HAT3.1, A NOVEL ARABIDOPSIS HOMEODOMAIN PROTEIN CONTAINING A CONSERVED CYSTEINE-RICH REGION

Homeodomain proteins have been shown to play a major role in the devel opment of various organisms. A novel Arabidopsis homeodomain protein h as been isolated based on its capability to interact with a DNA motif derived from the light-induced cab-E promoter of Nicotiana plumbaginif olia. The homeodomain of this protein, designated HAT3.1, differs subs tantially from those in other plant homeobox proteins identified so fa r. Furthermore, HAT3.1 is unique among other Arabidopsis proteins in t hat it does not contain a leucine zipper motif following the homeodoma in. HAT3.1 is further characterized by an N-terminal region that share s substantial sequence similarity with the maize homeodomain protein Z mhox1a. Within this conserved region, the presence of eight regularly spaced cysteine/histidine residues was observed reminiscent of other m etal-binding domains. Based on the strong evolutionary conservation of this domain, it is proposed that this region represents a novel prote in-motif which is denoted PHD-finger (plant homeo-domain-finger). In v itro DNA binding studies demonstrated that HAT3.1 is capable of intera cting with any DNA fragment larger than 100 bp. Interestingly, a delet ion of the N-terminal PHD-finger domain completely abolished DNA bindi ng, suggesting that this region may play an important functional role in protein-protein or protein-DNA interaction. HAT3.1 mRNA was primari ly detected in root tissue, implying a regulatory function of this pro tein in root development.