O(6)-alkylguanine-DNA alkyltransferases directly reverse the alkylation on the O(6) position of guanine in DNA. This group of proteins has been proposed to repair the damaged base in an extrahelical manner; however, the detailed mechanism is not understood. Here we applied a chemical disulfide crosslinking method to probe the damage-searching mechanism of two O(6)-alkylguanine-DNA alkyltransferases, the Escherichia coli C-Ada and the human AGT. Crosslinking reactions with different efficiency occur between the reactive Cys residues of both proteins and a modified cytosine bearing a thiol tether in various DNA probes. Our results indicate that it is not necessary for these proteins to actively flip out every base to find damage. Instead they can locate potential lesions by simply capturing a lesioned base that is transiently extrahelical or sensing the unstable nature of a damaged base pair.
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