The macrophage and B cell-specific transcription factor PU.1 is related to the ets oncogene

Genetic analysis of transcription factors implicated in B lymphocyte development

The transcription factors Oct-2, NF-kappa B and PU.1 have been implicated in regulating the development of B lymphocytes. Genetic approaches have been used to analyze the developmental functions of these regulatory proteins. Using gene targeting in murine embryonic stem cells, PU.1 is shown to be required for the development of progenitor B cells. Strikingly, PU.1 is also essential for the development of T lymphoid, granulocytic and monocytic progenitors. Transcription factors of the NF-kappa B/Rel family, which appear to regulate immunoglobulin kappa gene expression, are shown to be a target of the viral transforming protein (v-abl) which arrests B lineage development at the precursor B stage. This suggests a mechanism by which v-abl blocks precursor B cell differentiation. The Oct-2 transcription factor was considered to represent a development regulator of immunoglobulin gene expression. Using gene targeting in a murine B cell, Oct-2 is shown to be dispensable for immunoglobulin gene expression. This suggests the existence of an alternate pathway, involving the ubiquitous related protein, Oct-1, in immunoglobulin gene regulation.

Ets-1 activates the DRA promoter in B cells

The X box in promoters of class II major histocompatibility complex genes plays a crucial role in the B-cell-specific and gamma interferon-inducible expression of these genes. The sequence TTCC is located in the pyrimidine tract which extends 5' to and partially overlaps the X box of the DRA promoter. This sequence resembles the core binding site for the Ets family of DNA-binding proteins. In this study, we demonstrate that mutations within the pyrimidine tract which change the TTCC motif, but do not affect the binding of regulatory factor X to the X box, decrease the activity of the DRA promoter in B cells. Furthermore, using electrophoretic mobility shift assays and cotransfection experiments, we demonstrate that Ets-1, but not Ets-2 or PU.1, functionally interacts with the pyrimidine tract and activates the DRA promoter.

Inhibition of transcription factor binding to the HER2 promoter by triplex-forming oligodeoxyribonucleotides

We have identified a 28-bp homopurine/homopyrimidine sequence capable of triple helix (triplex) formation with G+T-rich oligodeoxyribonucleotides (oligos) within the critical proximal promoter of the HER2/neu/c-erbB2 (HER2) proto-oncogene. To investigate the possible therapeutic potential of triplex-forming oligos in HER2 overexpressing breast cancers, we have studied the ability of triplex formation to compete with and to inhibit the binding of a transcription factor to its consensus sequence at an adjacent site. Competition binding assays demonstrate that a triplex-forming oligo can inhibit transcription factor binding in a sequence-specific manner. Moreover, we find that the addition of both nucleotide and non-nucleotide 'tails' to triplex-forming oligos do not confer any enhancement of binding affinity, but provide additional inhibition of transcription factor binding, potentially by steric hindrance.

Identification and characterization of an Epstein-Barr virus nuclear antigen 2-responsive cis element in the bidirectional promoter region of latent membrane protein and terminal protein 2 genes

Epstein-Barr virus (EBV) transforms resting B cells in vitro very efficiently. The nuclear viral protein EBV nuclear antigen 2 (EBNA2) is absolutely required for this process and also acts as a transcriptional activator of cellular and viral genes. As shown previously, EBNA2 transactivates the promoters of the viral latent membrane proteins. It interacts indirectly with an EBNA2-responsive cis element of the terminal protein 1 (TP1) promoter. To identify the sequences mediating EBNA2 transactivation of the bidirectional promoter region driving expression of the latent membrane proteins LMP and TP2 in opposite directions, we assayed the effects of EBNA2 on the activities of promoter deletion and site-directed mutants of TP2 and LMP promoter luciferase reporter gene constructs by cotransfections into EBNA2-negative Burkitt's lymphoma cells. We were able to delineate an 80-bp EBNA2-responsive region (EBNA2RE) between -232 and -152 relative to the LMP RNA start site which could also mediate EBNA2-dependent activation on a heterologous promoter. Sequences of 20 and 32 bp located at the 5' and 3' ends, respectively, of the EBNA2RE were both essential for EBNA2 responsiveness. Full transactivation of the LMP and TP2 promoters seemed to require 20 bp of 5' adjacent sequences in addition to the 80-bp element. Electrophoretic mobility shift assays revealed specific protein-DNA complexes formed at the EBNA2RE. Oligonucleotides from -181 to -152 and -166 to -132 relative to the LMP RNA start site visualized one B-cell and one B-cell-plus-HL60-specific retarded protein-DNA complex, respectively. Additionally, an oligonucleotide from -253 to -210 revealed two specific protein-DNA complexes with nuclear extracts from different B and non-B cells, suggesting also the binding of ubiquitously expressed proteins on the EBNA2RE. Thus, these experiments defined a 80-bp cis element sufficient for conferring EBNA2 inducibility and demonstrated specific interactions of cellular proteins at DNA sequences within the EBNA2RE, which are critical for transactivation by EBNA2.

Constitutively expressed Oct-2 prevents immunoglobulin gene silencing in myeloma x T cell hybrids

Recent experiments involving disruption of the Oct-2 gene have shown that this largely B cell-restricted transcription factor is not required in the early stages of B cell development. However, B cells that lack Oct-2 may be blocked from differentiation past the surface immunoglobulin-positive stage. To identify a possible function for Oct-2 in the late stage immunoglobulin-secreting cell, we have used the method of somatic cell fusion. When the immunoglobulin-producing myeloma MPC11 is fused to a T lymphoma, Oct-2 production ceases, as does the expression of immunoglobulin, J chain, and several other B cell-specific gene products. In the present study, we show that by preventing the loss of Oct-2 in the hybrid cells, we can preserve expression of all other tested B cell-specific genes. These results establish a central role for Oct-2 in maintaining the genetic program of the immunoglobulin-secreting plasmacyte.

