DNA polymerase β fingers movement revealed by single-molecule FRET suggests a partially closed conformation as a fidelity checkpoint
The eukaryotic DNA polymerase {beta} plays an important role in cellular DNA repair as it fills gaps in single nucleotide gapped DNA that result from removal of damaged bases. Since defects in DNA repair may lead to cancer and genetic instabilities, Pol {beta} has been extensively studied, especially substrate binding and a fidelity-related conformational change called \"fingers closing\". Here, we applied single-molecule Forster resonance energy transfer to study the conformational dynamics of Pol {beta}. Using an acceptor labelled polymerase and a donor labelled DNA substrate, we measured distance changes associated with DNA binding and fingers movement. Our findings suggest that Pol {beta} does not bend its gapped DNA substrate to the extent related crystal structures indicate: instead, bending seems to be significantly less profound. Furthermore, we visualized dynamic fingers closing in single Pol {beta}-DNA complexes upon addition of complementary nucleotides and derived rates of conformational changes. Additionally, we provide evidence that the fingers close only partially when an incorrect nucleotide is bound. This \"ajar\" conformation found in Pol {beta}, a polymerase of the X-family, suggests the existence of an additional fidelity checkpoint similar to what has been previously proposed for a member of the A-family, the bacterial DNA polymerase I.