Interaction of the erythroid transcription factor cGATA-1 with a critical auto-regulatory element

A composite intronic element directs dynamic binding of the progesterone receptor and GATA-2

The progesterone receptor (PR) plays a pivotal role in proper development and function of the mammary gland and has also been implicated in mammary tumorigenesis. PR is a ligand-activated transcription factor; however, relatively, little is known about its mechanisms of action at endogenous target promoters. The aim of our study was to identify a natural PR-responsive gene and investigate its transcriptional regulation in the mammary microenvironment. Our experiments revealed FKBP5 as a direct target of the PR, because it exhibited a rapid activation by progestin that was cycloheximide independent and correlated with recruitment of RNA polymerase II to the promoter. Site-directed mutagenesis and chromatin immunoprecipitation assays showed that progestin responsiveness is mediated through a composite element in the first intron, to which the PR binds concomitantly with GATA-2. Mutational analysis of the element revealed that the GATA-2 site is essential for progestin activation. Direct binding of PR to DNA contributes to the efficiency of activation but is not sufficient, suggesting that the receptor makes important protein-protein interactions as part of its mechanism of action at the FKBP5 promoter. Using chromatin immunoprecipitation assays we also determined that the intronic region is in communication with the promoter, probably via DNA looping. Time course analysis revealed a cyclical pattern of PR recruitment to the FKBP5 gene but a persistent recruitment to the mouse mammary tumor virus promoter, indicating that receptor cycling is a gene-specific phenomenon rather than a characteristic of the receptor itself. Our study offers new insight in the nature of PR-regulated transcription in mammary cancer cells.

Crystal structures of multiple GATA zinc fingers bound to DNA reveal new insights into DNA recognition and self-association by GATA

The GATA family of transcription factors (GATA1-6) binds selected GATA sites in vertebrate genomes to regulate specific gene expression. Although vertebrate GATA factors have two highly conserved zinc finger motifs, how the two fingers act together to recognize functional DNA elements is not well understood. Here we determined the crystal structures of the C-terminal zinc finger of mouse GATA3 bound to DNA containing two variously arranged GATA binding sites. Our structures and accompanying biochemical analyses reveal two distinct modes of DNA binding by GATA to closely arranged sites. One mode involves cooperative binding by two GATA factors that interact with each other through protein-protein interactions. The other involves simultaneous binding of the N-terminal zinc finger (N-finger) and the C-terminal zinc finger of the same GATA factor. Our studies represent the first crystallographic analysis of GATA zinc fingers bound to DNA and provide new insights into the DNA recognition mechanism by the GATA zinc finger. Our crystal structure also reveals a dimerization interface in GATA that has previously been shown to be important for GATA self-association. These findings significantly advance our understanding of the structure and function of GATA and provide an important framework for further investigating the in vivo mechanisms of GATA-dependent gene regulation.

In vitro effects of budesonide on eosinophil-basophil lineage commitment

IL-5 is the primary cytokine that stimulates the production and survival of eosinophils and basophils from progenitor cells. The inhaled glucocorticoid, budesonide, has been shown to exert a therapeutic effect via suppression of eosinophil/basophil progenitors in vivo. Since various steroids have exhibited the ability to enhance eosinophil/basophil progenitor differentiation, we examined the effects of budesonide in vitro. Bone marrow and cord blood samples were obtained and cultured in the presence of IL-5 alone or IL-5 plus budesonide. Eosinophil/basophil colony-forming units were enumerated from cultured nonadherent mononuclear cells and from purified CD34⁺ cells. CD34⁺ cells with and without budesonide were also examined for up-regulation of ERK1/2, MAPK and GATA-1 using real time-PCR.

RESULTS

i) up-regulation of eosinophil/basophil colony-forming units is due to the direct effects of budesonide on IL-5-stimulated progenitors; ii) GATA-1 is likely involved in the early amplification of eosinophil/basophil progenitor commitment leading to increased differentiation. A potential transcriptional pathway has been identified which may mediate the effects of budesonide on eosinophil/basophil lineage commitment.

