Seed-specific transcriptional regulator that specifically binds DNA sequence 5'-AGAAnnTTCT-3' known as heat shock promoter elements (HSE). Seems to be specialized for the developmental expression of heat shock protein (HSP) genes during seed maturation. Activated by ABI3. Homotrimer. Starts to be expressed in seeds from 18 days after pollination (DAP) to reach the highest expression in dry seeds. Expression is strongly reduced after only 2 hours of seed imbibition, declines drastically after 6 hours, but can be detected at a very low level even after 10 hours. The hydrophobic-rich region (HR-A/B) corresponds to the oligomerization domain. Exhibits temperature-dependent phosphorylation. Belongs to the HSF family. Class A subfamily.