Northern blot analysis showed that Venus siRNAs arisen from the dsVenus accumulated at comparable level irrespective of co-infection with VIp19, VI⊿p19, V2Ip19 or V2I⊿p19, comfirming induction of RNA silencing by the dsVenus (Figure 3C). Quantitative RTPCR
analysis indicated that co-infection of the dsVenus construct decreased Venus accumulation in VIp19 and VI⊿p19 to half of that without dsVenus (Figure 3D). This result may indicate that neither VIp19 nor VI⊿p19 mRNAs were protected from RNA silencing. In contrast, co-infection of dsVenus decreased the accumulation of Venus in both V2I⊿p19 and V2Ip19 but the reduction was less in V2Ip19 than in V2I⊿p19 (Figure 3E). We confirmed these results by microscopic examination. After co-infection with dsVenus, fluorescence of Venus from V2Ip19 was clearly observed while weaker fluorescence was seen in VIp19 (Figure 3F). When p19 protein translation was inhibited by replacing the first codon with TTG (V2I⊿p19) we could observe very low fluorescence at the same level as that of VIp19 when coinfected with dsVenus. This result indicated polycistronic expression and translation of p19 protein suppressed its own mRNA gene silencing depending on the dose of p19.