A syndrome of tricuspid atresia in mice with a targeted mutation of the gene encoding Fog-2.

Tricuspid atresia (TA) is a common form of congenital heart disease, accounting for 1-3% of congenital cardiac disorders. TA is characterized by the congenital agenesis of the tricuspid valve connecting the right atrium to the right ventricle and both an atrial septal defect (ASD) and a ventricular septal defect (VSD). Some patients also have pulmonic stenosis, persistence of a left-sided superior vena cava or transposition of the great arteries. Most cases of TA are sporadic, but familial occurrences with disease in multiple siblings have been reported. Gata4 is a zinc-finger transcription factor with a role in early cardiac development. Gata4-deficient mice fail to form a ventral heart tube and die of circulatory failure at embryonic day (E) 8.5 (refs 6,7). Zfpm2 (also known as Fog-2) is a multi-zinc-finger protein that is co-expressed with Gata4 in the developing heart beginning at E8.5 (refs 8-10). Zfpm2 interacts specifically with the N-terminal zinc finger of Gata4 and represses Gata4-dependent transcription. Here we use targeted mutagenesis to explore the role of Zfpm2 in normal cardiac development. Zfpm2-deficient mice died of congestive heart failure at E13 with a syndrome of tricuspid atresia that includes an absent tricuspid valve, a large ASD, a VSD, an elongated left ventricular outflow tract, rightward displacement of the aortic valve and pulmonic stenosis. These mice also display hypoplasia of the compact zone of the left ventricle. Our findings indicate the importance of Zfpm2 in the normal looping and septation of the heart and suggest a genetic basis for the syndrome of tricuspid atresia.

Immune responses to transgene-encoded proteins limit the stability of gene expression after injection of replication-defective adenovirus vectors.

The use of replication-defective adenoviruses (RDAd) for human gene therapy has been limited by host immune responses that result in transient recombinant gene expression in vivo. It remained unclear whether these immune responses were directed predominantly against viral proteins or, alternatively, against foreign transgene-encoded proteins. In this report, we have compared the stability of recombinant gene expression in adult immunocompetent mice following intramuscular (i.m.) injection with identical RDAd encoding self (murine) or foreign (human) erythropoietin. Our results demonstrate that immune responses direct against foreign transgene-encoded proteins are the major determinants of the stability of gene expression following i.m. injection of RDAd. Moreover, we demonstrate long-term recombinant gene expression in immunocompetent animals following a single i.m. injection of RDAd encoding a self protein. These findings are important for the design of future preclinical and clinical gene therapy trials.

The T alpha 2 nuclear protein binding site from the human T cell receptor alpha enhancer functions as both a T cell-specific transcriptional activator and repressor.

T cell-specific expression of the human T cell receptor alpha (TCR-alpha) gene is regulated by the interaction of variable region promoter elements with a transcriptional enhancer that is located 4.5 kb 3' of the TCR-alpha constant region (C alpha) gene segment. The minimal TCR-alpha enhancer is composed of two nuclear protein binding sites, T alpha 1 and T alpha 2, that are both required for the T cell-specific activity of the enhancer. The T alpha 1 binding site contains a consensus cAMP response element (CRE), and binds a set of ubiquitous nuclear proteins. The T alpha 2 binding site does not contain known transcriptional enhancer motifs. However, it binds at least two nuclear protein complexes, one of which is T cell specific. We now report that although the T alpha 2 nuclear protein binding site displays transcriptional activator activity in the context of the TCR-alpha enhancer, this site alone can function as a potent, T cell-specific transcriptional repressor when positioned either upstream, or downstream of several heterologous promoter and enhancer elements. These results demonstrate that a single nuclear protein binding site can function as a T cell-specific transcriptional activator or repressor element, depending upon the context in which it is located.

An essential role for nuclear factor kappaB in promoting double positive thymocyte apoptosis.

