GT-2 - IN-VIVO TRANSCRIPTIONAL ACTIVATION ACTIVITY AND DEFINITION OF NOVEL TWIN DNA-BINDING DOMAINS WITH RECIPROCAL TARGET SEQUENCE SELECTIVITY

GT-2 is a novel DNA binding protein that interacts with a triplet of f unctionally defined, positively acting GT-box motifs (GT1-bx, GT2-bx, and GT3-bx) in the rice phytochrome A gene (PHYA) promoter. Data from a transient transfection assay used here show that recombinant GT-2 en hanced transcription from both homologous and heterologous GT-box-cont aining promoters, thereby indicating that this protein can function as a transcriptional activator in vivo. Previously, we have shown that G T-2 contains separate DNA binding determinants in its N- and C-termina l halves, with binding site preferences for the GT3-bx and GT2-bx prom oter motifs, respectively, Here, we demonstrate that the minimal DNA b inding domains reside within dual 90-amino acid polypeptide segments e ncompassing duplicated sequences, termed trihelix regions, in each hal f of the molecule, plus 15 additional immediately adjacent amino acids downstream, These minimal binding domains retained considerable targe t sequence selectivity for the different GT-box motifs, but this selec tivity was enhanced by a separate polypeptide segment farther downstre am on the C-terminal side of each trihelix region. Therefore, the data indicate that the twin DNA binding domains of GT-2 each consist of a general GT-box recognition core with intrinsic differential binding ac tivity toward closely related target motifs and a modifier sequence co nferring higher resolution reciprocal selectivity between these motifs .