Defining the sequence specificity of DNA-binding proteins by selecting binding sites from random-sequence...

We describe a new method for accurately defining the sequence recognition properties of DNA-binding proteins by selecting high-affinity binding sites from random-sequence DNA. The yeast transcriptional activator protein GCN4 was coupled to a Sepharose column, and binding sites were isolated by passing short, random-sequence oligonucleotides over the column and eluting them with increasing salt concentrations. Of 43 specifically bound oligonucleotides, 40 contained the symmetric sequence TGA(C/G)TCA, whereas the other 3 contained sequences matching six of these seven bases. The extreme preference for this 7-base-pair sequence suggests that each position directly contacts GCN4. The three nucleotide positions on each side of this core heptanucleotide also showed sequence preferences, indicating their effect on GCN4 binding. Interestingly, deviations in the core and a stronger sequence preference in the flanking region were found on one side of the central C . G base pair. Although GCN4 binds as a dimer, this asymmetry supports a model in which interactions on each side of the binding site are not equivalent. The random selection method should prove generally useful for defining the specificities of other DNA-binding proteins and for identifying putative target sequences from genomic DNA.