THE PLANT CELL, Vol 8, Issue 6 1041-1059, Copyright © 1996 by American Society of Plant Biologists
GT-2: In Vivo Transcriptional Activation Activity and Definition of Novel Twin DNA Binding Domains with Reciprocal Target Sequence Selectivity
M. Ni, K. Dehesh, J. M. Tepperman and P. H. Quail
Department of Plant Biology, University of California, Berkeley, California 94720
GT-2 is a novel DNA binding protein that interacts with a triplet of
functionally 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 enhanced
transcription from both homologous and heterologous GT-box-containing
promoters, thereby indicating that this protein can function as a
transcriptional activator in vivo. Previously, we have shown that GT-2
contains separate DNA binding determinants in its N- and C-terminal halves,
with binding site preferences for the GT3-bx and GT2-bx promoter motifs,
respectively. Here, we demonstrate that the minimal DNA binding domains
reside within dual 90-amino acid polypeptide segments encompassing
duplicated sequences, termed trihelix regions, in each half of the
molecule, plus 15 additional immediately adjacent amino acids downstream.
These minimal binding domains retained considerable target sequence
selectivity for the different GT-box motifs, but this selectivity was
enhanced by a separate polypeptide segment farther downstream 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 activity toward
closely related target motifs and a modifier sequence conferring higher
resolution reciprocal selectivity between these motifs.