The GATA site-dependent hemogen promoter is transcriptionally regulated by GATA1 in hematopoietic and leukemia cells

Hemgn (a gene symbol for hemogen in mouse, EDAG in human and RP59 in rat) encodes a nuclear protein that is highly expressed in hematopoietic tissues and acute leukemia. To characterize its regulatory mechanisms, we examined the activities of a Hemgn promoter containing 2975 bp of 5' flanking sequence and 196 bp of 5' untranslated region (5' UTR) sequence both in vitro and in vivo: this promoter is preferentially activated in a hematopoietic cell line, not in nonhematopoietic cell lines, and is sufficient to drive the transcription of a lacZ transgene in hematopoietic tissues in transgenic mice. Mutagenesis analyses showed that the 5' UTR including two highly conserved GATA boxes is critical for the promoter activity. GATA1, not GATA2, binds to the GATA binding sites and transactivates the Hemgn promoter in a dose-dependent manner. Furthermore, the expression of human hemogen (EDAG) transcripts were closely correlated with levels of GATA1 transcripts in primary acute myeloid leukemia specimens. This study suggests that the Hemgn promoter contains critical regulatory elements for its transcription in hematopoietic tissues and Hemgn is a direct target of GATA1 in leukemia cells.

Determinants of GATA-1 binding to DNA: the role of non-finger residues

Mammalian GATA transcription factors are expressed in various tissues in a temporally regulated manner. The prototypic member, GATA-1, is required for normal erythroid, megakaryocytic, and mast cell development. This family of DNA-binding proteins recognizes a consensus (A/T)GATA(A/G) motif and possesses homologous DNA binding domains consisting of two zinc fingers. The C-terminal finger of GATA-1 recognizes the consensus motif with nanomolar affinities, whereas the N-terminal finger shows a binding preference for a GATC motif, albeit with much reduced affinity (Kd approximately microm). The N-terminal finger of GATA-2 also shows a preference for an AGATCT binding site, with an increased affinity attributed to N- and C-terminal flanking basic residues (Kd approximately nm). To understand the differences in the binding specificities of the N- and C-terminal zinc fingers of GATA-1, we have constructed a series of swapped domain peptides. We show that the specificity for AGATAA over AGATCT arises from the C-terminal non-finger basic domain. Thus, the N-terminal finger binds preferentially to AGATAA once appended to the C-terminal arm of the C-terminal finger. We further show that this specificity arises from the highly conserved QTRNRK residues. The converse is, however, untrue in the case of the C-terminal finger; swapping of QTRNRK with the corresponding LVSKRA does not switch the DNA binding specificity from AGATAA to AGATCT. These results highlight the important role of residues adjacent to the CXXCX17CNAC zinc finger motif (i.e. non-finger residues) in the specific recognition of DNA residues.

A tissue-specific knockout reveals that Gata1 is not essential for Sertoli cell function in the mouse

The transcription factor Gata1 is essential for the development of erythroid cells. Consequently, Gata1 null mutants die in utero due to severe anaemia. Outside the haematopoietic system, Gata1 is only expressed in the Sertoli cells of the testis. To elucidate the function of Gata1 in the testis, we made a Sertoli cell-specific knockout of the Gata1 gene in the mouse. We deleted a normally functioning 'floxed' Gata1 gene in pre-Sertoli cells in vivo through the expression of Cre from a transgene driven by the Desert Hedgehog promoter. Surprisingly, Gata1 null testes developed to be morphologically normal, spermatogenesis was not obviously affected and expression levels of putative Gata1 target genes, and other Gata factors, were not altered. We conclude that expression of Gata1 in Sertoli cells is not essential for testis development or spermatogenesis in the mouse.

A nonsense mutation in zebrafish gata1 causes the bloodless phenotype in vlad tepes

Vlad tepes (vlt(m651)) is one of only five "bloodless" zebrafish mutants isolated through large-scale chemical mutagenesis screening. It is characterized by a severe reduction in blood cell progenitors and few or no blood cells at the onset of circulation. We now report characterization of the mutant phenotype and the identification of the gene mutated in vlt(m651). Embryos homozygous for the vlt(m651) mutation had normal expression of hematopoietic stem cell markers through 24 h postfertilization, as well as normal expression of myeloid and lymphoid markers. Analysis of erythroid development revealed variable expression of erythroid markers. Through positional and candidate gene cloning approaches we identified a nonsense mutation in the gata1 gene, 1015C --> T (Arg-339 --> Stop), in vlt(m651). The nonsense mutation was located C-terminal to the two zinc fingers and resulted in a truncated protein that was unable to bind DNA or mediate GATA-specific transactivation. A BAC clone containing the zebrafish gata1 gene was able to rescue the bloodless phenotype in vlt(m651). These results show that the vlt(m651) mutation is a previously uncharacterized gata1 allele in the zebrafish. The vlt(m651) mutation sheds new light on Gata1 structure and function in vivo, demonstrates that Gata1 plays an essential role in zebrafish hematopoiesis with significant conservation of function between mammals and zebrafish, and offers a powerful tool for future studies of the hematopoietic pathway.

