Professor Achillefs Kapanidis

Abstract:

The remarkable fidelity of most DNA polymerases depends on a series of early steps in the reaction pathway which allow the selection of the correct nucleotide substrate, while excluding all incorrect ones, before the enzyme is committed to the chemical step of nucleotide incorporation. The conformational transitions that are involved in these early steps are detectable with a variety of fluorescence assays and include the fingers-closing transition that has been characterized in structural studies. Using DNA polymerase I (Klenow fragment) labeled with both donor and acceptor fluorophores, we have employed single-molecule fluorescence resonance energy transfer to study the polymerase conformational transitions that precede nucleotide addition. Our experiments clearly distinguish the open and closed conformations that predominate in Pol-DNA and Pol-DNA-dNTP complexes, respectively. By contrast, the unliganded polymerase shows a broad distribution of FRET values, indicating a high degree of conformational flexibility in the protein in the absence of its substrates; such flexibility was not anticipated on the basis of the available crystallographic structures. Real-time observation of conformational dynamics showed that most of the unliganded polymerase molecules sample the open and closed conformations in the millisecond timescale. Ternary complexes formed in the presence of mismatched dNTPs or complementary ribonucleotides show unique FRET species, which we suggest are relevant to kinetic checkpoints that discriminate against these incorrect substrates.

Abstract:

The analysis of structure and dynamics of biomolecules is important for understanding their function. Toward this aim, we introduce a method called 'switchable FRET', which combines single-molecule fluorescence resonance energy transfer (FRET) with reversible photoswitching of fluorophores. Typically, single-molecule FRET is measured within a single donor-acceptor pair and reports on only one distance. Although multipair FRET approaches that monitor multiple distances have been developed, they are technically challenging and difficult to extend, mainly because of their reliance on spectrally distinct acceptors. In contrast, switchable FRET sequentially probes FRET between a single donor and spectrally identical photoswitchable acceptors, dramatically reducing the experimental and analytical complexity and enabling direct monitoring of multiple distances. Our experiments on DNA molecules, a protein-DNA complex and dynamic Holliday junctions demonstrate the potential of switchable FRET for studying dynamic, multicomponent biomolecules. © 2010 Nature America, Inc. All rights reserved.