Ets-1 activates the DRA promoter in B cells

The X box in promoters of class II major histocompatibility complex genes plays a crucial role in the B-cell-specific and gamma interferon-inducible expression of these genes. The sequence TTCC is located in the pyrimidine tract which extends 5' to and partially overlaps the X box of the DRA promoter. This sequence resembles the core binding site for the Ets family of DNA-binding proteins. In this study, we demonstrate that mutations within the pyrimidine tract which change the TTCC motif, but do not affect the binding of regulatory factor X to the X box, decrease the activity of the DRA promoter in B cells. Furthermore, using electrophoretic mobility shift assays and cotransfection experiments, we demonstrate that Ets-1, but not Ets-2 or PU.1, functionally interacts with the pyrimidine tract and activates the DRA promoter.

Multiple regulatory elements control the basal promoter activity of the human alpha 4 integrin gene

It has been suggested that expression of the genes encoding the alpha 4/beta 1 integrin increases during wound healing of the cornea. As a first step in understanding the mechanisms required to stimulate alpha 4 gene expression during this process, we defined the minimal upstream sequence required to direct basal promoter activity for this gene. Using deletion analyses of the alpha 4 gene upstream sequence, we identified two functionally important negative regulatory elements. Dimethylsulfate (DMS) methylation interference assays provided evidence for the binding of a single nuclear protein to tandemly repeated homologous cis-acting elements (designated alpha 4.1 and alpha 4.2) from the alpha 4 basal promoter that share the core sequence 5'-GTGGGT-3'. The formation of a protection only at alpha 4.1 in DNase I footprinting suggested that it is the primary target element for the binding of nuclear proteins. Three distinct nuclear proteins bound a double-stranded oligonucleotide bearing the DNA sequence of alpha 4.1 to produce specific DNA-protein complexes (R1 to R3) in gel-shift assays, from which that producing R3 was identified as the protein yielding DNase I protection at alpha 4.1. Detailed mutational analysis of alpha 4.1 and alpha 4.2 indicated that both elements positively regulate gene expression in primary cultures of corneal epithelial cells and Jurkat tissue culture cells, which is consistent with the deletion analysis. However, when transiently transfected into pituitary GH4C1, the alpha 4.2 mutants yielded increased chloramphenicol acetyl transferase activity therefore demonstrating that these elements have the ability to function either as positive or negative regulators of gene transcription in a manner that is dependent on the type of cell transfected.

A novel cysteine-rich sequence-specific DNA-binding protein interacts with the conserved X-box motif of the human major histocompatibility complex class II genes via a repeated Cys-His domain and functions as a transcriptional repressor

The class II major histocompatibility complex (MHC) molecules function in the presentation of processed peptides to helper T cells. As most mammalian cells can endocytose and process foreign antigen, the critical determinant of an antigen-presenting cell is its ability to express class II MHC molecules. Expression of these molecules is usually restricted to cells of the immune system and dysregulated expression is hypothesized to contribute to the pathogenesis of a severe combined immunodeficiency syndrome and certain autoimmune diseases. Human complementary DNA clones encoding a newly identified, cysteine-rich transcription factor, NF-X1, which binds to the conserved X-box motif of class II MHC genes, were obtained, and the primary amino acid sequence deduced. The major open reading frame encodes a polypeptide of 1,104 amino acids with a symmetrical organization. A central cysteine-rich portion encodes the DNA-binding domain, and is subdivided into seven repeated motifs. This motif is similar to but distinct from the LIM domain and the RING finger family, and is reminiscent of known metal-binding regions. The unique arrangement of cysteines indicates that the consensus sequence CX3CXL-XCGX1-5HXCX3CHXGXC represents a novel cysteine-rich motif. Two lines of evidence indicate that the polypeptide encodes a potent and biologically relevant repressor of HLA-DRA transcription: (a) overexpression of NF-X1 from a retroviral construct strongly decreases transcription from the HLA-DRA promoter; and (b) the NF-X1 transcript is markedly induced late after induction with interferon gamma (IFN-gamma), coinciding with postinduction attenuation of HLA-DRA transcription. The NF-X1 protein may therefore play an important role in regulating the duration of an inflammatory response by limiting the period in which class II MHC molecules are induced by IFN-gamma.

Vintage reds and whites: combinatorial transcription factor utilization in hematopoietic differentiation

Pluripotent hematopoietic stem cells can differentiate into a number of distinct specialized cell types; however, no single lineage-specific master regulators have been identified that can activate individual patterns of gene expression. Recent evidence suggests that such lineage determination is regulated by a combinatorial matrix of regulatory proteins with overlapping tissue specificities which cooperate to define individual cell types.