To examine the role of nuclear factor (NF)-kappaB in T cell development and activation in vivo, we produced transgenic mice that express a superinhibitory mutant form of inhibitor kappaB-alpha (IkappaB-alphaA32/36) under the control of the T cell-specific CD2 promoter and enhancer (mutant [m]IkappaB-alpha mice). Thymocyte development proceeded normally in the mIkappaB-alpha mice. However, the numbers of peripheral CD8(+) T cells were significantly reduced in these animals. The mIkappaB-alpha thymocytes displayed a marked proliferative defect and significant reductions in interleukin (IL)-2, IL-3, and granulocyte/macrophage colony-stimulating factor production after cross-linking of the T cell antigen receptor. Perhaps more unexpectedly, double positive (CD4(+)CD8(+); DP) thymocytes from the mIkappaB-alpha mice were resistant to alpha-CD3-mediated apoptosis in vivo. In contrast, they remained sensitive to apoptosis induced by gamma-irradiation. Apoptosis of wild-type DP thymocytes after in vivo administration of alpha-CD3 mAb was preceded by a significant reduction in the level of expression of the antiapoptotic gene, bcl-xL. In contrast, the DP mIkappaB-alpha thymocytes maintained high level expression of bcl-xL after alpha-CD3 treatment. Taken together, these results demonstrated important roles for NF-kappaB in both inducible cytokine expression and T cell proliferation after TCR engagement. In addition, NF-kappaB is required for the alpha-CD3-mediated apoptosis of DP thymocytes through a pathway that involves the regulation of the antiapoptotic gene, bcl-xL.

Evolutionarily conserved Ets family members display distinct DNA binding specificities.

Members of the Ets family of proto-oncogenes encode sequence-specific transcription factors that bind to a purine-rich motif centered around a conserved GGA trinucleotide. Ets binding sites have been identified in the transcriptional regulatory regions of multiple T cell genes including the T cell receptor alpha and beta (TCR-alpha and -beta) enhancers and the IL-2 enhancer, as well as in the enhancers of several T cell-trophic viruses including Maloney sarcoma virus, human leukemia virus type 1, and human immunodeficiency virus-2. T cells express multiple members of the Ets gene family including Ets-1, Ets-2, GABP alpha, Elf-1, and Fli-1. The different patterns of expression and protein-protein interactions of these different Ets family members undoubtedly contribute to their ability to specifically regulate distinct sets of T cell genes. However, previous studies have suggested that different Ets family members might also display distinct DNA binding specificities. In this report, we have examined the DNA binding characteristics of two Ets family members, Ets-1 and Elf-1, that are highly expressed in T cells. The results demonstrate that the minimal DNA binding domain of these proteins consists of adjacent basic and putative alpha-helical regions that are conserved in all of the known Ets family members. Both regions are required for DNA binding activity. In vitro binding studies demonstrated that Ets-1 and Elf-1 display distinct DNA binding specificities, and, thereby interact preferentially with different naturally occurring Ets binding sites. A comparison of known Ets binding sites identified three nucleotides at the 3' end of these sequences that control the differential binding of the Ets-1 and Elf-1 proteins. These results are consistent with a model in which different Ets family members regulate the expression of different T cell genes by binding preferentially to purine-rich sequences that share a GGA core motif, but contain distinct flanking sequences.

Cytotoxic T lymphocyte antigen 4 (CTLA4) blockade accelerates the acute rejection of cardiac allografts in CD28-deficient mice: CTLA4 can function independently of CD28.

Cytotoxic T lymphocyte antigen 4 (CTLA4) appears to negatively regulate T cell activation. One mechanism by which CTLA4 might antagonize T cell function is through inhibition of CD28 signaling by competing for their shared ligands B7-1 and B7-2. In addition, CTLA4 ligation could initiate a signaling cascade that inhibits T cell activation. To address whether CTLA4 could inhibit immune responses in the absence of CD28, rejection of heart allografts was studied in CD28-deficient mice. H-2(q) hearts were transplanted into allogeneic wild-type or CD28-deficient mice (H-2(b)). Graft rejection was delayed in CD28-deficient compared with wild-type mice. Treatment of wild-type recipients with CTLA4-immunoglobulin (Ig), or with anti-B7-1 plus anti-B7-2 mAbs significantly prolonged allograft survival. In contrast, treatment of CD28-deficient mice with CTLA4-Ig, anti-B7-1 plus anti-B7-2 mAbs, or a blocking anti-CTLA4 mAb induced acceleration of allograft rejection. This increased rate of graft rejection was associated with more severe mononuclear cell infiltration and enhanced levels of IFN-gamma and IL-6 transcripts in donor hearts of untreated wild-type and CTLA4-Ig- or anti-CTLA4 mAb-treated CD28-deficient mice. Thus, the negative regulatory role of CTLA4 extends beyond its potential ability to prevent CD28 activation through ligand competition. Even in the absence of CD28, CTLA4 plays an inhibitory role in the regulation of allograft rejection.

