Solubility and structure of domains of chicken erythrocyte chromatin containing transcriptionally competent and inactive genes

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Abstract
Chromatin generated bymicrococcal nuclease digestion of erythrocyte nuclei can be fractionated into 2 pools of differing solubility in solvents containing 0.15-0.25 M NaCl. A fixed percentage of the chromatin is soluble under these conditions, independent of the average size of the DNA in the unfractionated chromatin. Chromatin containing particular gene sequences is also distributed between soluble and insoluble fractions in a way that is independent of the average size of the starting material. The actual percentage of gene copies present in each fraction is not necessarily the same as for bulk chromatin. The transcriptionally active chicken erythrocyte adult .beta.-globin gene is more soluble than the bulk, while the ovalbumin gene in the same tissue is less soluble. These differences do not appear to be related to variations in content of RNA, core histones, or 2 classes on non-histone proteins. The soluble chromatin pool is somewhat depleted in histones H1 and H5 and contains lower MW DNA than precipitable chromatin. The soluble fraction can be made insoluble by addition of H1. If the precipitable chromatin fraction is redigested to reduce its size and then recombined with the soluble fraction and reprecipitated, distribution of globin gene is randomized. The partitioning of chromatin into soluble and insoluble pools in 0.15-0.25 M NaCl arises from redistribution of a limiting amount of histones H1 and H5 to the chromatin fractions containing the longest DNA.