DNA bending by GCN4 mutants bearing cationic residues

Transcription activation is thought to require DNA bending to promote the interaction of upstream activators and the basal transcription machinery. Previous experiments have shown that some members of the bZIP family of DNA binding proteins bend DNA, while others do not. We are exploring the possibility that electrostatic effects play a role in these differences. The yeast bZIP transcription factor GCN4 does not induce DNA bending in vitro. Substitution of basic residues for three neutral amino acids of GCN4 confers the ability to bend DNA. This result is consistent with a model of induced DNA bending wherein excess positive charge in proximity to one face of the double helix neutralizes local phosphate diester anions resulting in a laterally asymmetric charge distribution along the DNA. Previous data suggest that such an unbalanced charge distribution results in collapse of the DNA toward the neutralized surface. Interpretations of the present data are discussed. Our result supports the hypothesis that electrostatic interactions can play a key role in DNA bending by bZIP proteins.