Arabidopsis and the heat stress transcription factor world: how many heat stress transcription factors do we need?

Sequencing of the Arabidopsis genome revealed a unique complexity of the pl ant heat stress transcription factor (Hsf) family. By structural characteri stics and phylogenetic comparison, the 21 representatives are assigned to 3 classes and 14 groups. Particularly striking is the finding of a new class of Hsfs (AtHsfC1) closely related to Hsf1 from rice and to Hsfs identified from frequently found expressed sequence tags of tomato, potato, barley, a nd soybean. Evidently, this new type of Hsf is well expressed in different plant tissues. Besides the DNA binding and oligomerization domains (HR-A/B region), we identified other functional modules of Arabidopsis Hsfs by sequ ence comparison with the well-characterized tomato Hsfs. These are putative motifs for nuclear import and export and transcriptional activation (AHA m otifs). There is intriguing flexibility of size and sequence in certain par ts of the otherwise strongly conserved N-terminal half of these Hsfs. We ha ve speculated about possible exon-intron borders in this region in the anci ent precursor gene of plant Hsfs, similar to the exon-intron structure of t he present mammalian Hsf-encoding genes.