Monocyte maturation controls expression of equine infectious anemia virus

In vivo, equine infectious anemia virus (EIAV) replicates in tissues rich in macrophages, and it is widely believed that the tissue macrophage is the principal, if not sole, cell within the host that replicates virus. No viral replication has been detected in circulating peripheral blood monocytes. However, proviral DNA can be detected in these cells, and monocytes may serve as a reservoir for the virus. In this study, an in vitro model was developed to clarify the role of monocyte maturation in regulating EIAV expression. Freshly isolated, nonadherent equine peripheral blood monocytes were infected with a macrophage-tropic strain of EIAV, and expression of EIAV was monitored in cells held as nonadherent monocytes and cells allowed to adhere and differentiate into macrophages. A 2- to 3-day delay in viral antigen expression was observed in the nonadherent cells. This restriction of viral expression in monocytes was supported by nuclear run-on studies demonstrating that on day 5 postinfection, the level of actively transcribed viral messages was 4.7-fold lower in monocyte cultures than in macrophage cultures. Electrophoretic mobility shift assays identified three regions of the U3 enhancer that interacted with nuclear extracts from normal equine macrophages. Each region contained the core binding motif of a family of transcription factors that includes the product of the proto-oncogene ets. Antibodies to the Ets family member PU.1 caused a supershifting of retarded bands in an electrophoretic mobility shift assay. Transfection studies of ets motif mutants demonstrated that the U3 ets sites were important in the regulation of EIAV transcription in macrophages. Interactions between the ets motif and nuclear extracts from freshly isolated, nonadherent monocytes, macrophages adherent for 1 or 2 days, or macrophages adherent for 5 days gave different patterns of retarded bands, although the binding specificities were similar with all three extracts. The different complexes formed by monocyte and macrophage nuclear extracts may explain the enhanced ability of mature macrophages to support EIAV expression.

Gene structure and cell type-specific expression of the human ATP synthase alpha subunit

The gene structure of the human ATP synthase alpha subunit (hATP1) was determined by cloning and sequencing. This gene is approximately 14 kbp in length and contains 12 exons interrupted by 11 introns. Mapping of the clones of hATP1 and Southern blot analysis of the genomic gene showed that there were a single copy of bona fide hATP1 gene and two pseudogenes. Primer extension and S1 mapping analysis showed the presence of multiple transcription initiation sites of the hATP1 gene. No TATA box or CAAT box was found near the transcription initiation sites. Comparison with the bovine gene showed that the 5'-flanking region of the hATP1 gene has an unconserved guanine-cytosine (GC) rich region, including several binding motifs of transcriptional factors, such as Sp1, AP-2, and GCF. By functional assay of gene expression, the basal promoter activity was located near the GC rich region. Comparison of the 5'-upstream region of the hATP1 gene with those of the genes for bovine ATP synthase alpha, human beta, and human gamma subunits indicated three common sequences, suggesting that putative cis-elements coordinate the expressions of the three subunit genes for the ATP synthase. The enhancer activities derived from the 5'-deletion mutants of a hATP1-CAT chimeric gene were different in cell lines from four different human tissues, suggesting the existence of cell type-specific gene regulation.

Genomic structure analysis of promoter sequence of a mouse mu opioid receptor gene

We have isolated mouse mu opioid receptor genomic clones (termed MOR) containing the entire amino acid coding sequence corresponding to rat MOR-1 cDNA, including additional 5' flanking sequence. The mouse MOR gene is > 53 kb long, and the coding sequence is divided by three introns, with exon junctions in codons 95 and 213 and between codons 386 and 387. The first intron is > 26 kb, the second is 0.8 kb, and the third is > 12 kb. Multiple transcription initiation sites were observed, with four major sites confirmed by 5' rapid amplification of cDNA ends and RNase protection located between 291 and 268 bp upstream of the translation start codon. Comparison of the 5' flanking sequence with a transcription factor database revealed putative cis-acting regulatory elements for transcription factors affected by cAMP, as well as those involved in the action of gluco- and mineralocorticoids, cytokines, and immune-cell-specific factors.

Requirement of transcription factor PU.1 in the development of multiple hematopoietic lineages

The transcription factor PU.1 is a hematopoietic-specific member of the ets family. Mice carrying a mutation in the PU.1 locus were generated by gene targeting. Homozygous mutant embryos died at a late gestational stage. Mutant embryos produced normal numbers of megakaryocytes and erythroid progenitors, but some showed an impairment of erythroblast maturation. An invariant consequence of the mutation was a multilineage defect in the generation of progenitors for B and T lymphocytes, monocytes, and granulocytes. Thus, the developmental programs of lymphoid and myeloid lineages require a common genetic function likely acting at the level of a multipotential progenitor.

The activation antigen CD69

One of the earliest cell surface antigens expressed by T cells following activation is CD69, which is detectable within one h of ligation of the T cell receptor/CD3 complex. Once expressed, CD69 acts as a costimulatory molecule for T cell activation and proliferation. In addition to mature T cells, CD69 is inducibly expressed by immature thymocytes, B cells, natural killer (NK) cells, monocytes, neutrophils and eosinophils, and is constitutively expressed by mature thymocytes and platelets. Recently, cDNA clones encoding human and mouse CD69 were isolated and showed CD69 to be a member of the C-type lectin superfamily. Gene mapping studies have placed CD69 on distal mouse chromosome 6 and human chromosome 12p13, close to, if not in, the NK gene complex. The structure, chromosomal localization, expression and function of CD69 suggest that it is likely a pleiotropic immune regulator, potentially important not only in NK cell function but also in the activation and differentiation of a wide variety of hematopoietic cells.