GATA-4 interacts distinctively with negative and positive regulatory elements in the Fgf-3 promoter

GATA-4 binds two sites in the Fgf-3 promoter, PS4A and PS13, which function as positive and negative regulatory elements, respectively. In spite of their opposite functions, both PS4A and PS13 acted as potent enhancer elements when three copies of each were appended to a minimal tk promoter. Mutational analysis showed that the negative regulatory activity of PS13 was dependent on its close proximity to the major transcription initiation site (P3), since it was a stronger repressor when moved closer to P3, but had no significant activity when moved to more distal positions. While only the C-terminal zinc finger and the basic domain of GATA-4 were required for binding to PS13, this was insufficient for binding at PS4A. In addition to the PS4A GATA site, the presence of sequences located 10-12 bp distant was required for efficient binding. Both the sequence and location of this second site was crucial for binding and enhancer activity. Truncation deletions of GATA-4 showed that efficient binding to PS4A was dependent on both zinc fingers and the basic domain, suggesting a direct interaction between one zinc finger domain and a possible second site (AGACAA) that shows some similarity to a GATA motif. GATA-4 binding to PS4A through both zinc finger domains was essential for Fgf-3 promoter activity. The substitution in PS4A of a GATA-binding sequence similar to PS13, which only requires a single zinc finger domain, bound GATA-4 efficiently but did not activate the Fgf-3 promoter. These differences in GATA-4 binding were also reflected in DNA bending assays that suggested clear conformational differences between complexes formed on PS4A and PS13.

Hematopoietic regulatory domain of gata1 gene is positively regulated by GATA1 protein in zebrafish embryos

Expression of gata1 is regulated through multiple cis-acting GATA motifs. To elucidate regulatory mechanisms of the gata1 gene, we have used zebrafish. To this end, we isolated and analyzed zebrafish gata1 genomic DNA, which resulted in the discovery of a novel intron that was unknown in previous analyses. This intron corresponds to the first intron of other vertebrate Gata1 genes. GFP reporter analyses revealed that this intron and a distal double GATA motif in the regulatory region are important for the regulation of zebrafish gata1 gene expression. To examine whether GATA1 regulates its own gene expression, we microinjected into embryos a GFP reporter gene linked successively to the gata1 gene regulatory region and to GATA1 mRNA. Surprisingly, ectopic expression of the reporter gene was induced at the site of GATA1 overexpression and was dependent on the distal double GATA motif. Functional domain analyses using transgenic fish lines that harbor the gata1-GFP reporter construct revealed that both the N- and C-terminal zinc-finger domains of GATA1, hence intact GATA1 function, are required for the ectopic GFP expression. These results provide the first in vivo evidence that gata1 gene expression undergoes positive autoregulation.

GATA-1 bends DNA in a site-independent fashion

The DNA binding domain of GATA-1 consists of two adjacent homologous zinc fingers, of which only the C-terminal finger binds DNA independently. Solution structure studies have shown that the DNA is bent by about 15 degrees in the complex formed with the single C-terminal finger of GATA-1. The N-terminal finger stabilizes DNA binding at some sites. To determine whether it contributes to DNA bending, we have performed circular permutation DNA bending experiments with a variety of DNA-binding sites recognized by GATA-1. By using a series of full-length GATA-1, double zinc finger, and single C-terminal finger constructs, we show that GATA-1 bends DNA by about 24 degrees, irrespective of the DNA-binding site. We propose that the N- and C-terminal fingers of GATA-1 adopt different orientations when bound to different cognate DNA sites. Furthermore, we characterize circular permutation bending artifacts arising from the reduced gel mobility of the protein-DNA complexes.

DNA-induced conformational change of Gaf1, a novel GATA factor in Schizosaccharomyces pombe

A novel GATA factor in Schizosaccharomyces pombe, Gaf1, containing one zinc-finger motif was studied for conformational change that was induced by DNA-binding. Gaf1 was shown to bind to the upstream activation sequence of a gene in Saccharomyces cerevisiae containing GATA element by gel mobility shift assay. Circular dichroism spectra of Gaf1 indicated an increase of alpha-helix content of Gaf1 occurred upon binding to the upstream activation sequence. These results suggest that the binding of Gaf1 to the GATA element is required for the conformational change that may precede transactivation of the target gene(s).

