Optimal heat-induced expression of the Drosophila hsp26 gene requires a promoter sequence containing (CT)_n · (GA)_n repeats

We report here the analysis of the sequence requirements for the heat-induced expression of the Drosophila melanogaster hsp26 gene using germline transformation. Heat-induced expression is augmented fivefold by a homopurine/homopyrimidine region from -85 to -134 that is devoid of heat-shock elements but contains numerous (dC-dT) · (dG-dA) repeats. Sequences within this interval have been shown to assume a nuclease S₁-hypersensitive structure in vitro. In this paper, we extend those in vitro observations, demonstrating that the S₁-hypersensitive structure is triple-helical H-DNA formed by a symmetric (dC-dT) · (dG-dA) sequence. Thus, the sequences that form H-DNA in vitro are also required in vivo for optimal hsp26 transcription. However, mutational analysis and diethylpyrocarbonate modification experiments in isolated nuclei suggest that the (dC-dT) · (dG-dA) sequence does not form H-DNA in vivo and argue against a role for H-DNA in the heat-induced expression of hsp26.