Isolation and characterization of a fourth Arabidopsis thaliana G-box-binding factor, which has similarities to Fos oncoprotein.

Abstract

A fourth member of the Aralopm G-boxb ing factor (GBF) family of bZIP proteins, GBF4, has been solated and characterized. In a manner remst of the Fos-related oncoproteins of mam n systems, GBF4 cannot bind to DNA as a ho er, although it coins a basic region cpable of spc fi g the G-box and G-boxlike elements. However, GBF4 can interact with GBF2 and GBF3 to bind DNA as hetrdmers. Mutagenesis ofthe leuine zipper of GBF4 indiates that the mutation of a single amino add coanfs upon the protein the ability to r ize the G-box as a hodimer, apparently by altering the charge distribution within the leucne zipper.The mechanisms of regulated eukaryotic gene expression are topics of intense examination. These regulatory pathways require the coordination of highly specific DNA-protein and protein-protein interactions, many of which are not fully understood. One goal is to understand the individual contribution of seemingly similar DNA elements to regulated gene expression when these conserved elements are present in promoters responding to diverse regulatory controls. In plant systems, one example of this complexity is illustrated by the hexameric G-box (5'-CACGTG-3') and those DNA elements that contain small variations on this palindromic sequence, which we refer to collectively as G-box-like elements. In some cases these elements have been identified as targets for nuclear DNA-binding factors, examples of which include the light-regulated genes of ribulose-1,5-bisphosphate carboxylase/oxygenase (1), chlorophyll a/b binding proteins (2), and chalcone synthase (3), genes regulated by abscisic acid such as the Em gene of wheat (4) and the rabl6A gene of rice (5), as well as the stress-induced Adh gene of maize and Arabidopsis (6, 7). The identification of these nuclear factors has prompted the isolation and characterization of 10 DNAbinding proteins specific for the G-box and G-box-like elements from four different plant species (4,(8)(9)(10)(11).To understand how a seemingly ubiquitous element can be involved in such diverse regulatory pathways, it is essential to understand the individual contributions made by each of the relevant DNA-binding proteins. To date, all of the plant DNA-binding proteins that recognize the G-box belong to the bZIP family of transcription factors (12, 13). These proteins are characterized by the presence of the basic region, a subdomain of -20 residues rich in basic amino acids that mediates DNA binding. Immediately adjacent to the basic region is the leucine zipper, a dimerization motif defined by a 4-3 repeat of typically four or five leucine residues interspersed with other hydrophobic amino acids, which align in parallel to form a coiled.coil, with the leucine and additional hydrophobic residues forming a hydrophobic interface (14)(15)(16)(17)(18).A well-documented example of the intricate interactions between members of a family of related bZIP proteins is illustrated with the products ofthefos andjun oncogenes (12,13,19). There are three Jun-related proteins, each of which is capa...

A Rice Functional Transcriptional Activator, RISBZ1, Responsible for Endosperm-specific Expression of Storage Protein Genes through GCN4 Motif

“…8). The dimerization capability of these transcription factors may result from a lower negative charge distribution in the zipper region as suggested by Menkes and Cashmore (49), showing that a high negative charge distribution at the "g" position close to leucines (position d) prevented the subunits from forming a homodimer. It is noted that amino acid residues at positions d and g in zipper regions are highly conserved among RISBZ1, RISBZ2, BLZ1, OHP1, and O2.…”

Intracellular localization of GBF proteins and blue light‐induced import of GBF2 fusion proteins into the nucleus of cultured Arabidopsis and soybean cells

“…Subsequently, we have cloned and characterized four members of the Arabidopsis GBF family: GBF1, GBF2, GBF3 and GBF4 (Schindler etaL, 1992a;Menkens and Cashmore, 1994). GBF1 was identified through a DNA oligonucleotide screen of a cDNA expression library, while the others were isolated by homology to the basic region of GBF1.…”

“…GBF4 contains an acidic putative activation motif (Menkens and Cashmore, 1994). All these GBFs except GBF4 can homodimerize, and, except for the GBF1/ GBF4 combination, all four can freely heterodimerize with one another to form various functional DNA-binding complexes with potentially distinct binding affinities and activation abilities (Menkens and Cashmore, 1994;Schindler et al, 1992a).…”

Phosphorylation of the Parsley bZIP Transcription Factor CPRF2 Is Regulated by Light

“…However, G-and C-boxes are not only found in the promoters of lightregulated genes but also in promoters of genes that respond to other exogenic and endogenic stimuli such as stress, hormones, and cell cycle-related signals (6). Transcription factors containing a basic leucine-zipper motif (bZIP), 1 as, for example, the common plant regulatory factors (CPRFs) from parsley (7)(8)(9)(10)(11) and G-box binding factors from Arabidopsis (12)(13)(14), were shown to bind to the G-box or the C-box, respectively, in vitro as well as in vivo and form specific homo-and heterodimers (7,8,12,13). Since CPRF and G-box binding factors proteins, which have molecular masses between 35 and 45 kDa, are encoded by multigene families (15), it is difficult to define which and how many members directly act as transcription factors regulating a certain inducible gene.…”