DNA bending determines Fos-Jun heterodimer orientation

Heterodimeric transcription factors can bind to palindromic recognition elements in two opposite orientations with potentially distinct effects on transcriptional activity. We have determined the orientation of Fos-Jun binding at different AP-I sites using a novel gel-based fluorescence resonance energy transfer assay. The orientation preference of heterodimer binding varied over a greater than 10-fold range. Single base pair substitutions that alter bending of flanking sequences reversed the orientation of heterodimer binding. Single amino acid substitutions that reduce the difference in DNA bending between Fos and Jun also reduced the orientation preference. Consequently, indirect read-out mediated by differences in DNA structure can contribute to the structural organization of nucleoprotein complexes.