Cooperative interactions between transcription factors Sp1 and OTF-1.

We have examined whether the functional synergism between transcription factors Sp1 and OTF-1 involves cooperativity in binding. To demonstrate cooperativity, synthetic enhancers were constructed in which Sp1-binding sites were combined with various OTF-1-binding sites that differed in their binding affinities. The ability of these constructions to activate transcription from the human U2 small nuclear RNA promoter was measured. The results showed that an Sp1-binding site stimulated transcription 2-fold when combined with a high-affinity binding site for OTF-1. When combined with a low-affinity OTF-1-binding site, in contrast, a 20-fold stimulation of transcription was observed. The stimulatory effect of Sp1 was moreover influenced by the distance between the Sp1- and OTF-1-binding sites and the functional cooperation was mirrored by the cooperative formation of OTF-1- and Sp1-specific protein-DNA complexes in vitro. We conclude from these results that the functional cooperation between OTF-1 and Sp1 involves physical interactions between the two transcription factors resulting in cooperative binding. The results thus reveal a mechanism by which Sp1 can modulate transcription.