In vitro collapse of DNA by trivalent cations like spermidine produces torus (donut) shaped DNA structures thought to have a DNA organization similar to certain double stranded DNA bacteriophage and viruses. This has prompted our studies of these structures using freeze-etch low Pt-C metal (9Å) replica TEM. With a variety of DNAs the TEM and biochemical data support a circumferential DNA winding model for hydrated DNA torus organization. Since toruses are almost invariably oriented nearly horizontal to the ice surface one of the most accessible parameters of a torus population is annulus (ring) thickness. We have tabulated this parameter for populations of both nicked, circular (Fig. 1: n=63) and linear (n=40: data not shown) ϕX-174 DNA toruses. In both cases, as can be noted in Fig. 1, there appears to be a compact grouping of toruses possessing smaller dimensions separated from a dispersed population possessing considerably larger dimensions.
