Nucleotide and amino acid mutations of different SD passages
To develop a live attenuated lineage 1 PRRSV vaccine, we first isolated an SD strain and passaged it in Marc-145 cells. Compared to the parental virus SD, there were 75 nucleotide changes at the 125th passage (Table 2). Among these mutations were two nucleotide changes (at position 29 [C-T] and 36 [C-T]) in the 5’-UTR and two (at position 22 [T-C] and 69 [T-C]) in the 3’-UTR (Table 2). Other nucleotide mutations were observed in Nsps and structural proteins, 31 of which were missense mutations, causing a change in 31 amino acids in Nsp2-5, 9-12, GP2a-5, M, N and ORF5a (Table 2). The major changes in amino acids were located on Nsp2 and minor structural proteins GP2a-4 (Table 2). No nucleotide amino acid changes were observed from passages 125 through 150 (Table 2).