The structural features of protein-DNA interactions have been evaluated using a new information theory algorithm for the analysis of protein structure/function dependence: the so-called resonant recognition model. The physicochemical basis of this analysis was firstly validated with the trprepressor -operator interaction as a well-defined example. The amino acid and structural features predicted by these procedures to be crucial for repressor-operator interaction were found to be clustered around the known three-dimensional structure of the active site of the trp repressor, Similar methods of analysis have been extended to the less-well-defined example of the Ha-ras p21 protein family. The results of this analysis have indicated two distinct interactive regions in p21, one associated with the guanine-nucleotide-binding site, whilst the second is proposed to be associated with a binding site for an activator protein. These studies indicate that the p22 protein, besides the ability to function as a plasma-membrane-associated guanine-nucleotide-binding regulatory protein and bind free guanine nucleotides in the cytoplasm, has the structural ability to bind guanine incorporated in DNA. Thus, p21-related protcins may have the potential to function as an DNA-binding and regulating protein with the mode of upstream DNA binding closely related to their oncogenic function.In recent years, substantial scientific effort has been directed towards unravelling the cellular and molecular mechanisms which lead to uncontrolled proliferation associated with cell transformation. Regulation of gene expression represents the central role in these cellular processes of proliferation, differentiation and transformation. Specific protein-DNA interactions are known to pkay a critical role in gene regulation. Investigations in this field have largely been directed towards understanding these interactions through identification of the structural features of the proteins and DNA segments involved in these processes [l -41. As a generally accepted model. selective protein-DNA recognition is often discussed in terms of mutual structural complementarity, with the protein and DNA fitting together like a lock and key. Although, this model has been used [3 -51 prospectively with a number of examples of protein-DNA interactions, the key-and-lock model cannot predict thc critical shapes which allow the interaction, identify & I I O V D the amino acids or nucleotides which are crucial for defining this shape, or anticipate the overall physicochemical parameters which characterise the molecular forces responsiblc for selective rccognition betwcen proteins and certain sequence arrangements of DNA. Thus, this keyand-lock model has important empirical, but not predictive, capabilities relevant to the submolecular basis of protein-DNA recognition.To gain insight into the physicochemical and structural requirements which may characterise protein-DNA interacCorrespondence to M. T. W. Itearn, Department of Biochcmislry and Centre for Bioprocess Technology,...