Structural basis of DNA bending and oriented heterodimer binding by the basic leucine zipper domains of Fos and Jun.

Interactions among transcription factors that bind to separate sequence elements require bending of the intervening DNA and juxtaposition of interacting molecular surfaces in an appropriate orientation. Here, we examine the effects of single amino acid substitutions adjacent to the basic regions of Fos and Jun as well as changes in sequences flanking the AP-1 site on DNA bending. Substitution of charged amino acid residues at positions adjacent to the basic DNA-binding domains of Fos and Jun altered DNA bending. The change in DNA bending was directly proportional to the change in net charge for all heterodimeric combinations between these proteins. Fos and Jun induced distinct DNA bends at different binding sites. Exchange of a single base pair outside of the region contacted in the x-ray crystal structure altered DNA bending. Substitution of base pairs flanking the AP-1 site had converse effects on the opposite directions of DNA bending induced by homodimers and heterodimers. These results suggest that Fos and Jun induce DNA bending in part through electrostatic interactions between amino acid residues adjacent to the basic region and base pairs flanking the AP-1 site. DNA bending by Fos and Jun at inverted binding sites indicated that heterodimers bind to the AP-1 site in a preferred orientation. Mutation of a conserved arginine within the basic regions of Fos and transversion of the central C:G base pair in the AP-1 site to G:C had complementary effects on the orientation of heterodimer binding and DNA bending. The conformational variability of the Fos-Jun-AP-1 complex may contribute to its functional versatility at different promoters.