Complex pattern of immunoglobulin mu gene expression in normal and transgenic mice: nonoverlapping regulatory sequences govern distinct tissue specificities

YY1-mediated enhancer-promoter communication in the immunoglobulin μ locus is regulated by MSL/MOF recruitment

The rearrangement and expression of the immunoglobulin μ heavy chain (Igh) gene require communication of the intragenic Eμ and 3' regulatory region (RR) enhancers with the variable (VH) gene promoter. Eμ binding of the transcription factor YY1 has been implicated in enhancer-promoter communication, but the YY1 protein network remains obscure. By analyzing the comprehensive proteome of the 1-kb Eμ wild-type enhancer and that of Eμ lacking the YY1 binding site, we identified the male-specific lethal (MSL)/MOF complex as a component of the YY1 protein network. We found that MSL2 recruitment depends on YY1 and that gene knockout of Msl2 in primary pre-B cells reduces μ gene expression and chromatin looping of Eμ to the 3' RR enhancer and VH promoter. Moreover, Mof heterozygosity in mice impaired μ expression and early B cell differentiation. Together, these data suggest that the MSL/MOF complex regulates Igh gene expression by augmenting YY1-mediated enhancer-promoter communication.

Enhancing B-Cell Malignancies-On Repurposing Enhancer Activity towards Cancer

B-cell lymphomas and leukemias derive from B cells at various stages of maturation and are the 6th most common cancer-related cause of death. While the role of several oncogenes and tumor suppressors in the pathogenesis of B-cell neoplasms was established, recent research indicated the involvement of non-coding, regulatory sequences. Enhancers are DNA elements controlling gene expression in a cell type- and developmental stage-specific manner. They ensure proper differentiation and maturation of B cells, resulting in production of high affinity antibodies. However, the activity of enhancers can be redirected, setting B cells on the path towards cancer. In this review we discuss different mechanisms through which enhancers are exploited in malignant B cells, from the well-studied translocations juxtaposing oncogenes to immunoglobulin loci, through enhancer dysregulation by sequence variants and mutations, to enhancer hijacking by viruses. We also highlight the potential of therapeutic targeting of enhancers as a direction for future investigation.

Identifying cis-acting DNA elements within a control region

Computational methods can be used to identify DNA sequence motifs that have been conserved through evolution, as well as motifs that correspond to recognition sites for known DNA-binding proteins. These computational methods, when combined with chromatin immunoprecipitation and other basic experiments, can provide preliminary insight into the elements and factors that regulate a gene of interest. When pursuing a more complete understanding of a control region of interest, a comprehensive mutant analysis should generally be performed as a critical step toward more advanced functional studies. This article describes strategies for such a comprehensive analysis. It also summarizes the insights provided by a comprehensive mutant analysis versus a phylogenetic analysis.

Direct interactions of OCA-B and TFII-I regulate immunoglobulin heavy-chain gene transcription by facilitating enhancer-promoter communication

B cell-specific coactivator OCA-B, together with Oct-1/2, binds to octamer sites in promoters and enhancers to activate transcription of immunoglobulin (Ig) genes, although the mechanisms underlying their roles in enhancer-promoter communication are unknown. Here, we demonstrate a direct interaction of OCA-B with transcription factor TFII-I, which binds to DICE elements in Igh promoters, that affects transcription at two levels. First, OCA-B relieves HDAC3-mediated Igh promoter repression by competing with HDAC3 for binding to promoter-bound TFII-I. Second, and most importantly, Igh 3' enhancer-bound OCA-B and promoter-bound TFII-I mediate promoter-enhancer interactions, in both cis and trans, that are important for Igh transcription. These and other results reveal an important function for OCA-B in Igh 3' enhancer function in vivo and strongly favor an enhancer mechanism involving looping and facilitated factor recruitment rather than a tracking mechanism.

Role of defective Oct-2 and OCA-B expression in immunoglobulin production and Kaposi's sarcoma-associated herpesvirus lytic reactivation in primary effusion lymphoma

Primary effusion lymphoma (PEL) is a distinct type of B-cell non-Hodgkin lymphoma characterized by the presence of Kaposi's sarcoma-associated herpesvirus (KSHV/human herpesvirus 8). Despite having a genotype and gene expression signature of highly differentiated B cells, PEL does not usually express surface or cytoplasmic immunoglobulin (Ig). We show the lack of Oct-2 and OCA-B transcription factors to be responsible, at least in part, for this defect in Ig production. Like Ig genes, ORF50, the key regulator of the switch from latency to lytic reactivation, contains an octamer motif within its promoter. We therefore examined the impact of Oct-2 and OCA-B on ORF50 activation. The binding of Oct-1 to the ORF50 promoter has been shown to significantly enhance ORF50 transactivation. We found that Oct-2, on the other hand, inhibited ORF50 expression and consequently lytic reactivation by competing with Oct-1 for the octamer motif in the ORF50 promoter. Our data suggest that Oct-2 downregulation in infected cells would be favorable to KSHV in allowing for efficient viral reactivation.

