The activation domain of the maize transcription factor Opaque-2 resides in a single acidic region

TaFDL2-1A interacts with TabZIP8-7A protein to cope with drought stress via the abscisic acid signaling pathway

Drought has negative effects on cereal production. Studies have shown that many basic leucine zipper transcription factors (bZIP TFs) help to cope with drought stress. In this study, bZIP TF wheat (Triticum aestivum L.) FD-Like2 (TaFDL2) was isolated and functionally analyzed. Three homologs of TaFDL2 were identified and their expression was induced by drought and abscisic acid (ABA) treatment. TaFDL2-1A has transactivation activity and two activation domains, and the domain D region has different effects on the transcriptional activity of the two domains. Analysis of TaFDL2-1A overexpression plants indicated their enhanced drought tolerance and greater sensitivity to ABA. TabZIP8-7A was identified as a protein that interacts with TaFDL2-1A in the nucleus, and the overexpression of TabZIP8-7A conferred greater drought resistance and ABA sensitivity in Arabidopsis. Surprisingly, TaFDL2-1A × TabZIP8-7A double overexpression lines exhibited the highest drought resistance. Genetic and transcriptional regulation analyses demonstrated that stress-response gene transcription was initiated by TaFDL2-1A or TabZIP8-7A via the ABA signaling pathway. Importantly, TaFDL2-1A and TabZIP8-7A synergistically promoted ABA-inducible gene expression in a more efficient manner to form the transcriptional activation complex. Our findings provide new insights into the molecular mechanisms that allow bZIP TFs to regulate ABA signaling in response to drought stress.

Stress-Mediated cis-Element Transcription Factor Interactions Interconnecting Primary and Specialized Metabolism in planta

Plant specialized metabolites are being used worldwide as therapeutic agents against several diseases. Since the precursors for specialized metabolites come through primary metabolism, extensive investigations have been carried out to understand the detailed connection between primary and specialized metabolism at various levels. Stress regulates the expression of primary and specialized metabolism genes at the transcriptional level via transcription factors binding to specific cis-elements. The presence of varied cis-element signatures upstream to different stress-responsive genes and their transcription factor binding patterns provide a prospective molecular link among diverse metabolic pathways. The pattern of occurrence of these cis-elements (overrepresentation/common) decipher the mechanism of stress-responsive upregulation of downstream genes, simultaneously forming a molecular bridge between primary and specialized metabolisms. Though many studies have been conducted on the transcriptional regulation of stress-mediated primary or specialized metabolism genes, but not much data is available with regard to cis-element signatures and transcription factors that simultaneously modulate both pathway genes. Hence, our major focus would be to present a comprehensive analysis of the stress-mediated interconnection between primary and specialized metabolism genes via the interaction between different transcription factors and their corresponding cis-elements. In future, this study could be further utilized for the overexpression of the specific transcription factors that upregulate both primary and specialized metabolism, thereby simultaneously improving the yield and therapeutic content of plants.

Zea mays Taxilin protein negatively regulates opaque-2 transcriptional activity by causing a change in its sub-cellular distribution

Zea mays (maize) Opaque-2 (ZmO2) protein is an important bZIP transcription factor that regulates the expression of major storage proteins (22-kD zeins) and other important genes during maize seed development. ZmO2 is subject to functional regulation through protein-protein interactions. To unveil the potential regulatory network associated with ZmO2, a protein-protein interaction study was carried out using the truncated version of ZmO2 (O2-2) as bait in a yeast two-hybrid screen with a maize seed cDNA library. A protein with homology to Taxilin was found to have stable interaction with ZmO2 in yeast and was designated as ZmTaxilin. Sequence analysis indicated that ZmTaxilin has a long coiled-coil domain containing three conserved zipper motifs. Each of the three zipper motifs is individually able to interact with ZmO2 in yeast. A GST pull-down assay demonstrated the interaction between GST-fused ZmTaxilin and ZmO2 extracted from developing maize seeds. Using onion epidermal cells as in vivo assay system, we found that ZmTaxilin could change the sub-cellular distribution of ZmO2. We also demonstrated that this change significantly repressed the transcriptional activity of ZmO2 on the 22-kD zein promoter. Our study suggests that a Taxilin-mediated change in sub-cellular distribution of ZmO2 may have important functional consequences for ZmO2 activity.