Human HE2 (microB) and microA motifs show the same function as whole IgH intronic enhancer in transgenic mice

In the murine IgH gene intronic enhancer (ENHiH), two major functional domains were reported. One is the E4/octomer region and another includes the microA and microB motifs. In the human ENHiH, it was reported that the HE2, which corresponds to the murine microB, and E6 motifs play an important role in an enhancer activity and a tissue-specificity at cellular level. Here we examined the in vivo function of the E6, microA and HE2 motifs within the human ENHiH by using the transgenic mice technique. The microA and HE2 motifs together revealed almost the same enhancer function as the whole human ENHiH, but the E6 motif had lesser enhancer activity and tissue-specificity.

Inhibition of hematopoiesis by competitive binding of transcription factor PU.1

Transcription factors have been shown to play a role as "master switch" factors in the programming of hematopoietic cell commitment and differentiation. PU.1 is a hematopoietic-specific member of the Ets family of transcription factors. In human bone marrow CD34-enriched progenitor cells, PU.1 expression was upregulated during the early phases of granulocytic/monocytic differentiation, preceding expression of its target genes encoding CD11b and the macrophage-colony-stimulating factor receptor, whereas PU.1 was expressed at stable levels throughout erythroid differentiation. To study PU.1 function, we synthesized double-stranded phosphorothioate oligonucleotides containing a characterized PU.1 site and demonstrated their ability to specifically compete for PU.1 DNA binding. When added to CD34+ cells in vitro, wild-type PU.1-binding oligonucleotides significantly blocked hematopoietic colony formation, whereas mutated PU.1 oligonucleotides which no longer bind PU.1 had no specific inhibitory effect. These results demonstrate that PU.1 is developmentally upregulated during normal human myelopoiesis and that the function of PU.1 is critical for the development of in vitro hematopoiesis.

Involvement of the transcription factor PU.1/Spi-1 in myeloid cell-restricted expression of an interferon-inducible gene encoding the human high-affinity Fc gamma receptor

Induction by gamma interferon (IFN-gamma) of the gene encoding the human high-affinity Fc gamma receptor (Fc gamma R1) in myeloid cells requires an IFN-gamma response region (GRR) and a myeloid cell-activating transcription element (MATE). GRR and MATE interact with factors to form, respectively, an IFN-gamma-activating complex (GIRE-BP), depending on the phosphorylation of the 91-kDa protein (subunit of ISGF3), and a cell-type-specific complex (MATE-BP). Although GIRE-BP is detected in cells of different origins after IFN-gamma treatment, the presence of MATE-BP was found to be restricted to B- and myeloid cell lines. Sequence analysis of a cDNA encoding a polypeptide recognizing specifically the MATE motif led to the identification of this product as the proto-oncogene PU.1/Spi-1, a transcriptional activator expressed in myeloid and B cells. Expression of this factor in nonhematopoietic cells allowed IFN-gamma-induced expression of a reporter gene under control of the GRR and MATE sequences. The presence of these motifs in other gene promoters indicates that the binding of PU.1/Spi-1 and IFN regulatory proteins to their respective motifs could be part of a general mechanism leading to cell-type-restricted and IFN-induced gene expression.

PU.1 and GATA: components of a mast cell-specific interleukin 4 intronic enhancer

Interleukin 4 (IL-4), a critical immunoregulatory cytokine, is produced by a subset of T lymphocytes and cells of the mast cell/basophil lineage. There are cell-specific differences in the regulatory elements that control IL-4 transcription in these two cell types. A 683-bp Bgl II fragment, located within the second intron of the murine IL-4 gene, was previously shown to exhibit mast cell-specific enhancer activity. To define critical cis-acting elements that regulate this enhancer, a series of deletions from the 5' and 3' ends of the Bgl II fragment were generated. Their effect on enhancer activity was assessed in IL-4-producing mast cell lines in transient transfection assays. Two functionally independent subregions, E1 and E2, were defined in this analysis. Both are required for full enhancer activity. Sequences identical to previously defined DNA-binding sites for SP1 and GATA are present within E1, and an ets binding site is located within E2. Although mutation of the SP1 sites had no effect on enhancer function, alteration of either the GATA or ets site reduced enhancer activity by 50-60%. Proteins that associate with the IL-4 intronic GATA and ets sites were detected in mast cell nuclear extracts by mobility-shift assays. Specific antibodies identified these factors as GATA-1 and GATA-2 and the ets family member PU.1. GATA-1, GATA-2, and PU.1 exhibit cell-specific expression, suggesting that these proteins play a critical role in the lineage-restricted activity of the IL-4 intronic enhancer in mast cells.

Genomic organization and chromosomal localization of the mouse synexin gene

We have isolated and characterized the gene encoding mouse synexin, which consists of 14 exons and spans approximately 30 kbp of genomic DNA. The protein's unique N-terminal domain is encoded by six exons, and the C-terminal tetrad repeat, the site of the membrane-fusion and ion-channel domain, is encoded by seven exons. The first exon encodes the 5'-untranslated region. Analysis of synexin-gene expression in different mouse tissues shows that mRNA with exon 6 is only present in brain, heart and skeletal muscle. mRNA lacking exon 6 is expressed in all tissues we have examined. The initiation site for transcription was determined by primer-extension analysis and S1 nuclease mapping. Sequence analysis of the 1.3 kb 5'-flanking region revealed that the promoter has a TATA box located at position -25 and a number of potential promoter and regulatory elements. A CCAAT motif was not observed but CCATT is located in an appropriate position for the CCAAT motif upstream from the transcription-initiation start site. In addition, the 5'-flanking region contains two sets of palindromic sequences. Finally, we have determined that the functional synexin gene (Anx7) is located on mouse chromosome 14 and that a pseudogene (Anx7-ps1) is located on chromosome 10.

