Promoter sequence containing (CT)n.(GA)n repeats is critical for the formation of the DNase I hypersensitive sites in the Drosophila hsp26 gene.

We have analyzed P-element-transformed lines carrying hsp26/lacZ transgenes with various deletions and substitutions within the Drosophila melanogaster hsp26 promoter region in order to identify the sequences required for the formation of the DNase I hypersensitive sites (DH sites). DH sites are generally found associated with promoters and enhancer elements of active and inducible eukaryotic genes, and are thought to be nucleosome-free regions of DNA that interact with regulatory proteins and the transcriptional machinery. There are two major DH sites located within the promoter region of the hsp26 gene, centered at -50 and at -350 (relative to the hsp26 transcription start site). The sequences from -135 to -85, which contain (CT)n.(GA)n repeats, contribute significantly to the formation of the DH sites in the hsp26 promoter region. Deletion or substitution of this (CT)n region drastically reduces the accessibility of the DNA at these sites to DNase I. This reduction in accessibility was quantified by measuring the susceptibility of the DNA within nuclei to cleavage at a restriction site within the DH site. In addition to the (CT)n region and the promoter at -85 to +11 (region P), one of two other regions must be present for effective creation of the DH sites: sequences between -351 and -135 (region A), or sequences between +11 and +632 (region D). Disruption of the wild-type chromatin structure, as assayed by the loss of accessibility to the DH sites, is correlated with a decrease in inducible transcriptional activity, even when the TATA box and heat shock regulatory elements are present in their normal positions.