Selective DNA Binding and Association with the CREB Binding Protein Coactivator Contribute to Differential Activation of Alpha/Beta Interferon Genes by Interferon Regulatory Factors 3 and 7

Abstract

Recent studies implicate the interferon (IFN) regulatory factors (IRF) IRF-3 and IRF-7 as key activators of the alpha/beta IFN (IFN-α/β) genes as well as the RANTES chemokine gene. Using coexpression analysis, the human IFNB, IFNA1, and RANTES promoters were stimulated by IRF-3 coexpression, whereas the IFNA4, IFNA7, and IFNA14 promoters were preferentially induced by IRF-7 only. Chimeric proteins containing combinations of different IRF-7 and IRF-3 domains were also tested, and the results provided evidence of distinct DNA binding properties of IRF-3 and IRF-7, as well as a preferential association of IRF-3 with the CREB binding protein (CBP) coactivator. Interestingly, some of these fusion proteins led to supraphysiological levels of IFN promoter activation. DNA binding site selection studies demonstrated that IRF-3 and IRF-7 bound to the 5′-GAAANNGAAANN-3′ consensus motif found in many virus-inducible genes; however, a single nucleotide substitution in either of the GAAA half-site motifs eliminated IRF-3 binding and transactivation activity but did not affect IRF-7 interaction or transactivation activity. These studies demonstrate that IRF-3 possesses a restricted DNA binding site specificity and interacts with CBP, whereas IRF-7 has a broader DNA binding specificity that contributes to its capacity to stimulate delayed-type IFN gene expression. These results provide an explanation for the differential regulation of IFN-α/β gene expression by IRF-3 and IRF-7 and suggest that these factors have complementary rather than redundant roles in the activation of the IFN-α/β genes.

ACKNOWLEDGMENTS

We thank Paula Pitha, Luwen Zhang, Joseph Pagano, Xiang-Jiao Yang, and Illka Julkunen for reagents used in this study and members of the Molecular Oncology Group, Lady Davis Institute for Medical Research, for helpful discussions.

This research was supported by grants from the Cancer Research Society Inc. and the Medical Research Council of Canada. R.L. was supported in part by a Fraser Monat McPherson fellowship from McGill University, P.G. was supported by an FRSQ postdoctoral fellowship, Y.M. was supported by an MRC studentship, and J.H. was supported by an MRC senior scientist award.