Eon, S., Culard, F., Sy, D., Charlier, M. and Spotheim-Maurizot, M. Radiation Disrupts Protein–DNA Complexes through Damage to the Protein. The lac Repressor–Operator System. Radiat. Res. 156, 110–117 (2001).
Binding of a protein to its cognate DNA sequence is a key step in the regulation of gene expression. If radiation damage interferes with protein–DNA recognition, the entire regulation process may be perturbed. We have studied the effect of γ rays on a model regulatory system, the E. coli lactose repressor–operator complex. We have observed the disruption of the complex upon irradiation in aerated solution. The complex is completely restored by the addition of nonirradiated repressor, but not by the addition of nonirradiated DNA. Thus radiation disrupts the DNA–protein complex by affecting the binding ability of the protein. This interpretation is supported by the dramatic loss of binding ability of a free irradiated repressor toward nonirradiated DNA. Interestingly, the dose necessary for the disruption of the irradiated complex is higher than that for inducing the complete loss of the binding ability of the free irradiated repressor. This may be due to the protection of key amino acids by the bound DNA. As seen from calculations of the accessibility of amino acids to radiolytic OH·, the protection is due to both masking and conformational effects.