Specificity of protein-DNA recognition revealed by structure-based potentials: symmetric/asymmetric and cognate/non-cognate binding.

Asymmetric binding of protein homodimers to DNA, which has been observed in a number of protein-DNA complexes, leads to subtle structural differences between the two subunits. Such structural differences are frequently observed when the subunits form cognate and non-cognate protein-DNA complexes, respectively. Analysis of these structural effects on binding specificity should provide insight into the mechanism of protein-DNA recognition. We previously derived empirical potential functions for specific nucleotide base-amino acid interactions from statistical analyses of the structures of many protein-DNA complexes and used a combinatorial threading procedure to evaluate the fitness of the DNA sequences involved. We then introduced Z-scores to measure the specificity with which proteins bind to DNA within complexes, as compared to random DNA sequences. Here, we examined in detail the structural effects of asymmetric and cognate/non-cognate binding on specificity. Marked differences in the specificity of DNA binding were observed for the two subunits of lambda repressor, the glucocorticoid receptor, and for transcription factors containing a Zn(2)Cys(6) binuclear cluster domain, which are known to bind asymmetrically to DNA. Moreover, the differences in the specificity with which BamH1 and EcoRV endonucleases bind to their cognate and non-cognate DNA sequences were clearly detected using this approach; indeed, analysis of EcoRV binding enabled us to show the cooperative effect of sequence and structure on binding specificity. The present results demonstrate the utility of this approach when examining the structure-specificity relationship in protein-DNA recognition, as subtle structural differences in symmetric/asymmetric and cognate/non-cognate binding were clearly shown to cause marked differences in specificity. This method can also be used as a tool for checking new structures of protein-DNA complexes for their specificity.