A close association between sites of DNase I hypersensitivity and sites of enhanced cleavage by micrococcal nuclease in the 5′‐flanking region of the actively transcribed ovalbumin gene.

The organization of chromatin was analysed in a segment of the chicken ovalbumin gene extending 6.5 kb upstream from the start site of transcription. Nuclei of chicken oviduct cells and of erythrocytes, and preparations of ‘naked’ DNA were digested with DNase I and with micrococcal nuclease. The locations of specific nuclease cleavage sites were determined by analyzing the fragments obtained with an indirect end‐labeling technique. In oviduct nuclei there are four regions of DNase I hypersensitivity centered at approximately 0.15, 0.80, 3.2 and 6.0 kb upstream from the mRNA cap site. DNase I hypersensitive regions are absent from the 5′‐flanking regions of erythrocyte nuclei. Micrococcal nuclease cleavage sites were found that are unique to oviduct nuclei and others that are enhanced in oviduct nuclei, relative to erythrocyte nuclei and to naked DNA. The locations of these micrococcal nuclease cleavage sites are closely associated with the DNase I hypersensitive regions. Nuclease hypersensitivity in the 5′‐flanking region of oviduct nuclei reflects alterations in chromatin structure that are specifically correlated with gene expression. Our results suggest the presence at hypersensitive regions of specific proteins which alter the chromatin structure, making the DNA more accessible to nuclease attack.