Enforced expression of the transcriptional coactivator OBF1 impairs B cell differentiation at the earliest stage of development

OBF1, also known as Bob.1 or OCA-B, is a B lymphocyte-specific transcription factor which coactivates Oct1 and Oct2 on B cell specific promoters. So far, the function of OBF1 has been mainly identified in late stage B cell populations. The central defect of OBF1 deficient mice is a severely reduced immune response to T cell-dependent antigens and a lack of germinal center formation in the spleen. Relatively little is known about a potential function of OBF1 in developing B cells. Here we have generated transgenic mice overexpressing OBF1 in B cells under the control of the immunoglobulin heavy chain promoter and enhancer. Surprisingly, these mice have greatly reduced numbers of follicular B cells in the periphery and have a compromised immune response. Furthermore, B cell differentiation is impaired at an early stage in the bone marrow: a first block is observed during B cell commitment and a second differentiation block is seen at the large preB2 cell stage. The cells that succeed to escape the block and to differentiate into mature B cells have post-translationally downregulated the expression of transgene, indicating that expression of OBF1 beyond the normal level early in B cell development is deleterious. Transcriptome analysis identified genes deregulated in these mice and Id2 and Id3, two known negative regulators of B cell differentiation, were found to be upregulated in the EPLM and preB cells of the transgenic mice. Furthermore, the Id2 and Id3 promoters contain octamer-like sites, to which OBF1 can bind. These results provide evidence that tight regulation of OBF1 expression in early B cells is essential to allow efficient B lymphocyte differentiation.

Involvement of Oct-1 in the regulation of CDKN1A in response to clinically relevant doses of ionizing radiation

CDKN1A is a cyclin-dependent kinase inhibitor that plays a critical role in cell cycle checkpoint regulation. It is transcriptionally induced by TP53 (p53) following exposure to ionizing radiation (IR). Induction of CDKN1A after irradiation is closely related to IR-sensitivity of tumor cells, but the underlying mechanisms remain obscure because conventional reporter gene systems respond poorly to IR unless hyperlethal doses are used. Here, we performed a promoter analysis of the CDKN1A gene following irradiation with clinically relevant doses of IR using the adeno-associated virus-mediated reporter system which we have recently shown to be highly responsive to IR. We demonstrate that there are regulatory elements at -1.1 kb, -1.4 kb, and -1.8 kb, and deletion of these elements attenuate induction of the CDKN1A gene promoter in response to 0.2-2.0 Gy of IR. EMSA and ChIP assays showed that Oct-1 binds constitutively to the elements at -1.1 kb and -1.8 kb. Functional involvement of Oct-1 was confirmed by RNA interference targeting the Oct-1 gene, which suppressed both the basal and IR-inducible components of the CDKN1A expression. Thus, our results reveal that Oct-1 is crucial to the TP53-mediated regulation of the CDKN1A gene promoter following exposure to clinically relevant doses of IR.

The Ets factor Spi-B is a direct critical target of the coactivator OBF-1

OBF-1 (Bob.1, OCA-B) is a lymphoid-specific transcriptional coactivator that associates with the transcription factors Oct-1 or Oct-2 on the conserved octamer element present in the promoters of several ubiquitous and lymphoid-specific genes. OBF-1-deficient mice have B cell-intrinsic defects, lack germinal centers, and have severely impaired immune responses to T cell-dependent antigens. Crucial genes that are regulated by OBF-1 and that might explain the observed phenotype of OBF-1 deficiency have remained elusive to date. Here we have generated transgenic mice expressing OBF-1 specifically in T cells and examined these together with mice lacking OBF-1 to discover transcriptional targets of this coactivator. Using microarray analysis, we have identified the Ets transcription factor Spi-B as a direct target gene critically regulated by OBF-1 that can help explain the phenotype of OBF-1-deficient mice. Spi-B has been implicated in signaling pathways downstream of the B cell receptor and is essential for germinal center formation and maintenance. The present findings establish a hierarchy between these two factors and provide a molecular link between OBF-1 and B cell receptor signaling.

Transcriptional control of B cell activation

The developmental program that commits a hematopoietic stem cell to the B lymphocyte lineage employs transcriptional regulators to enable the assembly of an antigen receptor complex with a useful specificity and with signalling competence. Once a naive IgM+ B cell is generated, it must correctly integrate signals from the antigen receptor with those from cytokine receptors and co-receptors delivering T cell help. The B cell responds through the regulated expression of genes that implement specific cell expansion and differentiation, secretion of high levels of high-affinity antibody, and generation of long-term memory. The transcriptional regulators highlighted in this chapter are those for which genetic evidence of function in IgM+ B cells in vivo has been provided, often in the form of mutant mice generated by conventional or conditional gene targeting. A critical developmental step is the maturation of bone marrow emigrant "transitional" B cells into the mature, long-lived cells of the periphery, and a number of the transcription factors discussed here impact on this process, yielding B cells with poor mitogenic responses in vitro. For mature B cells, it is clear that not only the nature, but the duration and amplitude of an activating signal are major determinants of the transcription factor activities enlisted, and so the ultimate outcome. The current challenge is the identification of the target genes that are activated to implement the correct response, so that we may more precisely and safely manipulate B cell behavior to predictably and positively influence humoral immune responses.

