Evidence for hydrated spermidine-calf thymus DNA toruses organized by circumferential DNA wrapping

Abstract

In spermidine-condensed calf thymus DNA preparations, torus-shaped condensates were shown by transmission electron microscopy to exist under the hydrated conditions of the freeze fracture experiment. Using extremely low Pt metal deposition levels (9 A Pt/C) high-contrast replicas of the spermidine-DNA toruses were obtained that showed circumferential wrapping of single DNA double helix-size surface fibres. Stereoscopic analysis of high magnification stereomicrographs established some details of the three-dimensional organization of two DNA double helix sections winding circumferentially on the inner surface of one such torus. These measurements demonstrate the usefulness of stereoscopic analysis of these high macromolecular organization magnification. Measurements on a number of torus-shaped complexes (n = 16) yielded these average dimensions: inner circumference (1840 +/- 204 A) outer circumference (2800 +/- 222 A), torus ring thickness (143 +/- 18 A). These data support a continuous circumferential DNA-winding model of torus organization proposed by Marx & Reynolds.