DIRECT SENSING OF HEAT AND OXIDATION BY DROSOPHILA HEAT-SHOCK TRANSCRIPTION FACTOR

The heat shock transcription factor HSF activates expression of its ta rget genes in response to elevated temperatures and chemical or physio logical stress. A key step in the activation process involves the form ation of HSF homotrimers, leading to high-affinity DNA binding. The me chanism by which HSF trimerization and DNA binding is regulated by str ess signals has remained elusive. Here, we report that trimerization a nd DNA binding of purified Drosophila HSF can be directly and reversib ly induced in vitro by heat shock temperatures in the physiological ra nge and by an oxidant, hydrogen peroxide. Other inducers of the heat s hock response, including salicylate, dinitrophenol, ethanol, and arsen ite, have no effect on HSF trimerization in vitro, indicating that the se inducers act by indirect mechanisms.