Activation functions of transcription factor Sp1 at U2 snRNA and TATA box promoters.

To localize regions in the human transcription factor Sp1, which are involved in activating transcription of the U2 snRNA gene promoter and of a TATA box gene promoter, the activation potentials of GAL4/Sp1 chimeras were analyzed in mammalian cells. In vitro mutagenesis analysis of Sp1 showed that mutation of a hydrophobic amino acid residue in glutamine-rich activation domain A impairs stimulation of transcription from the TATA box promoter, but not from the U2 promoter. Furthermore, we found that similar parts of Sp1 are involved in synergistic activation of transcription together with the SV40 enhancer and with an enhancer which binds a single type of transcription factor. This suggests that the activating mechanism of Sp1 is the same with both enhancers. Interestingly, we found that the glutamine-rich domains A and B, that stimulate transcription from the TATA box promoter were not sufficient for U2 gene activation. Stimulation of U2 transcription required amino acid residues 231-485 of Sp1, which contain the glutamine-rich domain B and a serine/threonine-rich part. Since overlapping, but non-identical parts of Sp1 are required for activation of the two promoter types, we conclude that Sp1 activates the U2 snRNA and TATA box promoters by different mechanisms.