Analysis of the DNA of isolated nucleosomes suggests that virtually all genomic DNA sequences are organized in this basic chromatin subunit. In this report, we demonstrate that although histones reside on the transcriptionally active ovalbumin genes in the oviduct, the organization of proteins about this gene renders it highly sensitive to deoxyribonuclease I (deoxyribonucleate 5'-oligonucleotidohydrolase, EC 3.1.4.5). Treatment of oviduct nuclei from the laying hen with pancreatic deoxyribonuclease I results in the preferential digestion of over 70% of the ovalbumin sequences when only 10% of the total nuclear DNA has been solubilized. Treatment of liver nuclei does not reveal selective sensitivity of these genes to DNase I.Furthermore, regions of DNA not actively transcribed, such as the endogenous leukosis virus genes in the oviduct, are not selectively degraded by this enzyme. Similar digestions with micrococcal nuclease, however, reveal no specific digestion of transcriptionally active chromatin. These data confirm the observations of H. Weintraub and M. Groudine [(1976 result, at least in part, from the specific interaction of these proteins with DNA, in such a way as to permit the transcription of a given set of genes in one tissue while restricting their expression in other tissues. It is therefore likely that structural changes occur within the chromatin complex, which may be responsible for the induction and maintenance of the transcription of specific genes.One basic structural component of the chromosome results from the organization of histone and DNA into a regular array of nucleosomes (1-7). This basic unit of structure involves about 190 base pairs of DNA complexed with two each of the four major histones and encompasses virtually all genomic sequences. Analysis of the distribution of nucleosomes over transcribing sequences reveals that the template-active regions of the genome are also organized in this basic structure (8-10). The mere presence of nucleosomes along a given DNA sequence is therefore not sufficient to prohibit transcription.Considerable evidence exists to suggest that the templateactive regions of chromatin are structurally distinct from nontranscribing segments of the genome. These aspects of Abbreviations: Cot, initial concentration of total DNA (moles of nucleotide/liter) X time (seconds); COtI/2, value of initial concentration. of total DNA (moles of nucleotide/liter) X time; Rot, initial concentration of total RNA (moles of nucleotide/liter) X. time (seconds); Rot112, value of initial concentration of total RNA (moles of nucleotide/liter) X time (seconds).

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