Systemic delivery of recombinant proteins by genetically modified myoblasts.

The ability to stably deliver recombinant proteins to the systemic circulation would facilitate the treatment of a variety of acquired and inherited diseases. To explore the feasibility of the use of genetically engineered myoblasts as a recombinant protein delivery system, stable transfectants of the murine C2C12 myoblast cell line were produced that synthesize and secrete high levels of human growth hormone (hGH) in vitro. Mice injected with hGH-transfected myoblasts had significant levels of hGH in both muscle and serum that were stable for at least 3 weeks after injection. Histological examination of muscles injected with beta-galactosidase-expressing C2C12 myoblasts demonstrated that many of the injected cells had fused to form multinucleated myotubes. Thus, genetically engineered myoblasts can be used for the stable delivery of recombinant proteins into the circulation.

LKLF: A transcriptional regulator of single-positive T cell quiescence and survival.

Mature single-positive (SP) T lymphocytes enter a "resting" state in which they are proliferatively quiescent and relatively resistant to apoptosis. The molecular mechanisms regulating this quiescent phenotype were unknown. Here it was found that the expression of a Kruppel-like zinc finger transcription factor, lung Kruppel-like factor (LKLF), is developmentally induced during the maturation of SP quiescent T cells and rapidly extinguished after SP T cell activation. LKLF-deficient T cells produced by gene targeting had a spontaneously activated phenotype and died in the spleen and lymph nodes from Fas ligand-induced apoptosis. Thus, LKLF is required to program the quiescent state of SP T cells and to maintain their viability in the peripheral lymphoid organs and blood.

Regulation of the Ets-related transcription factor Elf-1 by binding to the retinoblastoma protein.

The retinoblastoma gene product (Rb) is a nuclear phosphoprotein that regulates cell cycle progression. Elf-1 is a lymphoid-specific Ets transcription factor that regulates inducible gene expression during T cell activation. In this report, it is demonstrated that Elf-1 contains a sequence motif that is highly related to the Rb binding sites of several viral oncoproteins and binds to the pocket region of Rb both in vitro and in vivo. Elf-1 binds exclusively to the underphosphorylated form of Rb and fails to bind to Rb mutants derived from patients with retinoblastoma. Co-immunoprecipitation experiments demonstrated an association between Elf-1 and Rb in resting normal human T cells. After T cell activation, the phosphorylation of Rb results in the release of Elf-1, which is correlated temporally with the activation of Elf-1-mediated transcription. Overexpression of a phosphorylation-defective form of Rb inhibited Elf-1-dependent transcription during T cell activation. These results demonstrate that Rb interacts specifically with a lineage-restricted Ets transcription factor. This regulated interaction may be important for the coordination of lineage-specific effector functions such as lymphokine production with cell cycle progression in activated T cells.

Function of GATA transcription factors in preadipocyte-adipocyte transition.

Genes that control the early stages of adipogenesis remain largely unknown. Here, we show that murine GATA-2 and GATA-3 are specifically expressed in white adipocyte precursors and that their down-regulation sets the stage for terminal differentiation. Constitutive GATA-2 and GATA-3 expression suppressed adipocyte differentiation and trapped cells at the preadipocyte stage. This effect is mediated, at least in part, through the direct suppression of peroxisome proliferator-activated receptor gamma. GATA-3-deficient embryonic stem cells exhibit an enhanced capacity to differentiate into adipocytes, and defective GATA-2 and GATA-3 expression is associated with obesity. Thus, GATA-2 and GATA-3 regulate adipocyte differentiation through molecular control of the preadipocyte-adipocyte transition.

Sequence-specific binding of human Ets-1 to the T cell receptor alpha gene enhancer.

