CLONING AND SEQUENCE ANALYSIS OF SEGMENT A cDNA OF A VERY VIRULENT INFECTIOUS BURSAL DISEASE VIRUS
By RT-PCR, the nucleotide sequences of the Harbin vvIBDV segment A, two ORFS for VP5 and precursor polyprotein were determined. The results showed that the cloned 3101bp segment A gene contains two overlapping open reading frames (ORFA1 and ORFA2). The large ORF of 3036bp encodes a polyprotein (VP2-4-3), whereas the small ORF is 435bp coding VP5. The deduced amino acid sequence of the Harbin strain was compared with those of other serotype 1 and 2 strains. The results showed that the sequence of the Chinese isolate, the Harbin strain, has variations for virulence and antigenecity. There are 25 to 267 nucleotide differences among segment A of Harbin strain and the other serotype I strains of IBDV. The polyprotein encoded by Harbin strain showed 17 to 40 amino acid differences from those of the other serotype I strains, with 12 substitutions unique to Harbin strain. The minimum amino acid divergence within polyprotein (1.7%) was observed among Harbin strain and P2, Cu-1 of serotype 1. The maximum amino acid difference was 3.9% observed between Harbin strain and Uk661. Within the 454 residues of VP2 of Harbin strain, there are 5 amino acid substitutions, representing 1.76% variation at the VP2 variable region between residues 206 to 350. Within the 267 residues of VP4, there are only 2 amino acid substitutions, representing 0.74% variation. Within the 289 residues of VP3, there are also 2 amino acid substitutions, representing 0.69% variation. Within the 145 residues of VP5, there are 2 amino acid substitutions, representing 1.38% variation. Harbin strain is neither similar to the vv strains from Europe, Japan and Israel nor to the variant strains from the United States and China Guanzhou. It is most closely related to the strains isolated earlier from Europe, however has distinct amino acid mutations in variable region, especially in two hydrophilic domains.