Lack of nucleosomal structure in a DNase-I-solubilized transcriptionally active chromatin fraction of Physarum polycephalum

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Abstract
Light treatment of nuclei of Physarum polycephalum microplasmodia with DNase I, at low MgCl2 concentration (.ltoreq. 3% DNA acid solubility, 0.1 mM MgCl2) selectively solubilizes a defined fraction of chromatin, in the form of a macromolecular complex. This fraction (up to 15% of the total chromatin) contains a full complement of the core histones and a reduced amount of histone H1, and is enriched in the high-mobility-group type of proteins. It is preferentially associated with nascent RNA and RNA polymerase B actively engaged in transcription. Digestion of DNAase-I-solubilized chromatin by micrococcal nuclease releases a size-heterogeneous population of cleavage products, indicative of lack of a typical nucleosomal packaging. The procedure used allows the isolation of structurally and functionally distinct regions of Physarum chromatin.