Involvement of the transcription factor PU.1/Spi-1 in myeloid cell-restricted expression of an interferon-inducible gene encoding the human high-affinity Fc gamma receptor

Induction by gamma interferon (IFN-gamma) of the gene encoding the human high-affinity Fc gamma receptor (Fc gamma R1) in myeloid cells requires an IFN-gamma response region (GRR) and a myeloid cell-activating transcription element (MATE). GRR and MATE interact with factors to form, respectively, an IFN-gamma-activating complex (GIRE-BP), depending on the phosphorylation of the 91-kDa protein (subunit of ISGF3), and a cell-type-specific complex (MATE-BP). Although GIRE-BP is detected in cells of different origins after IFN-gamma treatment, the presence of MATE-BP was found to be restricted to B- and myeloid cell lines. Sequence analysis of a cDNA encoding a polypeptide recognizing specifically the MATE motif led to the identification of this product as the proto-oncogene PU.1/Spi-1, a transcriptional activator expressed in myeloid and B cells. Expression of this factor in nonhematopoietic cells allowed IFN-gamma-induced expression of a reporter gene under control of the GRR and MATE sequences. The presence of these motifs in other gene promoters indicates that the binding of PU.1/Spi-1 and IFN regulatory proteins to their respective motifs could be part of a general mechanism leading to cell-type-restricted and IFN-induced gene expression.

Cell-specific expression of the macrophage scavenger receptor gene is dependent on PU.1 and a composite AP-1/ets motif

The type I and II scavenger receptors (SRs) are highly restricted to cells of monocyte origin and become maximally expressed during the process of monocyte-to-macrophage differentiation. In this report, we present evidence that SR genomic sequences from -245 to +46 bp relative to the major transcriptional start site were sufficient to confer preferential expression of a reporter gene to cells of monocyte and macrophage origin. This profile of expression resulted from the combinatorial actions of multiple positive and negative regulatory elements. Positive transcriptional control was primarily determined by two elements, located 181 and 46 bp upstream of the major transcriptional start site. Transcriptional control via the -181 element was mediated by PU.1/Spi-1, a macrophage and B-cell-specific transcription factor that is a member of the ets domain gene family. Intriguingly, the -181 element represented a relatively low-affinity binding site for Spi-B, a closely related member of the ets domain family that has been shown to bind with relatively high affinity to other PU.1/Spi-1 binding sites. These observations support the idea that PU.1/Spi-1 and Spi-B regulate overlapping but nonidentical sets of genes. The -46 element represented a composite binding site for a distinct set of ets domain proteins that were preferentially expressed in monocyte and macrophage cell lines and that formed ternary complexes with members of the AP-1 gene family. In concert, these observations suggest a model for how interactions between cell-specific and more generally expressed transcription factors function to dictate the appropriate temporal and cell-specific patterns of SR expression during the process of macrophage differentiation.

Cloning and characterization of the promoter region of the murine alpha-4 integrin subunit

To study the differential expression of the murine VLA-4 (alpha 4 beta 1) integrin, the 5'-flanking region of the gene for the alpha subunit (alpha 4m) was isolated and a cDNA for alpha 4m was obtained with reverse transcriptase polymerase chain reaction (RT-PCR). The cDNA sequence contained a difference in the signal peptide region compared to the previously described cDNA (Neuhaus et al., 1991). As a consequence, another start codon is predicted, resulting in a decrease in size of the signal peptide. This was confirmed by genomic sequencing. The promoter region was delimited by ribonuclease protection assay (RPA) and transfection experiments fusing 5'-upstream fragments to the luciferase gene. A fragment extending from -936 to +221 was capable of controlling the expected cell-type-specific expression. Sequence comparison of the mouse alpha 4m promoter region with the human alpha 4h promoter revealed little homology. Like most integrin subunits, alpha 4m lacks TATA anc CCAAT boxes. Putative recognition sites for DNA-binding nuclear factors (AP1, AP2, Sp1, and PU1) were identified. The characterization of the promoter region and further identification of the transcription regulatory elements should provide insight in the regulation of alpha 4m integrin gene expression.

Cell-specific expression of the macrophage scavenger receptor gene is dependent on PU.1 and a composite AP-1/ets motif

The type I and II scavenger receptors (SRs) are highly restricted to cells of monocyte origin and become maximally expressed during the process of monocyte-to-macrophage differentiation. In this report, we present evidence that SR genomic sequences from -245 to +46 bp relative to the major transcriptional start site were sufficient to confer preferential expression of a reporter gene to cells of monocyte and macrophage origin. This profile of expression resulted from the combinatorial actions of multiple positive and negative regulatory elements. Positive transcriptional control was primarily determined by two elements, located 181 and 46 bp upstream of the major transcriptional start site. Transcriptional control via the -181 element was mediated by PU.1/Spi-1, a macrophage and B-cell-specific transcription factor that is a member of the ets domain gene family. Intriguingly, the -181 element represented a relatively low-affinity binding site for Spi-B, a closely related member of the ets domain family that has been shown to bind with relatively high affinity to other PU.1/Spi-1 binding sites. These observations support the idea that PU.1/Spi-1 and Spi-B regulate overlapping but nonidentical sets of genes. The -46 element represented a composite binding site for a distinct set of ets domain proteins that were preferentially expressed in monocyte and macrophage cell lines and that formed ternary complexes with members of the AP-1 gene family. In concert, these observations suggest a model for how interactions between cell-specific and more generally expressed transcription factors function to dictate the appropriate temporal and cell-specific patterns of SR expression during the process of macrophage differentiation.

