COMPLEMENTARITY OF MICROSCOPIES FOR STRUC TURAL-ANALYSIS OF MC1-DNA MINICIRCLES

Electron microscopy of DNA, either free or complexed with ligands, all ows the analysis of local conformational variations along individual m olecules. Electron microscopy is unique, in that it has the capacity t o determine the average behaviour of a population of molecules observe d individually, and can thus provide a better appreciation of variabil ity within the series of molecules than biophysical or biochemical met hods. Very encouraging results have been obtained by cryoelectron and near-field microscopies, especially atomic force microscopy, in parall el with traditional techniques for visualizing DNA molecules adsorbed onto a support film. Differences in sample processing procedures and i mage formation modes render these 3 types of microscopies complementar y. The torsional stress of a DNA molecule together with a local curvat ure induced by the protein MC1 from archaebacteria, can be detected wi thin minicircles comprising 207 base pairs.