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Porcine reproductive and respiratory syndrome virus (PRRSV) is a member of the family Arteriviridae (1) and is characterized by respiratory disease in young pigs and severe reproductive failure in sows, including abortion, stillbirths and weak piglets. PRRS has caused immense economic losses in the pig industry and is considered to be one of the most important infectious diseases of pigs in the world (2). PRRSV has been recognized as one of the most important pathogens of pigs throughout the world. The virus was first confirmed in China in 1996 and since then the virus has become widespread all over the country (3). The PRRSV genome is a singlestranded, nonsegmented, positive-sense RNA, about 15.5 kb in length. The viral genomic RNA is polycistronic, comprising nine open-reading frames (ORFs) (4). Two large ORFs (ORF1a/ORF1b), occupying the 5`-terminal three-fourths of the genome, encode the viral replicase polyprotein, which is predicted to be autoproteolytically cleaved at 12 sites, ultimately producing 13 non-structural proteins (5), while the seven ORFs located at the 3`-terminus of the genome are postulated to encode viral structural proteins.
Among the nonstructural proteins, the multidomain protein nsp2 is the largest PRRSV replicative protein. Genetic analysis of both Coronaviridae and Arteriviridae (1) originally identified the protein to be spanning amino acids (aa) 384 to 1 363 (nsp2 aa 1 to 981); this was subsequently projected to comprise aa 384 to 1 578 (nsp2 aa 1 to 1 196) of strain VR-2332 ORF1a. Three major domains could be determined through the alignment of arterivirus nsp2 proteins: an N-terminal cysteine proteinase domain (PL2), a functionally unspecified middle region, and a hydrophobic transmembrane (TM) region near the C terminus (8, 9). However, the exact C-terminal cleavage sites have yet to be determined. The function of PRRSV nsp2 in the virus life cycle is currently not known, while Equine arteritis virus (EAV) nsp2 has been shown to be involved in the generation of double-membrane vesicles together with nsp3 and to serve as a cofactor for the nsp4 protein (10). Studies have identified the presence of a 51-nt deletion in the nsp2 gene of NA Type 1 viruses, which is located between nt 2 199 and 2 250 in ORF1a of the Lelystad virus genome (6, 7). Eighteen of the 20 NA-type 1 viruses possess the same 51-nt deletion (6).
Nucleotide and putative amino acid sequence analysis have revealed that Nsp2 is the most variable region of PRRSV (11, 12). Genetic variability, including point mutations and the insertion or deletion of nucleotides, has been demonstrated in the Nsp2 gene, not only between North American and European genotypes, but also among viruses within the same genotype (6). However, the biological and clinical significance of Nsp2 diversity remains to be elucidated. The aims of this study were: (ⅰ) Establish a genetic database for the nsp2 gene of PRRSV isolated from Ningxia Hui Nationality Autonomous Region (Ningxia) of China, (ⅱ) Identify relationships between sample date and the evolution of PRRSV in China, (ⅲ) Determine the phylogenetic relationship between the Ningxia strains and those from other regions of China.
Genetic Variation Analyses of nsp2 Gene of PRRSV in Ningxia Hui Autonomous Region of China
- Received Date: 10 December 2008
- Accepted Date: 12 April 2009
Abstract: To gain a better understanding of the genetic diversity and evolution of PRRSV in the Ningxia Hui Nationality Autonomous Region (Ningxia) of China, the nsp2 genes from a series of PRRSV strains collected from the region in 2007 were partially sequenced. These sequences were then analyzed along with the classical strain (ch-1a) and two other epidemic strains SD (3) and SD2006. Comparison of the nucleotide sequence with ch-1a indicated that nsp2 genes of seventeen Ningxia isolates (NX strain) have deletions of 87 nucleotides. Sequence analysis indicated that homology between the Ningxia strain and ch-1a was 60.3%-79.9% in the nucleotide sequence, and homology between the NX strains and SD strains was 80.3%-98.8% in the nucleotide sequence. The nsp2 genes of the seventeen isolates had 74.9%-100% nucleotide sequence identities with each other. This study was undertaken to assess the regional variation of prevalent PRRSV and to establish a sequence database for PRRSV molecular epidemiological studies.