Differential transcriptional regulation of c-myc promoter through the same DNA binding sites targeted by Sp1-like proteins.

Sp1 and Sp3 are closely related members of a gene family encoding proteins with very similar structural features. The zinc finger DNA binding domains of Sp1 and Sp3 are highly conserved and they bind to GC and GT box with comparable affinities. To begin to delineate the specific roles of these two members of the Sp1-like gene family, here we have analysed the DNA binding specificity and their effects on activation of human c-myc promoter. We found that both proteins bind to the same sites of c-myc promoter, upstream to both the P1 and P2 initiation sites. Cotransfection experiments, in mammalian and insect cells, indicated that Sp1 trans-activate c-myc promoter, whereas Sp3 did not. In addition, enforced expression of Sp3 repressed Sp1-mediated activation of c-myc. Finally, we found that Sp1 and E2F-1/DP-1 cooperatively trans-activate c-myc promoter. In contrast enforced expression of Sp3 fails to repress E2F-1/DP-1-mediated activation.