Zebrafish IRF1, IRF3, and IRF7 Differentially Regulate IFNΦ1 and IFNΦ3 Expression through Assembly of Homo- or Heteroprotein Complexes

In mammals, IFN regulatory factor (IRF)1, IRF3, and IRF7 are three critical transcription factors that are pivotal for cooperative regulation of the type I IFN response. In this study, we explored the relative contribution of zebrafish (Danio rerio) IRF1 (DrIRF1), IRF3 (DrIRF3), and IRF7 (DrIRF7) (DrIRF1/3/7) to zebrafish IFN Phi 1 (DrIFN Phi 1) and IFN Phi 3 (DrIFN Phi 3) (DrIFN Phi 1/3) activation. Following spring viremia of carp virus infection, DrIFN Phi 1/3 and DrIRF1/3/7 transcripts are significantly induced in zebrafish tissues, which correlates with the replication of spring viremia of carp virus. DrIRF1/3/7 selectively bind to the IRF-binding element/IFN-stimulated regulatory element sites of DrIFN Phi 1/3 promoters, with the exception that DrIRF3 has no preference for two IRF-binding element/IFN-stimulated regulatory element motifs within the DrIFN Phi 3 promoter. Consistently, DrIRF3 alone activates DrIFN Phi 1, but not DrIFN Phi 3; DrIRF7 predominantly stimulates DrIFN Phi 3; and DrIRF1 has similar potential to DrIFN Phi 1 and DrIFN Phi 3. Strikingly, DrIRF3 facilitates the binding of DrIRF1 and DrIRF7 to both zebrafish IFN promoters, and so does DrIRF7 for the binding of DrIRF1, particularly to the DrIFN Phi 3 promoter. These binding properties correlate with differential responses of DrIFN Phi 1 and DrIFN Phi 3 to the combinatory stimulation of DrIRF1/3/7, depending on their relative amounts. Similar to the dual roles of human IRF3 in regulating IRF7-activated IFN alpha genes, DrIRF3 exerts dual effects on DrIRF1-mediated DrIFN Phi 3 gene expression: an inhibitory effect at lower concentrations and a synergistic effect at higher concentrations. These data provide evidence that fish and mammals have evolved a similar IRF-dependent regulatory mechanism fine-tuning IFN gene activation.