Different sequence requirements for expression in erythroid and megakaryocytic cells within a regulatory element upstream of the GATA-1 gene

The lineage-restricted transcription factor GATA-1 is required for differentiation of erythroid and megakaryocytic cells. We have localized a 317 base pair cis-acting regulatory element, HS I, associated with a hematopoietic-specific DNase I hypersensitive site, which lies approx. 3.7 kilobases upstream of the murine hematopoietic-specific GATA-1 IE promoter. HS I directs high-level expression of reporter GATA-1/lacZ genes to primitive and definitive erythroid cells and megakaryocytes in transgenic mice. Comparative sequence analysis of HS I between human and mouse shows approx. 63% nucleotide identity with a more conserved core of 169 base pairs (86% identity). This core contains a GATA site separated by 10 base pairs from an E-box motif. The composite motif binds a multi-protein hematopoietic-specific transcription factor complex which includes GATA-1, SCL/tal-1, E2A, Lmo2 and Ldb-1. Point mutations of the GATA site abolishes HS I function, whereas mutation of the E-box motif still allows reporter gene expression in both lineages. Strict dependence of HS I activity on a GATA site implies that assembly of a protein complex containing a GATA-factor, presumably GATA-1 or GATA-2, is critical to activating or maintaining its function. Further dissection of the 317 base pair region demonstrates that, whereas all 317 base pairs are required for expression in megakaryocytes, only the 5′ 62 base pairs are needed for erythroid-specific reporter expression. These findings demonstrate differential lineage requirements for expression within the HS I element.

Positive and Negative Cis-Acting Elements Are Required for Hematopoietic Expression of Zebrafish GATA-1

GATA-1 is a transcription factor required for development of erythroid cells. The expression of GATA-1 is tightly restricted to the hematopoietic lineage. Using transgene constructs containing zebrafish GATA-1 genomic sequences and the green fluorescent protein (GFP) reporter gene, we previously showed that a 5.6-kb enhancer/promoter fragment is sufficient to direct erythroid-specific expression of the GFP. In this study, we used enhancer/promoter fragments containing various deletion and point mutations to further characterize the cis-acting elements controlling tissue-specific GATA-1 expression. We report here the identification of distinct cis-acting elements that cooperate to confer on GATA-1 its hematopoietic expression pattern. A CACCC box, located 142 bp upstream of the translation start codon, is critical for the initiation of GATA-1 expression. A distal double GATA element is required for maintaining and enhancing the hematopoietic expression of GATA-1. The erythroid-specific activity of the GATA-1 promoter is also enhanced by a 49-bp sequence element located 218 bp upstream of the CACCC element and a CCAAT box adjacent to the double GATA motif. Finally, the hematopoietic specificity of the GATA-1 promoter is secured by a negative cis-acting element that inhibits expression in the notochord.

Positive and Negative Cis-Acting Elements Are Required for Hematopoietic Expression of Zebrafish GATA-1

GATA-1 is a transcription factor required for development of erythroid cells. The expression of GATA-1 is tightly restricted to the hematopoietic lineage. Using transgene constructs containing zebrafish GATA-1 genomic sequences and the green fluorescent protein (GFP) reporter gene, we previously showed that a 5.6-kb enhancer/promoter fragment is sufficient to direct erythroid-specific expression of the GFP. In this study, we used enhancer/promoter fragments containing various deletion and point mutations to further characterize the cis-acting elements controlling tissue-specific GATA-1 expression. We report here the identification of distinct cis-acting elements that cooperate to confer on GATA-1 its hematopoietic expression pattern. A CACCC box, located 142 bp upstream of the translation start codon, is critical for the initiation of GATA-1 expression. A distal double GATA element is required for maintaining and enhancing the hematopoietic expression of GATA-1. The erythroid-specific activity of the GATA-1 promoter is also enhanced by a 49-bp sequence element located 218 bp upstream of the CACCC element and a CCAAT box adjacent to the double GATA motif. Finally, the hematopoietic specificity of the GATA-1 promoter is secured by a negative cis-acting element that inhibits expression in the notochord.

