A DNA base-specific sequence interposed between CRX and NRL contributes to RHODOPSIN expression.

The binding affinity of transcription factors (TFs) for their cognate DNA sequences controls gene expression. DNA determines the recruitment and positioning of TFs; whether it plays other roles is unknown. Here we found that the specific 22-bp sequence interposed between the CRX and NRL TFs in the proximal promoter of RHODOPSIN (RHO) largely controls the expression levels of RHO. Mutagenesis of this DNA-linker sequence resulted in wide variation in gene expression. In contrast, reciprocal exchange of human and murine RHO elements conferred species-specific expression levels. Targeting the DNA-linker with equal orthogonal DNA-binding proteins activates or represses RHO expression depending on its orientation relative to CRX and NRL binding sites. We conclude that DNA itself adds to TF activity through a code that determines optimal levels of RHO expression.