Complementarity of microscopies in the structural analysis of DNA minicircles associated to protein MC1

: Electron microscopy of DNA, either free or complexed with ligands, allows the analysis of local conformational variations along individual molecules. Electron microscopy is unique, in that it has the capacity to determine the average behaviour of a population of molecules observed individually, and can thus provide a better appreciation of variability within the series of molecules than biophysical or biochemical methods. Very encouraging results have been obtained by cryoelectron and near-field microscopies, especially atomic force microscopy, in parallel with traditional techniques for visualizing DNA molecules adsorbed onto a support film. Differences in sample processing procedures and image formation modes render these 3 types of microscopies complementary. The torsional stress of a DNA molecule together with a local curvature induced by the protein MC1 from archaebacteria, can be detected within minicircles comprising 207 base pairs.