GATA-1 dominantly activates a program of erythroid gene expression in factor-dependent myeloid FDCW2 cells

Erythrocyte development has previously been shown to depend upon the expression of the lineage-restricted trans-acting factor GATA-1. Despite predicted roles for this factor during early development, GATA-1-deficient cells in chimeric mice and embryonic stem cell cultures mature to a late proerythroblast stage and express at least certain genes that normally are thought to be regulated by GATA-1 (including erythroid Krüppel-like factor [EKLF] and the erythropoietin [Epo] receptor). Opportunities to test roles for GATA-1 in erythroid gene activation in these systems therefore are limited. In the present study, in an alternate approach to test the function of GATA-1, GATA-1 has been expressed together with the Epo receptor in myeloid FDCW2 cells and the resulting effects on cytokine-dependent proliferation and erythroid gene expression have been assessed. GATA-1 expression at low levels delayed FDCW2ER cell cycle progression at the G1 phase specifically during Epo-induced mitogenesis. Upon expression of GATA-1 at increased levels, proliferation in response to Epo, interleukin-3 (IL-3), and stem cell factor was attenuated and endogenous GATA-1, EKLF and betamaj-globin gene expression was activated. Friend of GATA-1 (FOG) transcript levels also were enhanced, and ets-1 and c-mpl but not Epo receptor gene expression was induced. Finally, in FDCW2 cells expressing increased levels of GATA-1 and a carboxyl-terminally truncated Epo receptor, Epo (with respect to IL-3 as a control) was shown to markedly promote globin transcript expression. Thus, novel evidence for select hierarchical roles for GATA-1 and Epo in erythroid lineage specification is provided.

A palindromic regulatory site within vertebrate GATA-1 promoters requires both zinc fingers of the GATA-1 DNA-binding domain for high-affinity interaction

GATA-1, a transcription factor essential for the development of the erythroid lineage, contains two adjacent highly conserved zinc finger motifs. The carboxy-terminal finger is necessary and sufficient for specific binding to the consensus GATA recognition sequence: mutant proteins containing only the amino-terminal finger do not bind. Here we identify a DNA sequence (GATApal) for which the GATA-1 amino-terminal finger makes a critical contribution to the strength of binding. The site occurs in the GATA-1 gene promoters of chickens, mice, and humans but occurs very infrequently in other vertebrate genes known to be regulated by GATA proteins. GATApal is a palindromic site composed of one complete [(A/T)GATA(A/G)] and one partial (GAT) canonical motif. Deletion of the partial motif changes the site to a normal GATA site and also reduces by as much as eightfold the activity of the GATA-1 promoter in an erythroid precursor cell. We propose that GATApal is important for positive regulation of GATA-1 expression in erythroid cells.

elt-2, a second GATA factor from the nematode Caenorhabditis elegans

We have previously shown that a tandem pair of (A/T)GATA(A/G) sequences in the promoter region of the Caenorhabditis elegans gut esterase gene (ges-1) controls the tissue specificity of ges-1 expression in vivo. The ges-1 GATA region was used as a probe to screen a C. elegans cDNA expression library, and a gene for a new C. elegans GATA-factor (named elt-2) was isolated. The longest open reading frame in the elt-2 cDNA codes for a protein of M(r) 47,000 with a single zinc finger domain, similar (approximately 75% amino acid identity) to the C-terminal fingers of all other two-fingered GATA factors isolated to date. A similar degree of relatedness is found with the single-finger DNA binding domains of GATA factors identified in invertebrates. An upstream region in the ELT-2 protein with the sequence C-X2-C-X16-C-X2-C has some of the characteristics of a zinc finger domain but is highly diverged from the zinc finger domains of other GATA factors. The elt-2 gene is expressed as an SL1 trans-spliced message, which can be detected at all stages of development except oocytes; however, elt-2 message levels are 5-10-fold higher in embryos than in other stages. The genomic clone for elt-2 has been characterized and mapped near the center of the C. elegans X chromosome, ELT-2 protein, produced by in vitro transcription-translation, binds to ges-1 GATA-containing oligonucleotides similar to a factor previously identified in C. elegans embryo extracts, both as assayed by electrophoretic migration and by competition with wild type and mutant oligonucleotides. However, there is as yet no direct evidence that elt-2 does or does not control ges-1.

