This V domain of L RNA-polymerase carries a new motif, GxxTx(n)HR, that is essential for mRNA cap formation. Nonsegmented negative-sense (NNS) RNA viruses, Mononegavirales, cap their mRNA by an unconventional mechanism. Specifically, 5'-monophosphate mRNA is transferred to GDP derived from GTP through a reaction that involves a covalent intermediate between the large polymerase protein L and mRNA. The V region is essential for this process [1].

1: A conserved motif in region v of the large polymerase proteins of nonsegmented negative-sense RNA viruses that is essential for mRNA capping. Li J, Rahmeh A, Morelli M, Whelan SP; J Virol. 2008;82:775-784. PMID:18003731