The BNoV RdRp gene contains highly conserved regions. VP1 is the major structural component of caliciviruses and is involved in receptor recognition, host specificity, strain antigenic diversity, and immunogenicity (Chen et al., 2004). VP2 is highly variable and can contain different types of mutations (Bok et al., 2009; Seah,Gunesekere,Marshall, & Wright, 1999). VP2 interacts withVP1 and, in turn, enhances the expression of capsid proteins (Vongpunsawad,Venkataram, & Estes, 2013). Due to the slightly different topologies between ORF2 and ORF3, different BNoV VP2 gene may have resulted from distinct evolutionary strategies (Kamel et al., 2009). Phylogenetic trees were constructed based on the BNoV VP1 andRdRp genes (Figure 2). The strain isolated in the present study and CH-HNSC-2018 were clustered in one branch, while strains isolated from Sichuan, southern China, were clustered in another branch, indicating that the BNoV strains in China are diverse. Different strains with different genotypes exist in China, making the prevention and control of viral spread difficult (Figure 2a). The sequence and phylogenetic analyses showed that Bo/CH/HB/BD/2019 belongs to GIII.2 BNoV (Figure 2b).
The VP1 protein includes a shell (S) domain, which is highly conserved among different NoVs, and a protruding (P) domain, with N-terminal P1 and C-terminal P1 and P2 parts (Chen et al., 2013). The highly variable region of the sequence (residues 279–406) forms the externally located P2 subdomain, whereas the central subdomain P1 (residues 226–278 and the C-terminal 124 residues) is located between the S and P2 domains (Venkataram,Hardy, & Estes, 2000). Comparing with previous studies, in both China and other countries, we found four new aa substitutions in VP1 , i.e., 225C, 246T, 624T, and 945T. The 225C residue is located in the conserved region and 246T is located in the P1 subdomain in the highly variable region. This may be an adaptation that is unique to BNoV circulating in cattle farms in Hebei, China. Whether these mutations in Bo/CH/HB/BD/2019 lead to changes in virulence, pathogenicity, and antigenicity remains to be explored.
In conclusion, in this study we isolated the Bo/CH/HB/BD/2019 strain from Hebei, sequenced its genome, and identified its genes. We analyzed its sequence homology with other BNoV strains and constructed phylogenetic trees to analyze its genetic evolution. Our results provide a reference for the development of a BNoV vaccine to prevent and control viral spread. Nevertheless, more data are still required, so in the future it will be necessary to further monitor and research BNoV and its subtypes.