Electron spectroscopic imaging of DNA.

Electron spectroscopic imaging (ESI) was carried out using a fixed beam electron microscope equipped with a parallel electron energy filter to form micrographs of purified plasmid DNA without the use of heavy metal stains and shadows. Inelastically scattered electrons that have ionized the phosphorus LII,III shell electrons were used to form phosphorus distribution maps of DNA and deoxyribonucleoprotein complexes. Signal-to-noise values of the net phosphorus content over single DNA molecules compared to two and four interwound DNA strands directly reflect the known stoichiometric levels of phosphorus content, illustrating that ESI can be used to determine the relative levels of nucleic acid in nucleoprotein complexes. An initial attempt to characterize nucleosomal and transcriptionally active chromatin from Saccharomyces cerevisiae with this technique reveals three distinct ultrastructural classes of the basic chromatin fiber.