Isolation and characterization of HsfA3, a new heat stress transcription factor of Lycopersicon peruvianum

Stress-induced transcription of heat shock proteins (Hsps) in eukaryotes is mediated by a conserved class of transcription factors called heat stress transcription factors (Hsfs). Here we report the isolation and functional c haracterization of HsfA3, a new member of the Hsf family. HsfA3 was cloned from a tomato heat stress cDNA library by yeast two-hybrid screening, using HsfA1 as a bait. HsfA3 is a single-copy gene with all the conserved sequen ce elements characteristic of a heat stress transcription factor. The const itutively expressed HsfA3 is mainly found in the cytoplasm under control co nditions and in the nucleus under heat stress conditions. Functionally, Hsf A3 behaves similarly to the already known members of tomato Hsf family. It is able to substitute yeast Hsf for viability functions and is a strong act ivator of Hsf-dependent reporter constructs both in tobacco protoplasts and yeast. Finally, similar to the AHA motifs in HsfA1 and HsfA2, the activato r function depends on four short peptide motifs with a central tryptophan r esidue found in the C-terminal domain of HsfA3.