Residues 41V and/or 210D in the NP protein enhance polymerase activities and potential replication of novel influenza (H7N9) viruses at low temperature

Wenfei Zhu, Xiaohui Zou, Jianfang Zhou, Jing Tang, Yuelong Shu

The influenza A (H7N9) virus emerged in the spring of 2013 in China. It contained six internal genes from Y280-like H9N2 viruses, which have co-circulated with G1-like lineage viruses throughout poultry in China. Accompanied with continuous reassortment among H7N9 and H9N2 viruses in poultry, it is possible for H7N9 viruses to acquire internal genes of G1-lineage viruses. Thus, it is important to evaluate potential impact of G1-like viruses on the H7N9 viruses. We used in vitro assays of polymerase activities and growth kinetics to evaluate the potential contribution of G1-like virus genes to the replication abilities of H7N9 viruses. Two mutations in the NP protein (41V and/or 210D) could enhance H7N9 RNP activities, especially at low temperature (33°C, which is similar to the temperature of human upper respiratory tract). Meanwhile, G1 viruses with V41I or D210E substitutions exhibited poor growth ability in the early infection stage at low temperature. The D210E substitution also reduced the replication ability of G1 virus at 12 and 24 hour post infection at 37°C. In both tested temperatures, V41I could compensate for the defective virus replication induced by the D210E mutation. Mutations 41V and/or 210D in the NP protein conferred improved RNP activity in H7N9 viruses and promoted the replication ability of H9N2 viruses, particularly at lower temperature. Substitutions at these two positions may promote the replication ability of H7N9 viruses in low temperature and thus might contribute to viral transmissibility. While these two residues have not yet been observed in H7N9 viruses, attention should be devoted to these two residues.