Molecular evolution of the Opaque-2 gene in Zea mays L

The Opaque-2 gene (O2) in maize encodes a transcriptional activator that controls the expression of various genes during kernel development, particularly some of the most abundant endosperm storage protein genes. Compared to its wild relative teosinte, maize has bigger and heavier kernels, with an increased proportion of starch and an altered distribution of the various storage protein categories. The molecular evolution of the O2 gene was investigated in connection with its possible involvement in the domestication process. Most of the coding sequence and parts of introns, 5'UTR, and 3' noncoding regions were sequenced in a set of cultivated and teosinte accessions. One hundred six polymorphic sites (5.4%) and 72 insertions/deletions, located mostly in noncoding regions, were found. Molecular diversity was quite high (pi = 0.0138, theta = 0.0167) compared to that of other transcription factors in maize. The synonymous and nonsynonymous diversity patterns along the coding sequence suggested that different regions are submitted to different functional constraints. Such an evolution would probably be favored by the observed rapid decay of linkage disequilibrium with distance. Cultivated accessions retained about 70% of the diversity observed in teosintes. Purifying selection was detected in both maize and teosintes. No conclusive evidence was obtained for a role of the O2 gene in the domestication process.

Interaction of maize Opaque-2 and the transcriptional co-activators GCN5 and ADA2, in the modulation of transcriptional activity

Maize Opaque-2 (ZmO2), a bZip class transcription factor has been shown to activate the transcription of a series of genes expressed in the maturation phase of endosperm development. Activation requires the presence of one or more enhancer binding sites, which confer the propensity for activation by ZmO2 on heterologous promoters and in heterologous plant cell types, such as tobacco mesophyll protoplasts. The region of ZmO2 required for conferring transcriptional activation has been localised to a stretch of acidic residues in the N-terminal portion of the ZmO2 sequence, which is conserved between O2-related bZip factor sequences. Previously we identified the maize homologues of yeast transcriptional co-activators GCN5 and ADA2 that are implicated in nucleosome modification and transcription. In the present study we have shown that transcriptional modulation by ZmO2 involves the intranuclear interaction of ZmO2 with ZmADA2 and ZmGCN5. Förster resonance energy transfer (FRET) based techniques have enabled us to estimate the intracellular site of these intermolecular interactions. As a functional readout of these intranuclear interactions, we used the ZmO2 responsive maize b-32 promoter to drive the beta-glucuronidase (GUS) in the presence and absence of ZmGCN5 and ZmADA2. Our results suggest that the likely recruitment of ZmADA2 and ZmGCN5 modulates the transactivation of b-32 promoter by ZmO2 and that there may be a competition between ZmGCN5 and ZmO2 for binding to the amino-terminal of ZmADA2. The results may be taken as a paradigm for other processes of transcriptional modulation in planta involving acidic activation domains.

Alteration of GCN5 levels in maize reveals dynamic responses to manipulating histone acetylation

The role played by histone acetyltransferase (HAT), GCN5, in transcriptional co-activation has been analysed in detail in yeast and mammals. Here, we present the cloning and expression pattern of Zmgcn5, the maize homologue. The enzymatic activity of the recombinant ZmGCN5 was analysed with histone and nucleosome substrates. In situ hybridisation of developing maize kernels using Zmgcn5 as probe shows that the transcript is concentrated in rapidly dividing cells. To investigate the role of ZmGCN5 in the transcription of specific plant genes, direct protein-protein interactions were tested. A cDNA clone encoding a putative interacting partner in GCN5-adapter complexes, ZmADA2, was isolated and the interaction between ZmGCN5 and ZmADA2 was confirmed by a GST-spin down experiment. Co-immunoprecipitation of the plant transcriptional activator Opaque-2 and ZmADA2 in nuclear extracts suggests ADA2/GCN5-containing complexes to mediate transcriptional activation by binding of this bZIP factor. For a more general analysis of the effects of histone acetylation on plant gene expression, 2500 ESTs spotted on filters were hybridised with cDNA probes derived either from maize cell lines treated with Trichostatin A (TSA), or from a transgenic line expressing the ZmGCN5 antisense transcript. Several sequences showing marked changes in abundance were confirmed by RNA blot analysis. Inhibition of histone deacetylation with TSA is accompanied by a decrease in the abundance of ZmGCN5 acetylase protein, but by increases in mRNAs for histones H2A, H2B, H3 and H4. The elevated histone mRNA levels were not reflected in increasing histone protein concentrations, suggesting hyperacetylated histones arising from TSA treatment may be preferentially degraded and substituted by de novo synthesised histones. The ZmGCN5 antisense material showed suppression of the endogenous ZmGCN5 transcript and the profiling analysis revealed increased mRNA levels for H2A, H2B and H4. Furthermore, in the antisense line, a reduction in the amount of the RPD3-type HD1B-I histone deacetylase protein was observed. A model for linked regulation of histone acetylation and histone mRNA transcription is discussed.

