Acts as a cysteine protease, methyltransferase and deubiquitinase. The cysteine protease activity cleaves the polyprotein giving rise to mature proteins. The methyltransferase domain is probably involved in viral RNA capping. The deubiquitylating activity counteracts the degradation of the viral polymerase mediated by the host ubiquitin-proteasome system. The polymerase is thus stabilized and infectivity is increased. Favors K63 poly-Ub linkage. RNA-directed RNA polymerase is responsible for the replication and transcription of the genome. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Interacts with host ubiquitin. The viral OTU domain (vOTU) is responsible for the deubiquitination activity. Both protease (PRO) and deubiquitination (DUB) activities rely on the single catalytic site of the cysteine proteinase. The switch in the PRO/DUB activities is due to the mobility of a GPP flap. The deubiquitinase activity is low compared to that of Bunyaviruses or coronaviruses. Belongs to the tymovirus NS35 RNA replicase polyprotein family.