Studies of DNA ‘breathing’ by polarization-sweep single-molecule fluorescence microscopy of exciton-coupled (iCy3)2dimer-labeled DNA fork constructs

Local fluctuations of the sugar-phosphate backbones and bases of DNA (often called DNA ‘breathing’) play a variety of critical roles in controlling the functional interactions of the DNA genome with the protein complexes that regulate it. Here we present a single-molecule fluorescence method that we have used to measure and characterize such conformational fluctuations at and near biologically important positions in model DNA replication fork constructs labeled with exciton-coupled cyanine [(iCy3) 2 ] dimer probes. Previous work has shown that the constructs that we test here exhibit a broad range of spectral properties at the ensemble level, and these differences can be structurally and dynamically interpreted using our present methodology at the single-molecule level. The (iCy3) 2 dimer has one symmetric (+) and one anti-symmetric (–) exciton with respective transition dipole moments oriented perpendicular to one another. We excite single molecule samples using a continuous-wave linearly polarized laser with polarization direction continuously rotated at the frequency 1 MHz. The ensuing fluorescence signal is modulated as the laser polarization alternately excites the symmetric and the anti-symmetric excitons of the (iCy3) 2 dimer probe. Phase-sensitive detection of the modulated signal provides information about the distribution of local conformations and conformational interconversion dynamics of the (iCy3) 2 probe. We find that at most construct positions that we examined the (iCy3) 2 dimer-labeled DNA fork constructs can adopt four topologically distinct conformational macrostates. These results suggest that in addition to observing DNA breathing at and near ss-dsDNA junctions, our new methodology should be useful to determine which of these pre-existing macrostates are recognized by, bind to, and are stabilized by various genome regulatory proteins.