Selective insolubility of active hsp70 gene chromatin.

A gentle chromatin fractionation procedure was used to investigate solubility properties of Drosophila hsp70 heat-shock genes. After a brief digestion of isolated nuclei with micrococcal nuclease, most DNA is readily solubilized under low-ionic-strength conditions that maintain native nucleosomal organization. Actively transcribing hsp70 genes, however, are found to be enriched in the insoluble nuclear residue. Inactive genes are not resistant to solubilization, showing a fractionation pattern similar to that of bulk DNA. The insolubility characteristic correlates well with two other structural features of active hsp70 chromatin: increased sensitivity to endonuclease attack and disruption of the nucleosomal repeat pattern. The 5'-flanking regulatory region of active hsp70 genes is particularly resistant to solubilization, suggesting a role for binding of transcription factors in mediating this effect.