Molecular characteristics of full-length genomic segment A of three infectious bursal disease viruses in China: two attenuated strains and one virulent field strain.

The full-length cDNA of genomic segment A of three infectious bursal disease viruses, two attenuated strains (HZ2 and JD1) and one virulent field strain (ZJ2000), was amplified in a single step by reverse transcription-polymerase chain reaction, cloned into pGEM-T Easy Vector, and sequenced. The full length of cloned segment A contains 3259 nucleotides, which includes two partially overlapping open reading fragments (ORFs) ORF1 and ORF2, flanked by 5' and 3' noncoding regions. These strains shared high sequence identity with each other either at the nucleotide or deduced amino acid level. Strains HZ2 and JD1 were highly related to two attenuated strains, CEF94 and P2, whereas ZJ2000 was closely related to two other virulent strains, Cu-1 and Harbin. Substitutions of four amino acids at positions 253, 279, 284, and 330, a common feature of attenuated and most virulent strains, were also observed in these three strains. Two major hydrophilic peaks were conserved in the three strains; however, there are two amino acid substitutions at positions 280 (N to S) and 290 (M to L) in the second minor hydrophilic peak for all three strains, which might have a critical influence on antigenicity. Two amino acid substitutions near the VP2-VP4 cleavage site were identified in virulent strain JZ2000, which might be involved in increasing the virulence of the virus. Phylogenetic analyses indicated that these three Chinese strains are most closely related to some European virulent strains but are distinct from very virulent infectious bursal disease virus and variant strains.