Homotypic interactions of chicken GATA-1 can mediate transcriptional activation

We used a one-hybrid system to replace precisely the finger II chicken GATA-1 DNA-binding domain with the binding domain of bacterial repressor protein LexA. The LexA DNA-binding domain lacks amino acids that function for transcriptional activation, nuclear localization, or protein dimerization. This allowed us to analyze activities of GATA-1 sequences distinct from DNA binding. We found that strong transcriptional activating sequences that function independently of finger II are present in GATA-1. Sequences including finger I contain an independent nuclear localizing function. Our data are consistent with cooperative binding of two LexA-GATA-1 hybrid proteins on a palindromic operator. The sensitivity of our transcription assay provides the first evidence that GATA-1 can make homotypic interactions in vivo. The ability of a non-DNA-binding form of GATA-1 to activate gene expression by targeting to a bound GATA-1 derivative further supports the notion that GATA-1-GATA-1 interactions may have functional consequences. A coimmunoprecipitation assay was used to demonstrate that GATA-1 multimeric complexes form in solution by protein-protein interaction. The novel ability of GATA-1 to interact homotypically may be important for the formation of higher-order structures among distant regulatory elements that share binding sites for this transcription factor. We also used the system to test the ability of GATA-1 to interact heterotypically with other activators.

The C-terminal zinc finger of GATA-1 or GATA-2 is sufficient to induce megakaryocytic differentiation of an early myeloid cell line

The GATA-1 and GATA-2 transcription factors, which each contain two homologous zinc fingers, are important hematopoietic regulators expressed within the erythroid, mast cell, and megakaryocytic lineages. Enforced expression of either factor in the primitive myeloid line 416B induces megakaryocytic differentiation. The features of their structure required for this activity have been explored. The ability of 12 GATA-1 mutants to promote 416B maturation was compared with their DNA-binding activity and transactivation potential. Differentiation did not require any of the seven serine residues that are phosphorylated in vivo, an N-terminal region bearing the major transactivation domain, or a C-terminal segment beyond the fingers. Removal of a consensus nuclear localization signal following the second finger did not block differentiation or nuclear translocation. The N-terminal finger was also dispensable, although its removal attenuated differentiation. In contrast, the C-terminal finger was essential, underscoring its distinct function. Remarkably, only 69 residues spanning the C-terminal finger were required to induce limited megakaryocytic differentiation. Analysis of three GATA-2 mutants led to the same conclusion. Endogenous GATA-1 mRNA was induced by most mutants and may contribute to differentiation. Because the GATA-1 C-terminal finger could bind its target site but not transactivate a minimal reporter, it may direct megakaryocytic maturation by derepressing specific genes and/or by interacting with another protein which provides the transactivation function.

Direct analysis of native and chimeric GATA specific DNA binding proteins from Aspergillus nidulans

In Aspergillus nidulans the regulatory gene areA is responsible for mediating nitrogen metabolite repression. The areA product (AREA) represents an example of the GATA family of DNA binding proteins, which are characterised by the presence of a GATA domain consisting of a zinc finger within a highly conserved region of 52 amino acids. Among the other transcription factors included in this family is the principal erythroid transcription factor, GATA-1, which contains two GATA domains. In order to demonstrate high specificity binding of native AREA to DNA containing the sequence -GATA-, and investigate the presence in A.nidulans of other proteins with related specificities, we have used gel mobility shift assays. Both AREA-dependent and independent complexes have been identified. Two strains bearing chimeric genes were also characterised. In these, the region encoding the native GATA domain of AREA was replaced by sequences from murine GATA-1 cDNA encoding either the equivalent C-terminal domain or both the N and C-terminal domains. Strains bearing the areA::NC-GATA construct, which includes the sequence encoding both the N and C-terminal domains of GATA-1, leads to a pronounced increase in one of two AREA-dependent complexes and implicates the N-terminal domain of GATA-1 in mediating protein-protein interactions.

Negative control of Xenopus GATA-2 by activin and noggin with eventual expression in precursors of the ventral blood islands

