Functions as a stress-inducible and DNA-binding transcription factor that plays a central role in the transcriptional activation of the heat shock response (HSR), leading to the expression of a large class of molecular chaperones heat shock proteins (HSPs) that protect cells from cellular insults' damage (PubMed:8455593). In unstressed cells, is present in a HSP90-containing multichaperone complex that maintains it in a non-DNA-binding inactivated monomeric form. Upon exposure to heat and other stress stimuli, undergoes homotrimerization and activates HSP gene transcription through binding to site-specific heat shock elements (HSEs) present in the promoter regions of HSP genes. Upon heat shock stress, forms a chromatin-associated complex with TTC5/STRAP and p300/EP300 to stimulate HSR transcription, therefore increasing cell survival (By similarity). Activation is reversible, and during the attenuation and recovery phase period of the HSR, returns to its unactivated form. Binds to inverted 5'-NGAAN-3' pentamer DNA sequences. Binds to chromatin at heat shock gene promoters. Also serves several other functions independently of its transcriptional activity. Involved in the repression of Ras-induced transcriptional activation of the c-fos gene in heat-stressed cells. Positively regulates pre-mRNA 3'-end processing and polyadenylation of HSP70 mRNA upon heat-stressed cells. Plays a role in nuclear export of stress-induced mRNA. Plays a role in the regulation of mitotic progression. Also plays a role as a negative regulator of non-homologous end joining (NHEJ) repair activity in a DNA damage-dependent manner. Involved in stress-induced cancer cell proliferation. Monomer; cytoplasmic latent and transcriptionally inactive monomeric form in unstressed cells. Homotrimer; in response to stress, such as heat shock, homotrimerizes and translocates into the nucleus, binds to heat shock element (HSE) sequences in promoter of heat shock protein (HSP) genes and acquires transcriptional ability. Low expression found in most tissues with the exception of blood and liver. Highest levels are found in the pigmental layer of retina and in the lymphoblastoid cell line MSB-1. Expressed during development. In unstressed cells, spontaneous homotrimerization is inhibited. Intramolecular interactions between the hydrophobic repeat HR-A/B and HR-C regions are necessary to maintain HSF1 in the inactive, monomeric conformation. In heat stressed cells, HSF1 homotrimerization occurs through formation of a three-stranded coiled-coil structure generated by intermolecular interactions between HR-A/B regions allowing DNA-binding activity. The D domain is necessary for translocation to the nucleus. 9aaTAD is a transactivation motif present in a large number of yeast and animal transcription factors. Belongs to the HSF family.