Cooperative binding of Stat1-2 heterodimers and ISGF3 to tandem DNA elements

Interferon (IFN)-alpha-activated Stat1 homodimers and Stat1-2 heterodimers bind to GAS elements, whereas the transcription factor ISGF3, which contains Stat1, Stat2 and p48, binds to ISREs. We now find that Stat1-2 dimers can form heterotetramers on tandem GAS sites and that the heterotetramers have a much higher binding affinity for a double GAS site than do heterodimers for a single site, suggesting cooperativity mediated through protein-protein interactions. Stat1-2 heterotetramers can also be detected with a single GAS site, again indicating cooperativity mediated through protein-protein interactions. Deleting 40 amino acid residues from the N-terminus of Stat1 abolished Stat1-Stat2 heterotetramer formation, but did not affect heterodimer formation and an N-terminal peptide containing the first 120 residues of Stat2 inhibited heterotetramer formation but did not affect heterodimer formation. Thus, the N-terminal regions of both Stat1 and Stat2 are important for cooperative DNA binding, and heterodimers probably interact with each other through these regions. Cooperative binding of ISGF3 was also observed using the tandem ISREs from the IFN-alpha responsive promoter of the 6-16 gene. A more abundant and larger complex was formed with a probe containing two ISREs than with a probe containing a single ISRE. The N-terminal regions of both Stat1 and Stat2 are important for the cooperative binding of ISGF3 to tandem ISREs but not to a single site. The cooperative DNA-binding activities of ISGF3 and Stat1-2 dimers are likely to contribute to the transcriptional activation of those IFN-alpha-responsive genes that have tandem DNA elements.