Rat liver chromatin. Fractionation into eu- and heterochromatin with localization of ribosomal genes.

Purified, normal rat liver chromatin was sheared under controlled conditions in a Virtis “60” homogenizer and then separated into template-active (euchromatin) and template-inactive (heterochromatin) fractions by glycerol gradient centrifugation. The euchromatin portions of the glycerol gradients possessed greater than 90% of the in vitro template activity when estimated with Escherichia coli RNA polymerase and contained more than 90% of the nascent RNA formed in vivo. Inhibitor studies performed in vivo indicated that the leading edge of the heterochromatin portion of the glycerol gradients contains the genes coding for ribosomal RNA. Recentrifugation of putative eu- and heterochromatin fractions demonstrated that the isolation procedure produces fractions which are distinct and are characterized by little or no cross contamination. The data suggest that controlled shearing in conjunction with gradient centrifugation provides a means of fractionating with great fidelity the transcribable portions of the rat liver genome.