Overlapping sites for constitutive and induced DNA binding factors involved in interferon-stimulated transcription.

A 14 bp interferon (IFN)-stimulated response element (ISRE) from 6-16, a human gene regulated by alpha-IFN, confers IFN inducibility on a heterologous thymidine kinase promoter. A 39 bp double-stranded oligonucleotide corresponding to a 5' region of 6-16 which includes the ISRE competes for factors required for gene expression by alpha-IFN in transfected cells and a single base change (A-11 to C) within the ISRE (GGGAAAATGAAACT) abolishes this competition. Band-shift assays performed with whole-cell extracts and the 39 bp oligonucleotide reveal specific complexes formed by rapidly induced and constitutive factors, both of which fail to bind to the A-11 to C oligonucleotide. A detailed footprinting analysis reveals that these two types of factors bind to overlapping sites within the ISRE, but in very different ways. These data were used to design oligonucleotides which decreased the formation of the inducible complex without affecting the constitutive one. Changes at the 5' margin of the ISRE and upstream of it markedly decrease formation of the induced but not the constitutive complex and also abolish the ability of the 39 bp sequence to function as an inducible enhancer with the thymidine kinase promoter. Thus, induction of 6-16 transcription in IFN-treated cells is likely to be stimulated by binding of the induced factor to the ISRE and upstream sequences, while the subsequent suppression of transcription may involve competition for the ISRE by the other class of factors.