Osmium tetroxide: a new probe for site-specific distortions in supercoiled DNAs

摘要

Supercoiled plasmids Col El and cDm 506 (a Col El derivative carrying the D. melanogaster histone gene repeat) were treated with 0s0₄ in presence of pyriaine and the reaction products were analyzed using different approaches. Gel electrophoresis showed that 0s0₄ binding to supercoiled DNA induced its relaxation without nicking. The amount of osmium bound to DNA (as determined electrochernjcally) increased with the extent of DNA relaxation. As a result of osmium modification of supercoiled cDm 506, a single denaturation “bubble” was observed in the electron microscope. Mapping of the osmium binding site by S1 nuclease cleavage followed by restriction enzyme digestion has revealed one major site in the intergenic spacer between the H1 and H3 histone genes of D. melanogaster. This site differs from the site cleaved by S1 nuclease in supercoiled DNA in the absence of osmium.