LAUSR

Infectious bursal disease virus (IBDV) causes an economically significant disease of chickens worldwide (Berg, 2000). The molecular basis for the virulence of very virulent IBDV (vvIBDV) is not fully understood. Previous studies have shown that genome segment A, especically VP2 protein, plays the most important role in the tropism and pathogenicity of serotype 1 IBDV (Brandt et al., 2001). VP2 is, however, unlikely to be the only factor for the virulence of vvIBDV (Boot et al., 2000). A chrono-phylogenetic study suggested that the worldwide expansion of vvIBDV likely started following the acquisition of a new vvIBDV-related segment B, whereas the vvIBDV-related segment A could have been introduced several years earlier (Hon et al., 2006). Another recent experimental study based on reverse genetics demonstrated that several domains of the IBDV polymerase may contribute to the virulence of vvIBDV (Nouën et al., 2012). Segment B, encodes VP1, an RNA-dependent RNA polymerase (RdRp) responsible for the replication of the genome and synthesis of mRNAs (von Einem et al., 2004). The IBDV RdRp could be divided into three domains according to the tri-dimensional structure: an N-terminal domain (residues 1–167), the central polymerase domain (residues 168–658), and a C-terminal domain (residues 659– 878) (Pan et al., 2007). To study the function of different domains of VP1 protein of vvIBDV for its virulence, we constructed and rescued two wild-type and four intra-segment B reassortant IBDVs derived from vvIBDV HuB-1 and attenuated IBDV Gt strains based on the tri-dimensional structure of VP1 protein (Figures S1 and S2 in Supporting Information). The study of viral replication showed that the substitution of the central activity domain and C-terminal domain of VP1 protein of the attenuated IBDV Gt with the corresponding domains of VP1 from vvIBDV HuB-1 had a positive effect on viral replication in CEF cells at the early stage of infection, while the substitution of the N-terminal domain of VP1 from attenuated IBDV Gt (GtN) with the corresponding domain of VP1 from vvIBDV HuB-1 (HuBN) reduced the replication of Gt in CEF cells (Figure S3 in Supporting Information). On the other hand, the substitution of HuBN with GtN reduced the replication of HuB-1 in chickens significantly (P<0.05), and enhanced its replication in chickens with the substitution of GtN with HuBN (Figure S3 in Supporting Information). These results indicated that the N-terminal domain of VP1 from vvIBDVs has a negative effect on viral replication in vitro while exhibits a positive effect on viral replication in