Establishment of cereal endosperm expression domains: identification and properties of a maize transfer cell-specific transcription factor, ZmMRP-1

In maize, cells at the base of the endosperm are transformed into transfer cells that facilitate nutrient uptake by the developing seed. ZmMRP-1 is the first transfer cell-specific transcriptional activator to be identified. The protein it encodes contains nuclear localization signals and a MYB-related DNA binding domain. A single gene copy is present in maize, mapping to a locus on chromosome 8. ZmMRP-1 is first expressed soon after fertilization, when the endosperm is still a multinuclear coenocyte. The transcript accumulates in the basal nucleocytoplasmic domain that gives rise to transfer cells after cellularization. The transcript can be detected throughout transfer cell development, but it is not found in mature cells. ZmMRP-1 strongly transactivates the promoters of two unrelated transfer cell-specific genes. The properties of ZmMRP-1 are consistent with it being a determinant of transfer cell-specific expression. Possible roles for ZmMRP-1 in the regulation of endosperm and transfer cell differentiation are discussed.

The transcriptional activation domain of the plant-specific Dof1 factor functions in plant, animal, and yeast cells

Maize Dof1, one of the plant-specific Dof transcription factors, is involved in light-regulated gene expression. To elucidate the molecular mechanism underlying the activity of Dof1, in vivo functional analyses were carried out using transient expression assays with maize mesophyll protoplasts. The results suggest that the Dof domain alone, the region conserved among Dof factors, can mediate interaction with DNA in vivo and distinct Dof1 activities in greening and etiolated protoplasts. A region rich in basic amino acids was identified as a nuclear localization signal. Deletion analysis defined the transcriptional activation domain of 48 amino acids located in the C-terminus of Dof1. This activation domain was also found to be functional in both human cells and yeast, implying that Dof1 may stimulate transcription through a mechanism evolutionarily conserved among eukaryotes. A computer homology search with known transcription factors revealed that the activation domain of Dof1 displayed only a limited similarity to Activation domain II of animal transcription factor GATA-4. Mutational analysis revealed the critical role of a tryptophan residue within the activation domain of Dof1, as had been shown in Activation domain II of GATA-4.

A rice functional transcriptional activator, RISBZ1, responsible for endosperm-specific expression of storage protein genes through GCN4 motif

The GCN4 motif, a cis-element that is highly conserved in the promoters of cereal seed storage protein genes, plays a central role in controlling endosperm-specific expression. This motif is the recognition site for a basic leucine zipper transcriptional factor that belongs to the group of maize Opaque-2 (O2)-like proteins. Five different basic leucine zipper cDNA clones, designated RISBZ1-5, have been isolated from a rice seed cDNA library. The predicted gene products can be divided into two groups based on their amino acid sequences. Although all the RISBZ proteins are able to interact with the GCN4 motif, only RISBZ1 is capable of activating (more than 100-fold expression) the expression of a reporter gene under a minimal promoter fused to a pentamer of the GCN4 motif. Loss-of-function and gain-of-function experiments using the yeast GAL4 DNA binding domain revealed that the proline-rich N-terminal domain (27 amino acids in length) is responsible for transactivation. The RISBZ1 protein is capable of forming homodimers as well as heterodimers with other RISBZ subunit proteins. RISBZ1 gene expression is restricted to the seed, where it precedes the expression of storage protein genes. When the RISBZ1 promoter was transcriptionally fused to the beta-glucuronidase reporter gene and the chimeric gene was introduced into rice, the beta-glucuronidase gene is specifically expressed in aleurone and subaleurone layer of the developing endosperm. These findings suggest that the specific expression of transcriptional activator RISBZ1 gene may determine the endosperm specificity of the storage protein genes.

Modulation of gene expression by DNA-protein and protein-protein interactions in the promoter region of the zein multigene family

A common cis-acting element in the promoter region of many genes expressed during endosperm development of cereal seeds, the prolamine-box or P-box, is only 20bp upstream of the alpha-class 22-kDa zein gene-specific cis element, the O2-box, which is recognized by the b-ZIP transcription factor, Opaque-2 (O2). The proximity of these two boxes has prompted a study of how two DNA-binding proteins of a different hierarchy might be involved in the activation and modulation of the 22-kDa zein-encoding genes. This was accomplished by utilizing a highly purified P-box-binding-factor-1 (PBF-1) and a bacterially expressed truncated form of the O2 protein. After adding the recombinant O2 to the purified fraction of PBF-1, binding studies were performed with a series of DNA probes combining the P- and O2-boxes from zein promoters. These studies have revealed an interesting inhibitory effect of PBF-1 over O2 function dependent on their ratio, consistent with its in-vivo properties and the developmental expression profiles of zein genes. We also could show that the P-box is specifically recognized by topoisomerase II and single-strand DNA-binding proteins, indicating a possible additional linkage between P-box and the scaffold-attachment-region (SAR).