Interferon regulatory factor 3 (IRF-3) in Japanese flounder, Paralichthys olivaceus: Sequencing, limited tissue distribution, inducible expression and induction of fish type I interferon promoter

Two cDNAs with different 3′-untranslated region (UTR) encoding an interferon regulatory factor 3 (IRF-3) were cloned from head kidney of Japanese flounder, Paralichthys olivaceus, by reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) methods. Sequence analysis reveals that they were generated by alternative polyadenylation. The predicted protein consists of 467 amino acid residues which shares the highest identity of 50.7–57.6% to fish IRF-3 and possesses a DNA-binding domain (DBD), an IRF association domain (IAD) and a serine-rich domain (SRD) of vertebrate IRF-3. The presence of these domains along with phylogenetic analysis places it into the IRF-3 group of the IRF-3 subfamily. RT-PCR analysis revealed that flounder IRF-3 was expressed constitutively in limited tissue types including head kidney, spleen, kidney, heart, gill, intestine and liver. A quantitative real time PCR assay was employed to monitor expression of IRF-3, type I interferon (IFN) and Mx in flounder head kidney and gill. All three genes were up-regulated by polyinosinic:polycytidylic acid (polyI:C) and lymphocystis disease virus (LCDV) with an earlier but slight and less persistent increase in transcription levels seen for the IRF-3. Finally, flounder IRF-3 was proved to induce fish type I IFN promoter in FG9307 cells, a flounder gill cell line, by a luciferase assay. These results provide insights into the roles of fish IRF-3 in the antiviral immunity.