DNA binding and transcription activation by chicken interferon regulatory factor-3 (chIRF-3)

Interferon regulatory factors (IRFs) are a family of transcription factors involved in the cellular response to interferons and viral infection. Previously we isolated an IRF from a chicken embryonic liver cDNA library. Using a PCR-based binding site selection assay, we have characterised the binding specificity of chIRF-3. The optimal binding site (OBS) fits within the consensus interferon-stimulated response element (ISRE) but the specificity of chIRF-3 binding allows less variation in nucleotides outside the core IRF-binding sequence. A comparison of IRF-1 and chIRF-3 binding to ISREs in electrophoretic mobility shift assays confirmed that the binding specificity of chIRF-3 was clearly distinguishable from IRF-1. The selection assay also showed that chIRF-3 is capable of binding an inverted repeat of two half OBSs separated by 10–13 nt. ChIRF-3 appears to bind both the OBS and inverted repeat sites as a dimer with the protein–protein interaction requiring a domain between amino acids 117 and 311. In transfection experiments expression of chIRF-3 strongly activated a promoter containing the OBS. The activation domain was mapped to between amino acids 138 and 221 and a domain inhibitory to activation was also mapped to the C-terminal portion of chIRF-3.