HEAT-STRESS RESPONSE AND HEAT-STRESS TRANSCRIPTION FACTORS

Expression of heat shock protein (HSP)-coding genes is controlled by h eat stress transcription factors (Hsfs). They are structurally and fun ctionally conserved throughout the eukaryotic kingdom. In addition to the DNA-binding domain with the helix-turn-helix motif essential for D NA recognition, three functional parts in the C-terminal activator dom ain were characterized: (i) the HR-A/B region is responsible for oligo merization and activity control, (ii) the nuclear localizing signal (N LS) formed by a cluster of basic amino acid residues which is required and sufficient for nuclear import and (iii) short C-terminal peptide motifs with a central Trp residue (AHA elements). These three parts ar e indispensible for the activator function. A peculiaritiy of plants i s the heat shock-inducible new synthesis of Hsfs. In tomato HsfAl is c onstitutively expressed, whereas Hsfs A2 and B1 are heat shock-inducib le proteins themselves. We used Hsf knock-out strains of yeast and tra nsient reporter assays in tobacco protoplasts for functional analysis of Hsf-coding cDNA clones and mutants derived from them. HsfA2, which in tomato cell cultures is expressed only after heat shock induction, tends to form large cytoplasmic aggregates together with other HSPs (h eat stress granules). In the transient expression assay its relatively low activator potential is evidently due to the inefficient nuclear i mport. However, the intramolecular shielding of the NLS can be release d either by deletion of a short C-terminal fragment or by coexpression with HsfA1, which forms hetero-oligomers with HsfA2.