β-Globin Gene Family in Murine Erythroleukemia Cells Resides within Two Chromatin Domains Differing in Higher Order Structure

The β-globin gene family is organized into two distinct chromatin domains which are digested at significantly different rates by DNase I. We have investigated the possibility that this differential DNase I sensitivity is based upon differences in the higher order structure of chromatin. When nuclei are digested under low ionic strength conditions known to unfold higher order chromatin structures, the differential sensitivity is lost. That is, the relatively DNase I resistant domain, containing the transcriptionally inactive embryonic and β-homologous globin genes, becomes sensitive. When chromatin is recondensed with either MgCl₂ or NaCl, thus indicating the higher order coiling of the chromatin fiber, the differential sensitivity is restored. Furthermore, the removal of histone H1, known to be essential for stabilization of higher order chromatin structures, results in the loss of differential DNase I sensitivity. In contrast to the DNase I resistant domain, the transcriptionally active adult β-globin genes show no increase in the rate of digestion when chromatin is unfolded, indicating that this domain may exist as an unfolded nucleo-somal chain. The data further suggest that this sensitive domain may be depleted of histone HI.