteins but not with the 18S rRNA. The strongest crosslinking signal was identified for ribosomal protein S25 (rpS25). rpS25 is known to be a neighbor of rpS5, which has been shown to interact with a related IGR-IRES by cryo-electron microscopy. Crosslinking analysis with site-directed mutants showed that nucleotides UU(6089-6090), which are located in the loop region in conserved domain 2b in the IRES, appear to interact with rpS25. rpS25 is specific to eukaryotes, which explains why there is no recognition of the IGR-IRES by prokaryotic ribosomes. Although the idea that the IGR-IRES element may be a relict of a primitive translation system has been postulated, our experimental data suggest that the IRES has adapted to eukaryotic ribosomal proteins.