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 investig ate the presence of unrestrained supercoiling in DNA sequences located in nontranscribed regions flanking the 3' ends of the pair of diverge nt heat shock protein 70 (hsp70) genes at locus 87A7 of Drosophila. Tw o 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 regar dless of the transcriptional status of the hsp70 genes. The steric acc essibility of the scs region before heat shock was 3-fold higher than either flanking region (consistent with its previously documented DNas e I hypersensitivity); this increased an additional 2-fold following h sp70 gene activation without a concomitant rise in the accessibility o f flanking regions. Most notably, a sequence which lies outside the pr esumed 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 displaye d a low level of tension that was also essentially unaffected by trans cription, consistent with data obtained previously for a similarly sit uated fragment at the centromere-distal scs' location. The existence o f a highly localized region of supercoiling within the scs and scs' se quences might be related to their activity in vivo as insulators of ch romosomal position effects in Drosophila.