Molecular cloning and characterization of interferon regulatory factor 7 (IRF-7) in Japanese flounder, Paralichthys olivaceus

Interferon regulatory factor (IRF) 7 in mammals is known to be a key player in regulating the type I interferon (IFN) response to viral infection as a transcription activator of IFNs and IFN-stimulated genes (ISGs). In this study, a full-length cDNA of Japanese flounder, Paralichthys olivaceus, (Po)IRF-7 was cloned and characterized. PoIRF-7 is 2032 bp in length, with an open reading frame (ORF) of 1293 bp that encodes 430 amino acid residues. The putative amino acid sequence shows the highest homology to fish IRF-7 with 51.5–76.3% identity and possesses a DNA-binding domain (DBD), an IRF association domain (IAD) and a serine-rich domain of vertebrate IRF-7. In addition, the tryptophan cluster of PoIRF-7 DBD consists of only four tryptophans, which is a characteristic unique to all fish IRF-7 members. The PoIRF-7 was expressed constitutively in all tested tissues of healthy flounders, with high levels in head kidney, spleen, gill, intestine and skin, and moderately expressed in FG9307 cells, a flounder gill epithelial cell line. Using a luciferase assay, PoIRF-7 was proved to be capable of activating fish type I IFN promoter in FG9307 cells. A quantitative real time PCR assay was employed to monitor the gene expression of PoIRF-7 and Mx in FG9307 cells and flounder head kidney and gill. Both genes were up-regulated by polyinosinic:polycytidylic acid (poly I:C) and lymphocystis disease virus (LCDV) though to a much lesser extent in FG9307 cells. Further, their transcription kinetics were similar in fish organs but different in FG9307 cells. These data provide insights into the functions of PoIRF-7 and imply a difference in PoIRF-7-related signaling pathways in antiviral response between cultured cells and live fish.