SEQUENCE COMPARISON OF OPEN READING FRAMES 2 TO 5 OF LOW AND HIGH VIRULENCE UNITED-STATES ISOLATES OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS

The sequences of ORFs 2 to 5 of five United States (US) porcine reprod uctive and respiratory syndrome virus (PRRSV) isolates with differing virulence were determined. The nucleotide and deduced amino acid seque nces of these isolates were compared with those of other known PRRSV i solates. The amino acid sequence identity between seven US PRRSV isola tes was 91-99 % in ORF 2, 86-98 % in ORF 3, 92-99 % in ORF 4 and 88-97 % in ORF 5. The low virulence US isolate had highest sequence variati on in ORFs 2 to 4 compared to the other US isolates. A hypervariable r egion with antigenic potential was identified within the major envelop e glycoprotein. Phylogenetic analysis of ORFs 2 to 7 indicated the exi stence of at least three minor genotypes within the major US genotype. The low virulence US isolate formed a branch distinct from the other US isolates. The results of this study have implications for both the taxonomy of PRRSV and vaccine development.