Identification of fidelity-determined residues of Porcine reproductive and respiratory syndrome virus through structural alignment.

Porcine reproductive and respiratory syndrome virus(PRRSV) is an economically important pathogen for global pork industry. As a positive-strand RNA virus, lacking exonuclease-mediated proofreading, its RNA-dependent RNA polymerase (RdRP) domain within the nonstructural protein 9(nsp9) plays a vital role in maintaining replication accuracy. To identify the residues of PRRSV that regulates replication fidelity, its RdRP structure was predicted by using Alpha Fold 2 and aligned with the solved structure of coxsackievirus B3 (CVB3) RdRP. This comparison identified conserved residues in PRRSV RdRP that are potentially involved in fidelity. Using site-directed mutagenesis, nucleoside analog sensitivity tests, and next-generation sequencing(NGS), it was found that the nsp9 K541R mutation enhances fidelity, as increasing viral resistance to mutagens like ribavirin, 5-Fluorouracil(5-FU), and 5-Azacytidine(5-AZC), as well as generating lower rate of non-contiguous junctions. In contrast, mutations at other positions, including A394G, L396S, and R401A, reduced fidelity and elevated frequency of recombination and mutation accumulation. Structural modeling revealed that the highly conserved residue K336 is spatially adjacent to the key fidelity site K541 but situated on the opposite side of the RNA channel. We found that K336R exhibits a dissociated "resistance-high recombination" phenotype. The findings reveal the importance of specific residues in PRRSV RdRP for replication fidelity and provide insights into the potential for improving the stability and safety of live attenuated vaccines through targeted modifications. Furthermore, the study emphasizes the structural conservation of fidelity determinants across RNA viruses, despite low sequence similarity, which can offer a framework for identifying fidelity key sites in other viral RdRPs.