Roles of histone acetylation modification in basal and inducible expression of hsp26 gene in D. melanogaster.

The promoter of the Drosophila hsp26 gene contains two DNase I-hypersensitive (DH) sites and a positioned nucleosome, and this open chromatin structure is required for heat-inducible expression. Histone acetylation modification participates in transcriptional regulation of genes by affecting the status of chromatin remodeling. In this study, we investigated the roles of histone acetylation modification on hsp26 expression in Drosophila. We showed that the histone deacetylase inhibitor (HDI) treatments of Drosophila larvae induced the histone H3 hyperacetylation at the promoter DH sites, which facilitated the binding of heat shock factor (HSF) to heat shock element (HSE). This resulted in a promoted transcription of hsp26 gene following the heat shock, and further increased the inducible expression of hsp26 gene. On the contrary, the HDI-induced histone H3 hyperacetylation in the middle nucleosome decreased the basal expression of hsp26 gene under the normal growth conditions. In addition, by following up the heat-shock time course, we showed that the histone acetylation level at the DH sites exhibited a drop-raise-drop change, while that at the positioned nucleosome underwent a raise-drop-raise-drop switchover. These results demonstrated the distinct roles played by histone acetylation modification in hsp26 gene basal and inducible expression regulation in D. melanogaster.