Splicing mediates the activity of four putative cellular internal ribosome entry sites.

A growing number of cellular mRNAs are thought to possess internal ribosome entry sites (IRESs), sequences that permit translation of a transcript independent of its 5' end and cap structure. Although dicistronic assays are the canonical method of testing sequences for IRES activity, they may produce false-positive results if unanticipated monocistronic RNAs arise from the dicistronic construct used. Using a dicistronic reporter system and a green fluorescent protein-tagged retrovirus to evaluate six previously reported cellular IRESs, we found that four contain 3' splice sites whose activity was required for apparent IRES function and which resulted in formation of monocistronic transcripts by splicing. Bioinformatic analysis revealed that the 3' splice sites identified in three of these putative IRESs are used in their native mRNAs and that the fourth is likely an artifactual sequence created during cDNA cloning. Our findings demonstrate a need for reexamination of other reported cellular IRESs by using careful RNA structural analysis to rule out splicing as the source of perceived IRES activity.