Effective Distance for DNA-Mediated Charge Transport between Repair Proteins

The stacked aromatic base pairs within the DNA double helix facilitate charge transport down its length in the absence of lesions, mismatches, and other stacking perturbations. DNA repair proteins containing [4Fe4S] clusters can take advantage of DNA charge transport (CT) chemistry to scan the genome for mistakes more efficiently. Here we examine the effective length over which charge can be transported along DNA between these repair proteins. We define the effective CT distance as the length of DNA within which two proteins are able to influence their ensemble affinity to the DNA duplex via CT. Endonuclease III, a DNA repair glycosylase containing a [4Fe4S] cluster, was incubated with DNA duplexes of different lengths (1.5-9 kb), and atomic force microscopy was used to quantify the binding of proteins to these duplexes to determine how the relative protein affinity changes with increasing DNA length. A sharp change in binding slope is observed at 3509 base pairs, or about 1.2 mu m, that supports the existence of two regimes for protein binding, one within the range for DNA CT, one outside of the range for CT; DNA CT between the redox proteins bound to DNA effectively decreases the ensemble binding affinity of oxidized and reduced proteins to DNA. Utilizing an Endonuclease III mutant Y82A, which is defective in carrying out DNA CT, shows only one regime for protein binding. Decreasing the temperature to 4 degrees C or including metallointercalators on the duplex, both of which should enhance base stacking and decrease DNA floppiness, leads to extending the effective length for DNA charge transport to similar to 5300 bp or 1.8 mu m. These results thus support DNA charge transport between repair proteins over kilobase distances. The results furthermore highlight the ability of DNA repair proteins to search the genome quickly and efficiently using DNA charge transport chemistry.