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.