The double IRES cassette system was used to examine suppression of RNA silencing mediated by siRNAs. Constructs were generated that contained one Venus gene at the 5′ end followed by one or two IRES cassettes with the p19 gene, a Tombusvirus suppressor of RNA silencing (VIp19 and V2Ip19) (Figure 1B). The dsVenus construct was made enabling transcription of doublestranded RNAs from a 256nt Venus sequence to induce RNA silencing of Venus, allowing us to examine whether IRES-mediated p19 protein protects its own mRNA
(VIp19 or V2Ip19) from Venus siRNAs. Controls for VIp19 and V2Ip19 were generated by replacing the start codon (ATG) of the p19 gene with a TTG codon to depress p19 translation (VI⊿p19 and V2I⊿p19). As a result of this modification, the longer V2Ip19 and V2I⊿p19 mRNAs were accumulated at lower amounts than the shorter VIp19 and VI⊿p19 mRNAs (Figure 3A). Western blot analysis of these constructs in the Nicotiana
benthamiana transient expression assay indicated that the amount of p19 protein from V2Ip19 was much higher than that from VIp19.