Revisiting the Roles of Tobamovirus Replicase Complex Proteins in Viral Replication and Silencing Suppression.

Tobamoviral replicase possesses an RNA-dependent RNA polymerase(RDR) domain and is translated from genomic (g)RNA via a stop codonreadthrough mechanism at a one-to-ten ratio relative to a shorter protein lacking the RDR domain. The two proteins share methyltransferase and helicasedomains and form a heterodimer implicated in gRNA replication. The shorter protein is also implicated in suppressing RNA silencing–based antiviral defenses. Using a stop codonmutant of Oilseed rape mosaic tobamovirus (ORMV), we demonstrate that the readthrough replicase (p182) is sufficient for gRNA replication and for subgenomic RNA transcription during systemic infection in Nicotiana benthamianaand Arabidopsis thaliana. However, the mutant virus displays milder symptoms and does not interfere with HEN1-mediated methylation of viral short interfering (si)RNAs or plant small (s)RNAs. The mutant virus tends to revert the stop codon, thereby restoring expression of the shorter protein (p125), even in the absence of plant Dicer...