Complex functional interactions at the early enhancer of the PQ strain of BK virus

BK virus is a human papovavirus that latently infects a majority of the world's population. There are more than 30 strains of the virus, most of which differ in the structure of the early enhancer region. The enhancer of the progenitor strain, WW, from which the other strains can be derived, consists of four conserved DNA domains, P, Q, R, and S. Rearrangement of the enhancer occurs upon passage in tissue culture and is thought to occur during virus replication. The strain under study, PQ, was selected upon passage of the Gardner strain (PPPQS) in the permissive cell line, Vero. Mutational analysis of the entire enhancer region demonstrates the importance of five cis-acting sequences: DNA sites B, C, and F, which have homology to the NF-1 protein binding sequence; one purine-rich motif designated A; and site D, which is similar to an SP-1 protein binding site. Two sites, B and C, appear to have a negative influence on gene activity. To study the functional interactions in more detail, promoter-enhancer constructions that contain different combinations of the five DNA sites linked to the chloramphenicol acetyltransferase gene were tested for early gene activity. The results reveal that the proteins binding to the enhancer functionally cooperate with each other. The effects of making mutations at the DNA sites are very similar to the effects of using excess enhancer DNA sequences to titrate the proteins that bind to the cis-acting DNA sites (in vivo competition). Moreover, the effects of changing the spacing between the DNA sites also demonstrate that there are cooperative interactions among the proteins that bind to the PQ strain enhancer. DNA sites B, C, and F are clearly protected from DNase I digestion by Vero cell nuclear proteins. In addition, mutation of each DNA site alters its sensitivity to DNase I in the presence of Vero cell proteins. Interestingly, mutation of site B affects protein binding to site B as well as to sites A, C, D, and F. These results suggest that cooperative functional and physical interactions occur at the early enhancer of the PQ strain.

Restriction of interferon gamma responsiveness and basal expression of the myeloid human Fc gamma R1b gene is mediated by a functional PU.1 site and a transcription initiator consensus

The restricted expression of the human Fc gamma R1b gene to myeloid cells is likely to be regulated by a combination of transcription factors that may not be solely expressed in myeloid cells, but act together to restrict the expression of the gene to myeloid cells. Low basal expression of the human Fc gamma R1b gene is specifically upregulated by interferon gamma (IFN-gamma). A 181-bp region of 5' flanking sequence contains several key regulatory motifs that include the extended gamma response region (XGRR) and the PIE region. The XGRR contains the 39-bp gamma response region originally defined in the highly homologous Fc gamma R1a gene. The XGRR is in close proximity to the 21-bp PIE motif that is conserved in the promoters of some other myeloid genes. The PIE motif contains a consensus site for the macrophage and B cell transcription factor, PU.1, and is adjacent to the cluster of transcription start sites. An active transcription initiator, Inr, consensus spans the start sites and appears to direct transcription initiation of this TATA-less gene. In this study, we demonstrate that the PIE region contains a functional PU.1 site that binds a human PU.1-like protein and that associated factors present in myeloid extracts also bind in this PIE region. Mutational analysis reveals an absolute requirement for an intact PU.1 box for both basal and IFN-gamma inducible expression of this gene. In addition, mutations in the Inr greatly reduce basal and inducible transcription. Insertion of a strong TATA box downstream from the Inr or at -30 bp from the transcription start sites restores basal and inducible activity in the presence of a mutated PU.1 site. We also demonstrate that indeed, when the XGRR is positioned in the context of a heterologous TATA containing promoter, it is able to respond equivalently to either IFN-alpha or IFN-gamma. However, IFN-alpha responsiveness does not occur in the context of the physiological Fc gamma R1b TATA-less basal promoter. Our results suggest that a human PU.1-like factor acts as a "bridging factor" between the upstream IFN-gamma enhancer and the Inr dependent preinitiation complex. These findings indicate that the structure of the basal promoter in combination with restricted activators like PU.1 are important in regulating the expression of this gene.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

The transcription factors Elk-1 and serum response factor interact by direct protein-protein contacts mediated by a short region of Elk-1

Transcriptional induction of the c-fos gene in response to epidermal growth factor stimulation is mediated in part by a ternary nucleoprotein complex within the promoter consisting of serum response factor (SRF), p62TCF/Elk-1 and the serum response element (SRE). Both SRF and p62TCF/Elk-1 contact the DNA and bind in a cooperative manner to the SRE. In this study, we demonstrate that SRF and Elk-1 interact directly in the absence of the SRE. A 30-amino-acid peptide from Elk-1 (B-box) is both necessary and sufficient to mediate protein-protein contacts with SRF. Moreover, the Elk-1 B-box is necessary to enable SRF-dependent binding of an alternative ETS domain (from the transcription factor PU.1) to the c-fos SRE. Mutations in either the Elk-1 B-box or the C-terminal half of the SRF DNA-binding domain (coreSRF) which show reduced ability to form ternary complexes also show greatly reduced protein-protein interactions in the absence of the SRE. Our results clearly demonstrate that direct protein-protein interactions between the transcription factors Elk-1 and SRF, in addition to DNA contacts, contribute to the formation of a ternary complex on the c-fos SRE. We discuss the wider applicability of our results in describing specific protein-protein interactions between short well-defined transcription factor domains.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

