Stop codon readthrough contexts influence reporter expression differentially depending on the presence of an IRES

Abstract

Background: Previously we reported the discovery of stop codon readthrough in AMD1 mRNA followed by ribosome stalling at the end of a conserved Open Reading Frame (ORF) that we termed AMD1. To explain the severe suppression of reporters fused to AMD1 tail we proposed a mechanism invoking ribosome queueing. To test this hypothesis, we placed the reporter stop codon in the context of readthrough permissive sequences in a dual reporter vector with downstream reporter expression driven by the EMCV IRES. In accordance with our hypothesis, we observed a striking disproportional reduction of upstream reporter activity in response to increased readthrough levels. Methods: We employ dual luciferase assays, western blotting and RT-qPCR to explore the effects of test sequences downstream to the reporter stop codon on its expression in dual and monocistronic reporter vectors. Results:  With the dual reporter system, the disproportionate reduction of upstream reporter activity is not specific to AMD1 tail and occurs as long as the readthrough stop codon context is present at the end of the reporter’s ORF. In a monocistronic vector without an IRES, the test sequences had distinct effects which were reflective of their properties e.g. AMD1 tail inhibitory effect. We further show with RT-qPCR that the EMCV IRES driven expression of a reporter is an accurate proxy of reporter RNA levels.  Conclusions: While our findings provide little new information regarding the functional role of AMD1 tail, they raise caution for the use of viral IRES elements in expression vectors for studying mechanisms of mRNA translation. These findings may also be pertinent to the natural properties of read through permissive sequences and of IRES elements, though these require a separate investigation.