Mutational analysis of the SDD sequence motif of a PRRSV RNA-dependent RNA polymerase

Zhou, Yan; Zheng, HaiHong; Gao, Fei; Tian, DeBin; Yuan, ShiShan

Motif C contains highly conserved amino acid residues that represent specific signatures of each major group of RNA viruses. In motif C, the sequence GDD is conserved among polymerases of positive-strand RNA viruses, MDD belongs to retroviruses, GDN and SDD are conserved signature motifs of nonsegmented-negative stranded and segmented-negative stranded RNA viruses, respectively [8] , whereas SDD is conserved in nidoviruses. High-resolution structures of RdRps of positive-and double-strand RNA viruses revealed clear similarities, despite sharing little sequence identity, suggesting an evolutionary link between the RdRps of these RNA viruses [15, 20, 31] . To investigate the role of the conserved C motif in the evolutionary process of the polymerase, we mutated amino acid residues of SDD to those conserved amino acids described above. When SDD was mutated to MDD or GDN, RdRp lost transcriptional activity. From the aspect of evolutionary relationship, nidoviruses, which are also positive single-stranded RNA viruses, are closely related to the positive stranded RNA viruses, but are further from the nonsegmented-negative stranded RNA viruses and retroviruses. However, it is unclear why segmented-negative stranded RNA viruses also have an SDD motif, but abrogated polymerase activity when was mutated to GDD [16, 18] .