Breaking DNA Double Helix by Bending Stress
Monday, September 17, 2007 - 10:30am - 11:30am
DNA double helix should experience local breaks under sufficient bending or/and unwinding torsional stress. Local distortions of DNA under negative torsional stress have been studied in details, but until now very little has been known about distortions by bending stress. We addressed this question in the current study by probing the structure of very small DNA circles. First, we developed an efficient method to obtain covalently closed DNA minicircles. To detect breaks of regular DNA structure in these minicircles we treated them by single strand-specific endonucleases. This method has been widely used to study local conformational changes in supercoiled plasmids for many years. We showed, in agreement with the previous data obtained on plasmid DNA, that sufficient torsional stress introduces DNA breaks in the minicircles. Choosing the experimental conditions where the influence of torsional stressed was minimized, we found that the double helix is broken by bending deformation in the minicircles of 64-65 bp, but not in the 85-86 bp minicircles. Our data suggest that two different single strand-specific endonucleases, used in the study, have different sensitivity to the breaks created by bending and torsional deformations. We speculate, using this observation, that torsional stress creates open regions in the double helix while strong DNA bending initiates formation of kinks, which preserves the base pairing.