Expression of the human T cell receptor (TCR) alpha gene is regulated by a T cell-specific transcriptional enhancer that is located 4.5 kilobases (kb) 3' to the C alpha gene segment. The core enhancer contains two nuclear protein binding sites, T alpha 1 and T alpha 2, which are essential for full enhancer activity. T alpha 1 contains a consensus cyclic adenosine monophosphate (cAMP) response element (CRE) and binds a set of ubiquitously expressed CRE binding proteins. In contrast, the transcription factors that interact with the T alpha 2 site have not been defined. In this report, a lambda gt11 expression protocol was used to isolate a complementary DNA (cDNA) that programs the expression of a T alpha 2 binding protein. DNA sequence analysis demonstrated that this clone encodes the human ets-1 proto-oncogene. Lysogen extracts produced with this cDNA clone contained a beta-galactosidase-Ets-1 fusion protein that bound specifically to a synthetic T alpha 2 oligonucleotide. The Ets-1 binding site was localized to a 17-base pair (bp) region from the 3' end of T alpha 2. Mutation of five nucleotides within this sequence abolished both Ets-1 binding and the activity of the TCR alpha enhancer in T cells. These results demonstrate that Ets-1 binds in a sequence-specific fashion to the human TCR alpha enhancer and suggest that this developmentally regulated proto-oncogene functions in regulating TCR alpha gene expression.

Cytostatic gene therapy for vascular proliferative disorders with a constitutively active form of the retinoblastoma gene product.

Vascular smooth muscle cell (SMC) proliferation in response to injury is an important etiologic factor in vascular proliferative disorders such as atherosclerosis and restenosis after balloon angioplasty. The retinoblastoma gene product (Rb) is present in the unphosphorylated and active form in quiescent primary arterial SMCs, but is rapidly inactivated by phosphorylation in response to growth factor stimulation in vitro. A replication-defective adenovirus encoding a nonphosphorylatable, constitutively active form of Rb was constructed. Infection of cultured primary rat aortic SMCs with this virus inhibited growth factor-stimulated cell proliferation in vitro. Localized arterial infection with the virus at the time of balloon angioplasty significantly reduced SMC proliferation and neointima formation in both the rat carotid and porcine femoral artery models of restenosis. These results demonstrate the role of Rb in regulating vascular SMC proliferation and suggest a gene therapy approach for vascular proliferative disorders associated with arterial injury.

Recent advances in skeletal-muscle-based gene therapy.

Gene transfer into skeletal muscle holds promise for the treatment of a variety of serum protein deficiencies, muscular dystrophies, and chronic ischemic limb syndromes. The past two years have seen the development of new and improved vectors for programming recombinant gene expression in skeletal muscle. Important advances include first, novel plasmid DNA, adenovirus, and adeno-associated virus vectors that can be used to stably express therapeutic levels of recombinant proteins in the skeletal muscle of immunocompetent hosts and second, the development of vector systems that enable regulated and tissue-specific transgene expression in skeletal muscle in vivo.

Adenovirus-mediated over-expression of the cyclin/cyclin-dependent kinase inhibitor, p21 inhibits vascular smooth muscle cell proliferation and neointima formation in the rat carotid artery model of balloon angioplasty.

Vascular smooth muscle cell (VSMC) proliferation after arterial injury is important in the pathogenesis of a number of vascular proliferative disorders, including atherosclerosis and restenosis after balloon angioplasty. Thus, a better understanding of the molecular mechanisms underlying VSMC proliferation in response to arterial injury would have important therapeutic implications for patients with atherosclerotic vascular disease. The p21 protein is a negative regulator of mammalian cell cycle progression that functions both by inhibiting cyclin dependent kinases (CDKs) required for the initiation of S phase, and by binding to and inhibiting the DNA polymerase delta co-factor, proliferating cell nuclear antigen (PCNA). In this report, we show that adenovirus-mediated over-expression of human p21 inhibits growth factor-stimulated VSMC proliferation in vitro by efficiently arresting VSMCs in the G1 phase of the cell cycle. This p21-associated cell cycle arrest is associated both with significant inhibition of the phosphorylation of the retinoblastoma gene product (Rb) and with the formation of complexes between p21 and PCNA in VSMCs. In addition, we demonstrate that localized arterial infection with a p21-encoding adenovirus at the time of balloon angioplasty significantly reduced neointimal hyperplasia in the rat carotid artery model of restenosis. Taken together, these studies demonstrate the important role of p21 in regulating Rb phosphorylation and cell cycle progression in VSMC, and suggest a novel cytostatic gene therapy approach for restenosis and related vascular proliferative disorders.

Transcriptional targeting of replication-defective adenovirus transgene expression to smooth muscle cells in vivo.

