Low temperature regulation of the Arabidopsis CBF family of AP2 transcriptional activators as an early step in cold-induced COR gene expression

Cold-induced expression of the Arabidopsis COR (cold-regulated) genes is me diated by a DNA regulatory element termed the CRT (C-repeat)/DRE (dehydrati on-responsive element). Recently, we identified a transcriptional activator , CBF1, that binds to the CRT/DRE and demonstrated that its overexpression in transgenic Arabidopsis plants at non-acclimating temperatures induces CO R gene expression and increases plant freezing tolerance. Here we report th at CBF1 belongs to a small family of closely related proteins which include s CBF2 and CBF3. DNA sequencing of an 8.7 kb region of the Arabidopsis geno me along with genetic mapping experiments indicated that the three CBF gene s are organized in direct repeat on chromosome 4 at 72.8 cM, closely linked to molecular markers PG11 and m600. Like CBF1, both CBF2 and CBF3 activate d expression of reporter genes in yeast that contained the CRT/DRE as an up stream activator sequence. The transcript levels for all three CBF genes in creased within 15 min of transferring plants to low temperature, followed b y accumulation of COR gene transcripts at about 2 h. CBF transcripts also a ccumulated rapidly in response to mechanical agitation. The promoter region s of the CBF genes do not contain the CRT sequence, CCGAC, and overexpressi on of CBF1 did not have a detectable effect on CBF3 transcript revels, sugg esting that the CBF gene family is not subject to autoregulation. We propos e that cord-induced expression of CRT/DRE-containing COR genes involves a l ow temperature-stimulated signalling cascade in which CBF gene induction is an early event.