Transcripts expressed using a bicistronic vector pIREShyg2 are sensitized to nonsense-mediated mRNA decay

Compared with wild-type βc, the mRNA levels of βc79 were less than one tenth and decayed faster. Both translation inhibition and Upf1 knockdown led to significantly greater up-regulation of βc79 than wild-type βc. However, the use of a monocistronic pMT21 vector abolished the up-regulatory effects of translation inhibition and Upf1 knockdown on both wild-type βc and βc79, suggesting that the NMD is attributable to a structural determinant in pIREShyg2. The elimination of the intron and the proximal 3' 17 PTCs did not alter the greater effects of translation inhibition on βc79, suggesting that the first PTC, which determines 3'UTR length, was sufficient to enhance NMD efficiency. Thus, transcripts of PTC-harboring genes with longer 3'UTR are more efficiently degraded by the vector-dependent NMD than those of wild-type genes with relatively shorter 3'UTR, resulting in minimized expression of truncated mutants.We conclude that pIREShyg2, which sensitizes its bicistronic transcripts to NMD, may be useful for studying NMD but should be avoided when maximum expressions of PTC-harboring genes are required.Expression vectors containing an internal ribosome entry site (IRES) element have been widely used as bicistronic vectors that provide co-expression of two unrelated reading frames from a single transcript unit [1-6]. A reading frame in a multiple cloning site downstream of a promoter is called the first cistron, and the second cistron is downstream of an IRES element. pIREShyg2 is a bicistronic expression vector that possesses an intervening sequence (IVS) between the first cistron and an IRES element derived from encephalomyocarditis virus, and a hygromycin resistance gene in the second cistron, which serves as a selection marker for stable transfection. It has been shown that the first cistron gene is expressed at levels comparable to those achieved in a monocistronic vector and initiation of translation is cap-dependent [7]. However, the present study is the first to