The assembly of multicomponent complexes at promoters, enhancers, and silencers 
likely entails perturbations in the path of the DNA helix. We present evidence 
that YY1, a ubiquitously expressed DNA-binding protein, regulates the activity 
of the c-fos promoter primarily through an effect on DNA structure. YY1 binds to 
and induces a phased DNA bend at three sites in this promoter. By use of a 
truncated c-fos promoter activity containing a single functional YY1 site, we 
show that YY1 represses promoter activity but that repression does not appear to 
be an intrinsic property of the protein in this context. Moreover, when the 
orientation of the YY1 site is reversed, YY1 activates the same promoter. 
Repression by YY1 is also alleviated by changing the relative phasing of 
factor-binding sites on either side of YY1. We conclude that the principal 
function of YY1 in this promoter is to bend DNA to regulate contact between 
other proteins. Thus, YY1 represents a new class of transcription factors that 
influences promoter function by affecting promoter structure rather than by 
directly contacting the transcriptional machinery. We provide evidence that the 
product of the male sex determination gene SRY may also belong to this class of 
structural factors.