Differences in the circular dichroism spectra of of eu- and heterochromatin fractions from rat liver.

Significant differences are observed between the CD spectra of eu- and heterochromatin prepared by fractionating mechanically sheared rat liver chromatin on linear sucrose density gradients. Heterochromatin has a broad positive CD at 270--280 nm, a cross-over point at 256 nmn, and a negative shoulder at 248 nm. Euchromatin displays a shift in the positive peak to 267 nm and the cross-over point to 254 nm, as well as an intensification of the 267 nm peak. The negative shoulder at 248 nm is absent in euchromatin. These spectral differences between eu- and heterochromatin are present in both low and high ionic strength media. Mechanical shearing does not induce CD alteration and anomalous light scattering is absent in our system. The greater RNA content of euchromatin compared to heterochromatin cannot account for the spectral differences. These results suggest that the DNA conformation of euchromatin is distinct from that of heterochromatin and they may provide clues to the reasons for the greater transcription of euchromatin.