To increase our understanding of haematopoiesis during early vertebrate development, we have studied the expression pattern of the transcription factor GATA-2 in Xenopus embryos, and asked how this is regulated. We show that the blood island precursors of the ventral mesoderm express GATA-2 RNA at neural tube stages, some 5 hours before globin RNA is detected in their derivatives. Prior to this however, GATA-2 is expressed much more widely within the embryo. Maternal transcripts are uniformly distributed, and zygotic transcription is activated during gastrulation throughout ventral and lateral regions of the embryo, with expression highest in the sensorial ectoderm and only weak in the ventral mesoderm. The domain of GATA-2 expression in neurulae outlines the region of the neural plate and suggests a possible wider role in dorsoventral patterning. To identify the signals involved in regulating this pattern of expression, we performed experiments with embryo explants. GATA-2 is activated autonomously in isolated animal caps and this activation is suppressed by the mesoderm-inducing factor activin, but not by FGF. Thus, the down-regulation of GATA-2 observed in the region of the Spemann organiser may be a response to an activin-like signal emanating from the dorsal-vegetal region or Nieuwkoop centre. GATA-2 activation in animal caps and ventral marginal zones was suppressed by co-culturing with dorsal marginal zones, suggesting that a signal from the Spemann organiser is involved in suppression of GATA-2 in the dorsal region of the embryo. Expression of a candidate for this signal, noggin, had the same effect. Taken together, the observations presented here suggest that GATA-2 activation occurs by default in the absence of signals, that the restriction of its expression within the early embryo is controlled by negative signals emanating from the Nieuwkoop centre and the organiser, and that noggin and activin-like molecules play a role in these signalling pathways.

Embryonic expression and cloning of the murine GATA-3 gene

We describe the embryonic expression pattern as well as the cloning and initial transcriptional regulatory analysis of the murine (m) GATA-3 gene. In situ hybridization shows that mGATA-3 mRNA accumulation is temporally and spatially regulated during early development: although found most abundantly in the placenta prior to 10 days of embryogenesis, mGATA-3 expression becomes restricted to specific cells within the embryonic central nervous system (in the mesencephalon, diencephalon, pons and inner ear) later in gestation. GATA-3 also shows a restricted expression pattern in the peripheral nervous system, including terminally differentiating cells in the cranial and sympathetic ganglia. In addition to this distinct pattern in the nervous system, mGATA-3 is also expressed in the embryonic kidney and the thymic rudiment, and further analysis showed that it is expressed throughout T lymphocyte differentiation. To begin to investigate how this complex gene expression pattern is elicited, cloning and transcriptional regulatory analyses of the mGATA-3 gene were initiated. At least two regulatory elements (one positive and one negative) appear to be required for appropriate tissue-restricted regulation after transfection of mGATA-3-directed reporter genes into cells that naturally express GATA-3 (T lymphocytes and neuroblastoma cells). Furthermore, this same region of the locus confers developmentally appropriate expression in transgenic mice, but only in a subset of the tissues that naturally express the gene.

DNA-binding specificities of the GATA transcription factor family

Members of the GATA family of transcription factors, which are related by a high degree of amino acid sequence identity within their zinc finger DNA-binding domains, each show distinct but overlapping patterns of tissue-restricted expression. Although GATA-1, -2, and -3 have been shown to recognize a consensus sequence derived from regulatory elements in erythroid cell-specific genes, WGATAR (in which W indicates A/T and R indicates A/G), the potential for more subtle differences in the binding preferences of each factor has not been previously addressed. By employing a binding selection and polymerase chain reaction amplification scheme with randomized oligonucleotides, we have determined the binding-site specificities of bacterially expressed chicken GATA-1, -2, and -3 transcription factors. Whereas all three GATA factors bind an AGATAA erythroid consensus motif with high affinity, a second, alternative consensus DNA sequence, AGATCTTA, is also recognized well by GATA-2 and GATA-3 but only poorly by GATA-1. These studies suggest that all three GATA factors are capable of mediating transcriptional effects via a common erythroid consensus DNA-binding motif. Furthermore, GATA-2 and GATA-3, because of their distinct expression patterns and broader DNA recognition properties, may be involved in additional regulatory processes beyond those of GATA-1. The definition of an alternative GATA-2-GATA-3 consensus sequence may facilitate the identification of new target genes in the further elucidation of the roles that these transcription factors play during development.

NMR structure of a specific DNA complex of Zn-containing DNA binding domain of GATA-1

The three-dimensional solution structure of a complex between the DNA binding domain of the chicken erythroid transcription factor GATA-1 and its cognate DNA site has been determined with multidimensional heteronuclear magnetic resonance spectroscopy. The DNA binding domain consists of a core which contains a zinc coordinated by four cysteines and a carboxyl-terminal tail. The core is composed of two irregular antiparallel beta sheets and an alpha helix, followed by a long loop that leads into the carboxyl-terminal tail. The amino-terminal part of the core, including the helix, is similar in structure, although not in sequence, to the amino-terminal zinc module of the glucocorticoid receptor DNA binding domain. In the other regions, the structures of these two DNA binding domains are entirely different. The DNA target site in contact with the protein spans eight base pairs. The helix and the loop connecting the two antiparallel beta sheets interact with the major groove of the DNA. The carboxyl-terminal tail, which is an essential determinant of specific binding, wraps around into the minor groove. The complex resembles a hand holding a rope with the palm and fingers representing the protein core and the thumb, the carboxyl-terminal tail. The specific interactions between GATA-1 and DNA in the major groove are mainly hydrophobic in nature, which accounts for the preponderance of thymines in the target site. A large number of interactions are observed with the phosphate backbone.

