IRESs have been incorporated into virus vectors to facilitate polycistronic expression of multiple genes in eukaryotic cells (Thompson 2012). In particular, an IRES element derived from the coat protein gene of the crucifer Tobacco Mosaic Virus (cr-TMV) has
been used for polycistronic gene expression in plants (Ha et al. 2010; Skulachev et al. 1999). Yamamoto et al used the cr-TMV IRES to selectively express a firefly luciferase gene from a single mRNA that also encoded a Renilla luciferase gene positioned 5′ to the firefly gene (Yamamoto et al. 2003). RNA silencing, also referred to as RNA interference, involves post-transcriptional degradation of RNAs mediated by short (21–24nt) interfering RNAs (siRNAs) in a sequence-dependent manner (Baulcombe 2004).
RNA silencing is a host defense system against invading viruses (Pumplin and Voinnet 2013). To defeat this defense system, many plant viruses encode RNA silencing suppressors (Voinnet et al. 1999). The p19 protein of Tombusvirus, an RNA silencing suppressor that is widely used in transgenic experiments, binds viral siRNAs and abrogates siRNA-mediated degradation of viral RNAs (Vargason et al. 2003). Until now, the suppressor activity of the p19 protein has been used to maintain expression of genes by co-expression of p19 as a separate mRNA (Voinnet et al. 2003). In this report, we first attempted to improve IRESmediated polycistronic expression by a transient
expression system using Nicotiana benthamiana. In addition, we established a novel mRNA self-protection system from RNA silencing using two IRES cassettes harboring the p19 gene.