A cDNA Encoding an HD-Zip Protein from Sunflower

The homeodomain is a 61-amino acid protein motif present in a number of eukaryotic transcription factors (Gehring, 1987). This motif, first recognized to be encoded by some Drosophiln genes involved in development (Gehring, 1987), folds into a characteristic helix-tum-helix structure that participates in DNA binding (Kissinger et al., 1990). In plants, different strategies have led to the isolation and characterization of homeoboxes (DNA sequences encoding homeodomains). In Arabidopsis thalinna, degenerate oligonucleotides deduced from conserved regions of animal homeodomains have been used to screen cDNA libraries, resulting in the isolation of severa1 clones that represent four to five different genes coding unequivocally for homeodomains (Ruberti et al., 1991; Mattson et al., 1992; Schena and Davis, 1992; Carabelli et al., 1993). The encoded proteins are related to each other and contain a Leu zipper motif adjacent to the homeodomain. One of the members of this family of proteins, named HD-Zip proteins, is a developmental regulator (Schena et al., 1993). We have undertaken the study of homeobox-containing genes in sunflower. For this purpose we have used a degenerate oligonucleotide [ 5 C TT G G ATC C G C N C ( T G ) NC(GT)(GA)TT(TC)TG(AG)AACCA-3’1 derived from the conserved sequence WFQNRRA from helix 3 of the homeodomain to screen a stem cDNA library. Portions of cDNA clones were amplified by PCR using this degenerate oligonucleotide together with X sequencing primers (Gonzalez and Chan, 1993). One of the clones isolated was used to screen the original stem cDNA library in X gtlO (Table I). A cDNA clone (named Hahb-1, after Helianthus nnnuus Homeobox) containing a 1.2-kb insert was subcloned into pUC119 and sequenced. Hahb-1 contains a typical HD-Zip domain. The spacing between the Leu zipper and the homeodomain is the same in a11 HD-Zip proteins analyzed to date, suggesting a role for the Leu zippers in the formation of dimers that position two homeodomains for DNA binding. This structure has been described only for plant proteins. The Leu zipper of Hahb-1 has five Leu and one Val (in position d3). Another interesting feature in the deduced protein sequence is the presence of an acidic domain adjacent to the amino terminus of the homeodomain. Acidic domains are present in many transcription factors (Ptashne and Gann, 1990) and are lo-