Mutations in the genome of porcine reproductive and respiratory syndrome virus responsible for the attenuation phenotype

Although live-attenuated vaccines have been used for some time to control c linical symptoms of the porcine reproductive and respiratory syndrome (PRRS ), the molecular bases for the attenuated phenotype remain unclear. We had previously determined the genomic sequence of the pathogenic PRRSV 16244B. Limited comparisons of the structural protein coding sequence of an attenua ted vaccine strain have shown 98% homology to the pathogenic 16244B. Here w e have confirmed the attenuated phenotype and determined the genomic sequen ce of that attenuated PRRSV vaccine and compared it to its parental VR-2332 and the 16244B strains. The attenuated vaccine sequence was colinear with that of the strain 16244B sequence containing no gaps and 212 substitutions over 15,374 determined nucleotide sequence. We identified nine amino acid changes distributed in Nsp1 beta, Nsp2, Nsp10, ORF2, ORF3, ORF5 and ORF6. T hese changes may provide the molecular bases for the observed attenuated ph enotype.