DNA-binding specificities of the GATA transcription factor family

Members of the GATA family of transcription factors, which are related by a high degree of amino acid sequence identity within their zinc finger DNA-binding domains, each show distinct but overlapping patterns of tissue-restricted expression. Although GATA-1, -2, and -3 have been shown to recognize a consensus sequence derived from regulatory elements in erythroid cell-specific genes, WGATAR (in which W indicates A/T and R indicates A/G), the potential for more subtle differences in the binding preferences of each factor has not been previously addressed. By employing a binding selection and polymerase chain reaction amplification scheme with randomized oligonucleotides, we have determined the binding-site specificities of bacterially expressed chicken GATA-1, -2, and -3 transcription factors. Whereas all three GATA factors bind an AGATAA erythroid consensus motif with high affinity, a second, alternative consensus DNA sequence, AGATCTTA, is also recognized well by GATA-2 and GATA-3 but only poorly by GATA-1. These studies suggest that all three GATA factors are capable of mediating transcriptional effects via a common erythroid consensus DNA-binding motif. Furthermore, GATA-2 and GATA-3, because of their distinct expression patterns and broader DNA recognition properties, may be involved in additional regulatory processes beyond those of GATA-1. The definition of an alternative GATA-2-GATA-3 consensus sequence may facilitate the identification of new target genes in the further elucidation of the roles that these transcription factors play during development.

Regulation of the beta-globin locus

Transcription of the human beta-globin gene cluster depends upon upstream regulatory sequences, which are collectively termed the locus control region. Recent studies have provided new insights into how the individual genes of the cluster are regulated through development. The crux of transcriptional activation is how the locus control region communicates with the gene-proximal regulatory elements.

Enhancer-dependent transcription of the epsilon-globin promoter requires promoter-bound GATA-1 and enhancer-bound AP-1/NF-E2

We analyzed epsilon-globin transcription in erythroid cells and in erythroid extracts to determine the requirements for enhancer-dependent expression of this gene. Mutations that abolished GATA-1 binding at a single position in the promoter prevented interaction with enhancers, whereas elimination of a second more distal promoter GATA-1 site had no effect. Deletion or mutation of the GATA-1 sites in either the human beta-globin locus control region DNase-hypersensitive site II enhancer or the chicken beta A/epsilon-globin enhancer did not diminish the ability of the enhancers to interact with the promoter. In contrast, mutation of the AP-1/NF-E2 sites in these enhancers resulted in elimination of enhancement. In vitro transcription of these constructs was promoter dependent and was not sensitive to abolition of GATA-1 binding in the promoter, consistent with the role of GATA-1 solely as a mediator of the enhancer effect. Thus, GATA-1 regulates the response of the epsilon-globin gene to enhancers through a specific site in the promoter and requires enhancer AP-1/NF-E2 binding to transduce the enhancer effect on transcription.

Enhancer-dependent transcription of the epsilon-globin promoter requires promoter-bound GATA-1 and enhancer-bound AP-1/NF-E2

We analyzed epsilon-globin transcription in erythroid cells and in erythroid extracts to determine the requirements for enhancer-dependent expression of this gene. Mutations that abolished GATA-1 binding at a single position in the promoter prevented interaction with enhancers, whereas elimination of a second more distal promoter GATA-1 site had no effect. Deletion or mutation of the GATA-1 sites in either the human beta-globin locus control region DNase-hypersensitive site II enhancer or the chicken beta A/epsilon-globin enhancer did not diminish the ability of the enhancers to interact with the promoter. In contrast, mutation of the AP-1/NF-E2 sites in these enhancers resulted in elimination of enhancement. In vitro transcription of these constructs was promoter dependent and was not sensitive to abolition of GATA-1 binding in the promoter, consistent with the role of GATA-1 solely as a mediator of the enhancer effect. Thus, GATA-1 regulates the response of the epsilon-globin gene to enhancers through a specific site in the promoter and requires enhancer AP-1/NF-E2 binding to transduce the enhancer effect on transcription.