Red-grouper nervous necrosis virus B1 protein inhibits fish IFN response by targeting Ser5-phosphorylated RNA polymerase II to promote viral replication.

Nervous necrosis virus (NNV) could infect more than 200 fish species worldwide, with almost 100% mortality in affected larvae and juvenile fish. Among different genotypes of NNV, the red-grouper nervous necrosis virus (RGNNV) genotype is the most widely reported with the highest number of susceptible species. Interferon (IFN) is a crucial antiviral cytokine and RGNNV needs to develop some efficient strategies to resist host IFN-stimulated antiviral immune. Although considerable researches on RGNNV, whether RGNNV B1 protein participates in regulating the host's IFN response remains unknown. Here, we reported that B1 protein acted as a transcript inhibition factor to suppress fish IFN production. We firstly found that ectopic expression of B1 protein significantly decreased IFN and IFN-stimulated genes (ISGs) mRNA levels and IFNφ1 promoter activity induced by polyinosinic:polycytidylic acid [poly (I:C)]. Further studies showed that B1 protein inhibited the IFNφ1 promoter activity stimulated by the key RIG-I-like receptors (RLRs) factors, including MDA5, MAVS, TBK1, IRF3, and IRF7 and decreased their protein levels. Moreover, B1 protein significantly inhibited the activity of constitutively active cytomegalovirus (CMV) promoter, which suggested that B1 protein was a transcription inhibitor. Western blot indicated that B1 protein decreased the Ser5 phosphorylation of RNA polymerase II (RNAP II) C-terminal domain (CTD). Together, our data demonstrated that RGNNV B1 protein was a host transcript antagonist, which intervened RNAP II Ser5-phosphorylation, inhibiting host IFN response and facilitating RGNNV replication.