Gene transfer using replication-defective adenoviruses (RDAd) holds promise for the treatment of vascular proliferative disorders, but is potentially limited by the capacity of these viruses to infect multiple cell lineages. We have generated an RDAd vector, designated AdSM22-lacZ, which encodes the bacterial lacZ reporter gene under the transcriptional control of the smooth muscle cell (SMC)-specific SM22alpha promoter. Here, we show that in vitro AdSM22-lacZ programs expression of the lacZ reporter gene in primary rat aortic SMCs and immortalized A7r5 SMCs, but not in primary human umbilical vein endothelial cells (HUVECs) or NIH 3T3 cells. Consistent with these results, after intraarterial administration of AdSM22-lacZ to control and balloon-injured rat carotid arteries, beta-galactosidase activity was detected within SMCs of the tunica media and neointima, but not within endothelial or adventitial cells. Moreover, intravenous administration of AdSM22-lacZ did not result in lacZ gene expression in the liver or lungs. Finally, we have shown that direct injection of AdSM22-lacZ into SMC-containing tissues such as the ureter and bladder results in high-level transgene expression in visceral SMCs. Taken together, these results demonstrate that transgene expression after infection with an RDAd vector can be regulated in an SMC lineage-restricted fashion by using a transcriptional cassette containing the SMC-specific SM22alpha promoter. The demonstration of an efficient gene delivery system targeted specifically to SMCs provides a novel means to restrict expression of recombinant gene products to vascular or visceral SMCs in vivo.

Dilated cardiomyopathy in transgenic mice expressing a dominant-negative CREB transcription factor in the heart.

Idiopathic-dilated cardiomyopathy (IDC) is a common primary myocardial disease of unknown etiology characterized by progressive biventricular failure, cardiac dilatation, and premature mortality. Here we show that transgenic mice expressing a dominant-negative form of the CREB transcription factor (CREBA133) under the control of the cardiac myocyte-specific alpha-MHC promoter develop dilated cardiomyopathy that closely resembles many of the anatomical, physiological, and clinical features of human IDC. Between 2 and 20 wk of age, these mice develop four chamber cardiac dilatation, decreased systolic and diastolic left ventricular function, and attenuated contractile responses to the beta-adrenergic agonist, isoproterenol. Histologically, the CREBA133 hearts demonstrated both atrophic and hypertrophied fibers as well as significant interstitial fibrosis. These anatomical and hemodynamic changes were associated with hepatic congestion and peripheral edema, intracardiac thrombi, and premature mortality. Taken together, these results implicate CREB as an important regulator of cardiac myocyte function and provide a genetic model of dilated cardiomyopathy which should facilitate studies of both the pathogenesis and therapy of this clinically important disorder.

Transcriptional regulation of T-cell genes during T-cell development.

The expression of many T cell specific genes has been shown to be regulated at the transcriptional level. Recent studies of T-cell specific promoters and enhancers have allowed the identification of a number of transcription factors that appear to play distinct but complementary roles in regulating gene expression during T-cell development and activation.

Regulation of the human T-cell receptor alpha gene enhancer: multiple ubiquitous and T-cell-specific nuclear proteins interact with four hypomethylated enhancer elements.

Transcription of human T-cell receptor (TCR) alpha genes is regulated by a T-cell-specific transcriptional enhancer that is located 4.5 kilobases 3' of the C alpha gene segment. Previous studies have demonstrated that this enhancer contains at least five nuclear protein-binding sites called T alpha 1 to T alpha 5. In the studies described in this report, we have determined the molecular requirements for human TCR alpha enhancer function. In vitro mutagenesis and deletion analyses demonstrated that full enhancer activity is retained in a 116-base-pair fragment containing the T alpha 1 and T alpha 2 nuclear protein-binding sites and that both of these sites are required for full enhancer function. Functional enhancer activity requires that the T alpha 1 and T alpha 2 binding sites be separated by more than 15 and fewer than 85 base pairs. However, the sequence of this spacer region and the relative phase of the two binding sites on the DNA helix do not affect enhancer function. Deletion and mutation analyses demonstrated that the T alpha 3 and T alpha 4 nuclear protein-binding sites are not necessary or sufficient for TCR alpha enhancer activity. However, a fragment containing these two sites was able to compensate for T alpha 1 and T alpha 2 mutations that otherwise abolished enhancer activity. Electrophoretic mobility shift analyses of the TCR alpha enhancer binding proteins revealed that the T alpha 1, T alpha 3, and T alpha 4 binding proteins are expressed in a variety of T-cell and non-T-cell tumor cell lines. In contrast, one of the two T alpha 2 binding activities was detected only in T-cell nuclear extracts. The activity of the TCR alpha enhancer does not appear to be regulated solely at the level of DNA methylation on that the enhancer sequences were found to be identically hypomethylated in B and T cells as compared with fibroblasts. Taken together, these results suggest that TCR alpha enhancer activity is regulated by the interaction of multiple T-cell-specific and ubiquitous nuclear proteins with partially redundant cis-acting enhancer elements that are hypomethylated in cells of the lymphoid lineage.

