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 sequenc e elements require bending of the intervening DNA and juxtaposition of interacting molecular surfaces in an appropriate orientation. Here, w e examine the effects of single amino acid substitutions adjacent to t he basic regions of Fos and Jun as well as changes in sequences flanki ng the AP-1 site on DNA bending. Substitution of charged amino acid re sidues at positions adjacent to the basic DNA-binding domains of Fos a nd Jun altered DNA bending. The change in DNA bending was directly pro portional to the change in net charge for all heterodimeric combinatio ns between these proteins. Fos and Jun induced distinct DNA bends at d ifferent binding sites. Exchange of a single base pair outside of the region contacted in the x-ray crystal structure altered DNA bending. S ubstitution of base pairs flanking the AP-1 site had converse effects on the opposite directions of DNA bending induced by homodimers and he terodimers. 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. D NA bending by Fos and Jun at inverted binding sites indicated that het erodimers bind to the AP-I site in a preferred orientation. Mutation o f a conserved arginine within the basic regions of Fos and transversio n of the central C:G base pair in the AP-1. site to G:C had complement ary effects on the orientation of heterodimer binding and DNA bending. The conformational variability of the Fos-Jun-AP-l complex may contri bute to its functional versatility at different promoters.