Cloning, expression and sequence analysis of the classical swine fever virus nucleocapsid protein.

The DNA complementary to the 5'-terminal 1929 nucleotides of classical swine fever virus (CSFV; alias hog cholera virus, HCV) LPC vaccine strain RNA was cloned and sequenced. The sequence encompasses a 5'-noncoding region (NCR) of 264 nucleotides and an open reading frame (ORF) of 1665 nucleotides. The cloned sequence contains genes of four viral proteins, P23, nucleocapsid (core) protein, E0 and part of E1 proteins. Alignment of the 5'-terminal 1929 nucleotides of LPC strain with other strains of CSFV showed well conservation and a homology as high as 84-95% was found between these strains. The cDNA of CSFV-LPC core was cloned into an expression vector, and a fusion protein of 38.5 kDa was obtained which reacted strongly to CSFV antiserum. Purification of the core fusion protein was achieved by a single-step affinity chromatography and the purified product could be recognized by the sera of CSFV-infected swine in ELISA assay. Phylogenetic analysis of the 5'-terminal 1929 nucleotides between pestiviruses revealed that the 5'-end region seems to be suitable for differentiation of different strains of CSFV.