Reverse genetic manipulation of the overlapping coding regions for structural proteins of the type II porcine reproductive and respiratory syndrome virus

The overlapping genomic regions coding for structural proteins of porcine reproductive and respiratory syndrome virus (PRRSV) poses problems for molecular dissection of the virus replication process. We constructed five mutant full-length cDNA clones with the overlapping regions unwound and 1 to 3 restriction sites inserted between two adjacent ORFs (ORF1/2, ORF4/5, ORF5/6, ORF 6/7 and ORF7/3′ UTR), which generated the recombinant viruses. Our findings demonstrated that 1) the overlapping structural protein ORFs can be physically separated, and is dispensable for virus viability; 2) such ORF separations did not interrupt the subgenomic RNA synthesis; 3) the plaque morphology, growth kinetics, and antigenicity of these mutant viruses were virtually indistinguishable from those of the parental virus in cultured cells; and 4) these mutant viruses remained genetic stable in vitro. This study lays a foundation for further molecular dissection of PRRSV replication process, and development of genetically tagged vaccines against PRRS.