Active genes are packaged into an altered nucleosome structure forming a chromosomal domain defined by increased sensitivity to nucleases. This structure, reflecting a potential for transcription, contains sites hypersensitive to nuclease digestion adjacent to the coding regions and may also be distinguished by specific non-histone proteins, variant or modified histones or modified DNA. Its formation, by unfolding of a tightly packed chromatin fibre by factors which might affect DNA supercoiling, may be the first step in gene activation.

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The globin gene is preferentially sensitive to digestion by DNase I in erythrocyte chromatin but not in brain, fibroblast, or oviduct chromatin. Elution of the erythrocyte chromatin with 0.35 M NaCl leads to no detectable change in the gross structure of individual nucleosomes; however, in this depleted chromatin the globin gene is no longer preferentially sensitive to DNase I. Reconstitution of the depleted chromatin with either the entire 0.35 M NaCl fraction or a subclass from this fraction greatly enriched in two high mobility group proteins (nos. 14 and 17) results in the successful reconstitution of DNase I sensitivity of the globin gene. For all of these preparations, the inactive ovalbumin gene exhibited no preferential sensitivity to DNase I. Reconstitution of the erythrocyte 0.35 M NaCl fraction with depleted brain chromatin resulted in no preferential sensitivity of the globin gene in brain chromatin; however, reconstitution of the brain 0.35 M NaCl fraction with depleted erythrocyte chromatin led to successful reconstitution of DNase I sensitivity of the globin gene. Thus, the eluted proteins responsible for conferring DNase I sensitivity are probably not tissue-specific and probably do not recognize specific DNA sequences.The globin gene in chicken erythrocyte nuclei is preferentially sensitive to digestion by DNase I but not to digestion by staphylococcal nuclease (1). The resistance of the globin gene to staphylococcal nuclease suggests that the globin gene is packaged into nucleosome-like particles; however, its sensitivity to DNase I indicates that these particles are conformationally different from most nucleosomes. The sensitivity of the globin gene to DNase I is tissue specific in that globin chromatin is preferentially digested in erythrocytes, but not in chicken oviduct. Conversely, the ovalbumin gene is preferentially digested in the chicken oviduct but not in erythrocytes (1,2). The sensitivity to DNase I of actively transcribed genes seems to be general, because sequences coding for nuclear RNA (1) and the least abundant class of mRNAs (3, 4) are also preferentially digested. Moreover, the latter genes are digested at about the same rate as the ovalbumin gene in chicken oviduct nuclei (3). This indicates that the sensitive chromatin structure may be fairly universal and independent of the frequency at which an active gene is transcribed. In addition to these genes, various actively transcribed, integrated viral genes (exogenous and endogenous) are sensitive to DNase I (5-7), and ribosomal DNA is also preferentially sensitive (8-10).The sensitivity of actively transcribed genes to DNase I probably reflects an altered nucleosome structure that is required for RNA chain elongation. It is also likely that DNase I sensitivity reflects a potential for a gene to be copied, because the globin gene remains sensitive in mature erythrocytes that are no longer active in transcription (1, 11), because the ovalbumin gene remains sensitive in the hormone-withdrawn chicken oviduct (12), and beca...

Nucleosomes from actively transcribed genes (active nucleosomes) contain nonhistone proteins HMG 14 and 17 and are preferentially sensitive to digestion by DNAse I. Active nucleosomes isolated by chromatography on an HMG 14 and 17 glass bead affinity column were analyzed with respect to overall structure, accessory nonhistone components and modifications to the DNA and histones. The experiments lead to the following conclusions: the DNA in the active nucleosome is undermethylated compared to bulk DNA; topoisomerase I is a non-stoichiometric component of the active nucleosome fraction; the level of histone acetylation is enriched in active nucleosomes, but the extent of enrichment cannot account for HMG binding; and the two histone H3 molecules in the active nucleosome can dimerize more readily and are, therefore, probably closer together than those in the bulk of the nucleosomes. Additionally it is shown that HMG 14 and 17 prefer to bind to single- vs. double-stranded nucleic acids. The role of HMG 14 and 17 in producing a highly DNAse I sensitive structure and correspondingly helping to facilitate transcription is discussed in terms of these properties.