Solenoidal model for superstructure in chromatin.

TLDR

The authors propose that the 110‑Å x‑ray reflection series in chromatin arises from the spacing between turns of a solenoidal supercoil rather than from nucleosome spacing along the nucleofilament. Experiments show that micrococcal‑nuclease‑treated chromatin forms ~100‑Å filaments that, in 0.2 mM Mg²⁺, condense into a solenoidal supercoil with ~110‑Å pitch that requires histone H1 and matches the ~300‑Å thread seen in nuclei.

Abstract

Chromatin prepared by brief digestion of nuclei with micrococcal nuclease, and extracted in 0.2 mM EDTA, appears in the electron microscope as filaments of about 100 A diameter which coil loosely. In 0.2 mM Mg++ these "nucleofilaments" condense into a supercoil or solenoidal structure of pitch about 110 A corresponding to the diameter of a nucleofilament. It is proposed that the x-ray reflections at orders of 110 A observed in chromatin originate in the spacing between turns of the solenoid rather than that between nucleosomes along the nucleofilament. The solenoidal structure appears to need histone H1 for its stabilization. Under certain conditions, isolated nucleosomes can also aggregate into a similar structure. The solenoidal structure can be correlated with the "thread" of diameter about 300 A observed by other workers in nuclei.