The sequence of ORFs 2 to 4 of a U.S. isolate of porcine reproductive and respiratory syndrome virus (PRRSV), ATCC VR2385, was determined by analysis of a cDNA λ library. The cDNA clones containing PRRSV specific sequences were selected using a VR2385 ORF 4 specific PCR probe and sequenced. The ORFs 2, 3 and 4 overlapped each other and encoded polypeptides with predicted Mr of 29.5 kDa (ORF 2), 28.7 kDa (ORF 3) and 19.5 kDa (ORF 4), respectively. No overlap was found between ORFs 4 and 5, and instead there was a 10 bp sequence which separated these two ORFs. The nucleic acid homology with corresponding ORFs of the European PRRSV isolate Lelystad virus (LV) was 65% for ORF 2, 64% for ORF 3 and 66% for ORF 4. Comparison of the ORF 4 sequences of VR2385 with that of another U.S. isolate MN-1b revealed only 86% amino acid sequence homology and the presence of deletions in the ORF 4 of MN-1b. Our results further strengthen the observation that there is sequence variation between US and European PRRSV isolates.
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