Sp1 acts as a repressor of the human adenine nucleotide translocase-2 (ANT2) promoter

The human adenine nucleotide translocator-2 promoter is activated by adjace nt Sp1 activation elements centered at nucleotides -79 and -68 (Abox and Bb ox, respectively), and is repressed by Sp1 bound to a GC element (Cbox) tha t lies adjacent to transcription start. Here, we address the mechanism of t his unique Sp1-mediated repression using transfected Drosophila SL2 and mam malian cell lines. We show that repression is not due to steric interferenc e with assembly of the transcription machinery, as Sp1 bound to the Cbox ca n, under certain conditions, activate the promoter. Furthermore, ectopic ex pression of Sp1 deletion mutants in SL2 cells demonstrates that both the Sp 1-mediated repression and activation require the D transactivation domain o f Sp1 bound to the Cbox. In addition, repression of ABbox-mediated activati on is eliminated by separating the Abox and Bbox. Thus, for Cbox-bound Sp1 to repress, Sp1 must be precisely positioned at the region of the ABboxes. Together, these data suggest that the D transactivation domain mediates int eractions by Sp1 complexes on separate GC elements that results in repressi on of the activating Sp1 species.