Specialized chromatin structure domain boundary elements flanking a Drosophila heat shock gene locus are under torsional strain in vivo.

An in vivo assay employing psoralen cross-linking was used to investigate the presence of unrestrained supercoiling in DNA sequences located in nontranscribed regions flanking the 3' ends of the pair of divergent heat shock protein 70 (hsp70) genes at locus 87A7 of Drosophila. Two of the regions examined contain sequences comprising the previously defined specialized chromatin structure elements (scs and scs'). Both of these putative chromosomal domain boundaries exhibited very similar levels of unrestrained negative supercoiling that remained high regardless of the transcriptional status of the hsp70 genes. The steric accessibility of the scs region before heat shock was 3-fold higher than either flanking region (consistent with its previously documented DNase I hypersensitivity); this increased an additional 2-fold following hsp70 gene activation without a concomitant rise in the accessibility of flanking regions. Most notably, a sequence which lies outside the presumed 87A7 domain, as defined by the centromere-proximal scs element, exhibited no detectable torsional tension regardless of gene activity in the domain. A sequence located just inside the scs region displayed a low level of tension that was also essentially unaffected by transcription, consistent with data obtained previously for a similarly situated fragment at the centromere-distal scs' location. The existence of a highly localized region of supercoiling within the scs and scs' sequences might be related to their activity in vivo as insulators of chromosomal position effects in Drosophila.