Novel TGACG-Motif Binding Protein of Soybean

The promoters of a variety of plant genes are characterized by the presence of TGACG motif-containing sequences. These genes often exhibit quite diverse expression characteristics and in many case the TGACG-motif has been demonstrated to be essential for expression. Here we report the isolation and characterization of a soybean cDNA that encodes a novel basic/leucine zipper (bZIP) protein, STF1, that specifically interacts with Hex (TGACGTGG) and CRE (TGACGTCA) sequences. This protein contains a bZIP motif at C-teminus and an acidic domain at N-terminus. DNA binding specificities, heterodimer formation, and expression characteristics of STF1 were compared with a soybean TGA1 protein, STGA1. The soybean STF1 interacts with TGACG-sequences containing an ACGT core, while STGA1 requires TGACG as a sufficient binding sequence. The flanking sequences to the TGACG motif affected DNA binding of STF1 siginificantly. The STF1 mRNA is found mainly in dark grown soybean seedling with higher expression in apical and elongating hypocotyl, while STGA1 mRNA is highly abundant in roots of light grown plants. Furthermore, we demonstrate that STF1 heterodimerzes with G-box binding factorss (GBFs) which was not observed with TGA1. The fact that STF1 possesses both distinct DNA binding speficities and heterodimerization properties suggest that STF1 belongs to a new family of plant bZIP proteins which recognize the Hex/CRE motif.