p53 mutations in human tumors with chimeric EWS/FLI-1 genes

The Ewing family of tumors is recurrently characterized at the molecular level by the presence of a fusion transcript between the EWS gene on chromosome 22 and either the FLI-I or ERG genes, 2 closely related members of the Ets family of transcription factors. We have investigated 12 primary human tumors, 11 xenografts and 11 cell lines, which have been shown to express chimeric EWS transcripts in search of p53 mutations. Fragments of exons 5 to 8 and the corresponding consensus splice sequences were amplified by PCR and analyzed by denaturing gradient gel electrophoresis (DGGE). In 12 of 34 samples p53 mutations were detected (including 4 samples with multiple p53 mutations). The distribution of the mutations in the various samples was as follows: primary tumors 2/12; cell lines 5/11; xenografts 5/11. No correlation between the presence or absence of p53 mutations and the presence of a specific EWS chimeric transcript was observed. In addition, we observed that p53 mutations were almost always associated with a second hit (either deletion or second mutation) on the other p53 allele.

ERP, a new member of the ets transcription factor/oncoprotein family: cloning, characterization, and differential expression during B-lymphocyte development

The ets gene family encodes a group of proteins which function as transcription factors under physiological conditions and, if aberrantly expressed, can cause cellular transformation. We have recently identified two regulatory elements in the murine immunoglobulin heavy-chain (IgH) enhancer, pi and microB, which exhibit striking similarity to binding sites for ets-related proteins. To identify ets-related transcriptional regulators expressed in pre-B lymphocytes that may interact with either the pi or the microB site, we have used a PCR approach with degenerate oligonucleotides encoding conserved sequences in all members of the ets family. We have cloned the gene for a new ets-related transcription factor, ERP (ets-related protein), from the murine pre-B cell line BASC 6C2 and from mouse lung tissue. The ERP protein contains a region of high homology with the ETS DNA-binding domain common to all members of the ets transcription factor/oncoprotein family. Three additional smaller regions show homology to the ELK-1 and SAP-1 genes, a subgroup of the ets gene family that interacts with the serum response factor. Full-length ERP expresses only negligible DNA-binding activity by itself. Removal of the carboxy terminus enables ERP to interact with a variety of ets-binding sites including the E74 site, the IgH enhancer pi site, and the lck promoter ets site, suggesting a carboxy-terminal negative regulatory domain. At least three ERP-related transcripts are expressed in a variety of tissues. However, within the B-cell lineage, ERP is highly expressed primarily at early stages of B-lymphocyte development, and expression declines drastically upon B-cell maturation, correlating with the enhancer activity of the IgH pi site. These data suggest that ERP might play a role in B-cell development and in IgH gene regulation.

Putative nuclear localization signals (NLS) in protein transcription factors

We have recognized about ten distinct forms of strongly basic hexapeptides, containing at least four arginines and lysines, characteristic of nuclear proteins among all eukaryotic species, including yeast, plants, flies and mammals. These basic hexapeptides are considered to be different versions of a core nuclear localization signal, NLS. Core NLSs are present in nearly all nuclear proteins and absent from nearly all "nonassociated" cytoplasmic proteins that have been investigated. We suggest that the few (approximately 10%) protein factors lacking a typical NLS core peptide may enter the nucleus via their strong crosscomplexation with their protein factor partners that possess a core NLS. Those cytoplasmic proteins found to possess a NLS-like peptide are either tightly associated with cell membrane proteins or are integral components of large cytoplasmic protein complexes. On the other hand, some versions of core NLSs are found in many cell membrane proteins and secreted proteins. It is hypothesized that in these cases the N-terminal hydrophobic signal peptide of extracellular proteins and the internal hydrophobic domains of transmembrane proteins are stronger determinants for their subcellular localization. The position of core NLSs among homologous nuclear proteins may or may not be conserved; however, if lost from an homologous site it appears elsewhere in the protein. This search provides a set of rules to our understanding of the nature of core nuclear localization signals: (1) Core NLS are proposed to consist most frequently of an hexapeptide with 4 arginines and lysines; (2) aspartic and glutamic acid residues as well as bulky amino acids (F, Y, W) need not to be present in this hexapeptide; (3) acidic residues and proline or glycine that break the alpha-helix are frequently in the flanking region of this hexapeptide stretch; (4) hydrophobic residues ought not to be present in the core NLS flanking region allowing for the NLS to be exposed on the protein. In this study we attempt to classify putative core NLS from a wealth of nuclear protein transcription factors from diverse species into several categories, and we propose additional core NLS structures yet to be experimentally verified.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

The transcription factors Elk-1 and serum response factor interact by direct protein-protein contacts mediated by a short region of Elk-1

Transcriptional induction of the c-fos gene in response to epidermal growth factor stimulation is mediated in part by a ternary nucleoprotein complex within the promoter consisting of serum response factor (SRF), p62TCF/Elk-1 and the serum response element (SRE). Both SRF and p62TCF/Elk-1 contact the DNA and bind in a cooperative manner to the SRE. In this study, we demonstrate that SRF and Elk-1 interact directly in the absence of the SRE. A 30-amino-acid peptide from Elk-1 (B-box) is both necessary and sufficient to mediate protein-protein contacts with SRF. Moreover, the Elk-1 B-box is necessary to enable SRF-dependent binding of an alternative ETS domain (from the transcription factor PU.1) to the c-fos SRE. Mutations in either the Elk-1 B-box or the C-terminal half of the SRF DNA-binding domain (coreSRF) which show reduced ability to form ternary complexes also show greatly reduced protein-protein interactions in the absence of the SRE. Our results clearly demonstrate that direct protein-protein interactions between the transcription factors Elk-1 and SRF, in addition to DNA contacts, contribute to the formation of a ternary complex on the c-fos SRE. We discuss the wider applicability of our results in describing specific protein-protein interactions between short well-defined transcription factor domains.

