Expression of heat shock protein (HSP)-coding genes is controlled by heat stress transcription factors (Hsfs). They are structurally and functionally conserved throughout the eukaryotic kingdom. In addition to the DNA-binding domain with the helix-turn-helix motif essential for DNA recognition, three functional parts in the C-terminal activator domain were characterized: (i) the HR-A/B region is responsible for oligomerization and activity control, (ii) the nuclear localizing signal (NLS) 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 are indispensible for the activator function. A peculiaritiy of plants is the heat shock-inducible new synthesis of Hsfs. In tomato HsfA1 is constitutively expressed, whereas Hsfs A2 and B1 are heat shock-inducible proteins themselves. We used Hsf knock-out strains of yeast and transient 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 (heat stress granules). In the transient expression assay its relatively low activator potential is evidently due to the inefficient nuclear import. However, the intramolecular shielding of the NLS can be released either by deletion of a short C-terminal fragment or by coexpression with HsfA1, which forms hetero-oligomers with HsfA2.