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

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

The mutants could not produce sg positive strands or sg negative strands (data not show), but could produce genomic negative strands ( Figure 2 ). The results demonstrated that mutation of the SDD motif did not completely abolish the function of RdRp, which was different from the results of EAV [3] and other viruses [1, 19, [32] [33] [34] [35] . This could be because the primary function of replication is not determined by the SDD motif in PRRSV, and the SDD motif is specifically involved in transcription. In fact, viral RdRps function with other factors, such as viral and host factors, in natural infection, in which RdRps combine to differentiate genomic RNA replication from sg mRNA transcription. For positive stranded RNA viruses, replication occurs in membrane-bound, multiprotein complexes termed the replication/transcription complex (RTC). However, the domains of RdRp that interact with the other factors, and the process of transcription and replication in the RTC are still unknown. The fact that conservation of the SDD motif is not required for the replication of RdRp in PRRSV, suggested that transcription and replication of PRRSV are two independent processes and may be uncoupled. It is reported that nsp1 of EAV is essential for sg mRNAs production and is dispensable for genome replication [30] . In the synthesis of sg mRNAs, nsp1 combines with the viral RdRp complex to engage in transcription; however, the mechanism is still unknown, and we speculated that the SDD motif of RdRp could interact with nsp1 in this process.