Transcriptionally active chromatin is sensitive to Neurospora crassa and S₁ nucleases

We have examined the distribution of Neurospora crassa and S₁ nuclease cleavage products in the chromatin of the 87A7 heat shock locus of Drosophila melanogaster. Both of these nucleases generate single and double-strand breaks in chromatin at specific sites in the 87A7 locus. Before heat induction, we find that the 5′ ends of the two 87A7 hsp 70 genes contain N. crassa and S₁ nuclease hypersensitive sites, while there are only a few cleavage products from elsewhere in the locus. With N. crassa nuclease, we observe one major 5′ fragment, and this is derived from cleavage in a DNA segment mapping about 90 to 115 base-pairs from the beginning of the transcription unit. With S₁ nuclease, we find two 5′ cleavage products. The first maps about 120 to 130 base-pairs from the beginning of the gene. Interestingly, this site is also sensitive to S₁ nuclease in supercoiled but not linear naked DNA. The other fragment maps very close to the transcription start site (~0 to -15 base-pairs). After heat induction, there is a transition in the chromatin architecture of 87A7. First, there is a marked reduction in the yield of the prominent 5′ N. crassa and S₁ nuclease fragments. Second, the entire hsp 70 gene, as well as the spacer DNA just downstream from the 3′ end of the gene, becomes highly sensitive to both of these nucleases.