The human beta 2 integrin CD18 promoter consists of two inverted Ets cis elements

To define the minimal promoter responsible for expression of CD18 in myeloid and lymphoid cells, we generated 5' and 3' deletion constructs of a segment extending 785 bp upstream and 19 bp downstream of a major transcription start site and determined their effects on driving expression of the luciferase reporter gene in transfected hematopoietic cell lines. A region extending from nucleotides (nt) -302 to +19 was sufficient for cell-restricted and phorbol ester-inducible expression. DNase I footprinting of this region revealed two adjacent protected segments extending from nt -81 to -68 (box A) and -55 to -41 (box B). When a construct of 47 nt in length containing box A and box B and lacking other 3' or 5' elements was cloned into a promoterless vector, it conferred tissue-specific and phorbol ester-inducible expression. Gel retardation revealed that the protein components of two major protein-DNA complexes that form on both box A and box B and are required for transcriptional activation are members of the Ets oncoprotein family; one is related to the GA-binding protein (GABP), and the other is related to PU.1/Spi-1. The minimal CD18 promoter, lacking TATA, CAAT, and initiator elements and consisting primarily of Ets repeats, may exemplify an emerging class of promoters with which the concerted binding of Ets factors is necessary and sufficient to mediate transcriptional activation through direct recruitment of the basal transcription machinery.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

Analysis of the 5' long terminal repeat of bovine syncytial virus

Four molecular clones of the bovine syncytial virus (BSV) were determined to be replication competent by the initiation of cytopathic infections and production of viable virus following transfection of viral DNA into permissive cells. The nucleotide (nt) sequence of the 5' long terminal repeat (LTR) of the infectious clone, BSV-11, was determined and analyzed to identify regions common to retroviral LTRs and elements with the potential for involvement in transcriptional regulation.

The human beta 2 integrin CD18 promoter consists of two inverted Ets cis elements

To define the minimal promoter responsible for expression of CD18 in myeloid and lymphoid cells, we generated 5' and 3' deletion constructs of a segment extending 785 bp upstream and 19 bp downstream of a major transcription start site and determined their effects on driving expression of the luciferase reporter gene in transfected hematopoietic cell lines. A region extending from nucleotides (nt) -302 to +19 was sufficient for cell-restricted and phorbol ester-inducible expression. DNase I footprinting of this region revealed two adjacent protected segments extending from nt -81 to -68 (box A) and -55 to -41 (box B). When a construct of 47 nt in length containing box A and box B and lacking other 3' or 5' elements was cloned into a promoterless vector, it conferred tissue-specific and phorbol ester-inducible expression. Gel retardation revealed that the protein components of two major protein-DNA complexes that form on both box A and box B and are required for transcriptional activation are members of the Ets oncoprotein family; one is related to the GA-binding protein (GABP), and the other is related to PU.1/Spi-1. The minimal CD18 promoter, lacking TATA, CAAT, and initiator elements and consisting primarily of Ets repeats, may exemplify an emerging class of promoters with which the concerted binding of Ets factors is necessary and sufficient to mediate transcriptional activation through direct recruitment of the basal transcription machinery.

A transcriptional regulatory element is associated with a nuclease-hypersensitive site in the pol gene of human immunodeficiency virus type 1

Analysis of the chromatin organization of the integrated human immunodeficiency virus type 1 (HIV-1) genome has previously revealed a major constitutive DNase I-hypersensitive site associated with the pol gene (E. Verdin, J. Virol. 65:6790-6799, 1991). In the present report, high-resolution mapping of this site with DNase I and micrococcal nuclease identified a nucleosome-free region centered around nucleotides (nt) 4490 to 4766. A 500-bp fragment encompassing this hypersensitive site (nt 4481 to 4982) exhibited transcription-enhancing activity (two- to threefold) when it was cloned in its natural position with respect to the HIV-1 promoter after transient transfection in U937 and CEM cells. Using in vitro footprinting and gel shift assays, we have identified four distinct binding sites for nuclear proteins within this positive regulatory element. Site B (nt 4519 to 4545) specifically bound four distinct nuclear protein complexes: a ubiquitous factor, a T-cell-specific factor, a B-cell-specific factor, and the monocyte/macrophage- and B-cell-specific transcription factor PU.1/Spi-1. In most HIV-1 isolates in which this PU box was not conserved, it was replaced by a binding site for the related factor Ets1. Factors binding to site C (nt 4681 to 4701) had a DNA-binding specificity similar to that of factors binding to site B, except for PU.1/Spi-1. A GC box containing a binding site for Sp1 was identified (nt 4623 to 4631). Site D (nt 4816 to 4851) specifically bound a ubiquitously expressed factor. These results identify a transcriptional regulatory element associated with a nuclease-hypersensitive site in the pol gene of HIV-1 and suggest that its activity may be controlled by a complex interplay of cis-regulatory elements.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

Regulation of gene expression at early stages of B-cell differentiation

The phenotype of B lymphocytes at various stages of differentiation is, in part, controlled at the transcriptional level. Recently, a number of B-cell lineage and stage-specific transcription factors have been identified as candidate determinants for the developmental regulation of gene expression in B lymphocytes.