Identification and functional characterization of the human T-cell receptor beta gene transcriptional enhancer: common nuclear proteins interact with the transcriptional regulatory elements of the T-cell receptor alpha and beta genes.

A transcriptional enhancer has been mapped to a region 5.5 kilobases 3' of the C beta 2 gene in the human T-cell receptor (TCR) beta-chain locus. Transient transfections allowed localization of enhancer activity to a 480-base-pair HincII-XbaI restriction enzyme fragment. The TCR beta enhancer was active on both the minimal simian virus 40 promoter and a TCR beta variable gene promoter in both TCR alpha/beta + and TCR gamma/delta + T cells. It displayed significantly less activity in Epstein-Barr virus-transformed B cells and K562 chronic myelogenous leukemia cells and no activity in HeLa fibroblasts. DNA sequence analysis revealed that the enhancer contains a consensus immunoglobulin kappa E2 motif, as well as an AP-1-binding site and a cyclic AMP response element. DNase I footprint analyses using Jurkat T-cell nuclear extracts allowed the identification of five nuclear protein-binding sites, T beta 1 to T beta 5, within the enhancer element. Deletion and in vitro mutagenesis studies demonstrated that the T beta 2- and T beta 3- and T beta 4-binding sites are each required for full transcriptional enhancer activity. In contrast, deletion of the T beta 1- and T beta 5-binding sites had essentially no effect on enhancer function. Electrophoretic mobility shift assays demonstrated that TCR alpha/beta + and TCR gamma/delta + T cells expressed T beta 2-, T beta 3-, and T beta 4-binding activities. In contrast, non-T-cell lines, in which the enhancer was inactive, each lacked expression of at least one of these binding activities. TCR alpha and beta gene expression may be regulated by a common set of T-cell nuclear proteins in that the T beta 2 element binding a set of cyclic AMP response element-binding proteins that are also bound by the T alpha 1 element of the human TCR alpha enhancer and the decamer element present in a large number of human and murine TCR beta promoters. Similarly, the T beta 5 TCR beta-enhancer element and the T alpha 2 TCR alpha-enhancer element bind at least one common T-cell nuclear protein. Taken together, these results suggest that TCR beta gene expression is regulated by the interaction of multiple T cell nuclear proteins with a transcriptional enhancer element located 3' of the C beta 2 gene and that some of these proteins may be involved in the coordinate regulation of TCR alpha and beta gene expression.

Elf-1 binds to a critical element in a second CD4 enhancer.

The coordinated expression of CD4 and CD8 during T-cell development is tightly coupled with the maturation state of the T cell. Additionally, the mutually exclusive expression of these receptors in mature T cells is representative of the functional T-cell subclasses (CD4+ helper T cells versus CD8+ cytotoxic T cells). We have studied the regulation CD4 gene transcription during T-cell development in an attempt to gain an understanding of the molecular mechanisms involved in T-cell development and differentiation. Here we present the identification of a second transcriptional enhancer in the murine CD4 locus 24 kb upstream of the CD4 promoter. This enhancer is active in mature T cells and is especially active in CD4+ helper T cells. A number of nuclear proteins bind to elements in the minimal CD4 enhancer that includes consensus sites for AP-1, Sp1, Gata, and Ets transcription factor families. We find that the Ets consensus site is crucial for enhancer activity and that the recently identified Ets factor, Elf-1, which is expressed at high levels in T cells and involved in the regulation of several other T-cell-specific genes, is a dominant protein in T-cell nuclear extracts that binds to this site.