Thermal denaturation and circular dichroism properties of sheared and unsheared chromatins

Abstract

Nicolini et al. [I] recently reported that the circular dichroism spectrum of chromatin solubilized by strong shear forces differs markedly from the circular dichroism spectrum (corrected for differential scattering) of ‘unsheared’ chromatin prepared simply by suspending a swollen chromatin gel. Their results suggest that chromatin solubilized by shearing may not be appropriate to use for physical measurements. Unfortunately the huge size of the chromatin fragments in the ‘unsheared’ preparation of Nicolini et al. [ I] seriously complicates study of some physical properties (such as circular dichroism) and makes hydrodynamic investigations virtually impossible. Noll et al. [2] introduced the use of a short exposure of nuclei to micrococcal nuclease as an alternative to shear solubilization. This procedure avoids subjecting a chromatin gel to shear forces and has the additional benefit of solubilizing only chromatin. One disadvantage of the nuclease procedure is that the DNA molecules in the released chromatin fragments vary widely in the molecular weight [2] . This variation would greatly complicate hydrodynamic studies of chromatin as well as fractionation schemes based on chromatography or sedimentation. Strong shear forces can be used to reduce the size heterogeneity of DNA samples, but Noll et al. [2] reported that shearing nuclease-released chromatin results in a