Identification of a POU factor involved in regulating the neuron-specific expression of the gene encoding diapause hormone and pheromone biosynthesis-activating neuropeptide in Bombyx mori

Diapause hormone (DH) and PBAN (pheromone biosynthesis-activating neuropeptide) are two important insect neuropeptides regulating development and reproduction respectively. In the present study, we report two Bombyx mori transcription factors interacting specifically with the promoter of Bom-DH-PBAN (where Bom-DH stands for B. mori DH); we named them DHMBP-1 and -2 (DH-modulator-binding proteins 1 and 2). The developmental changes of DHMBP-1/-2 are closely correlated with that of Bom-DH-PBAN mRNA throughout the pupal stage. Competition assays indicate that DHMBP-1 from Chinese B. mori possesses binding characteristics similar to those of the POU-M1 protein from Japanese B. mori. POU-M1 cDNAs were cloned from various tissues of Chinese B. mori and were found to be distinct from the previously published POU-M1 in amino acid residues 108-136 because of insertion mutations. Owing to this difference in amino acid residues, we named this cDNA POU-M2. Even though POU-M2 differs from POU-M1 at the N-terminal, the POU domain and the binding properties of both POU-M1 and -M2 are the same. Functional analysis showed that overexpression of POU-M2 in the Bombyx cell line BmN activated the promoter of Bom-DH-PBAN, but failed to activate a promoter in which the POU-binding element was mutated. The transcriptional activity of POU-M2 is probably regulated by other factors binding to the upstream of the promoter sequence. We show that the POU-M2-binding site was able to activate the transcription of a heterologous promoter of the gene encoding B. mori larval serum protein. POU-M1 was found to exhibit the same transcriptional activities as POU-M2. Taken together, these results demonstrate that POU-M2 plays an important role in the transcriptional regulation of the Bom-DH-PBAN gene.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

Embryonic lethality, decreased erythropoiesis, and defective octamer-dependent promoter activation in Oct-1-deficient mice

Oct-1 is a sequence-specific DNA binding transcription factor that is believed to regulate a large group of tissue-specific and ubiquitous genes. Both Oct-1 and the related but tissue-restricted Oct-2 protein bind to a DNA sequence termed the octamer motif (5'-ATGCAAAT-3') with equal affinity in vitro. To address the role of Oct-1 in vivo, an Oct-1-deficient mouse strain was generated by gene targeting. Oct-1-deficient embryos died during gestation, frequently appeared anemic, and suffered from a lack of Ter-119-positive erythroid precursor cells. This defect was cell intrinsic. Fibroblasts derived from these embryos displayed a dramatic decrease in Oct-1 DNA binding activity and a lack of octamer-dependent promoter activity in transient transfection assays. Interestingly, several endogenous genes thought to be regulated by Oct-1 showed no change in expression. When crossed to Oct-2(+/-) animals, transheterozygotes were recovered at a very low frequency. These findings suggest a critical role for Oct-1 during development and a stringent gene dosage effect with Oct-2 in mediating postnatal survival.

B cell development and immunoglobulin transcription in Oct-1-deficient mice

The POU domain transcription factors Oct-1 and Oct-2 interact with the octamer element, a motif conserved within Ig promoters and enhancers, and mediate transcription from the Ig loci. Inactivation of Oct-2 by gene targeting results in normal B cell development and Ig transcription. To study the role of Oct-1 in these processes, the lymphoid compartment of RAG-1(-/-) animals was reconstituted with Oct-1-deficient fetal liver hematopoietic cells. Recipient mice develop B cells with levels of surface Ig expression comparable with wild type, although at slightly reduced numbers. These B cells transcribe Ig normally, respond to antigenic stimulation, undergo class switching, and use a normal repertoire of light chain variable segments. However, recipient mice show slight reductions in serum IgM and IgA. Thus, the Oct-1 protein is dispensable for B cell development and Ig transcription.

Evidence that the mouse 3' kappa light chain enhancer confers position-independent transgene expression in T- and B-lineage cells

One of the major obstacles for successful application of murine leukemia virus (MLV) vectors to genetic therapy of lymphocyte disorders is low levels of transgene expression or the eventual loss of expression. To overcome this problem, an improved retroviral vector was constructed utilizing the myeloproliferative sarcoma virus (MPSV) long terminal repeat (LTR), which provided a significantly higher level of transgene expression in human lymphoid cells than did MLV vectors. Nevertheless, transgene expression remained low in a large percentage of transduced cells. To address whether lymphocyte enhancer elements might improve transgene expression mediated by retroviral vectors in lymphocytes, we cloned the mouse immunoglobulin 3' kappa light chain enhancer gene (mE3') into the MPSV vector. We found that the mE3' conferred a higher, more uniform and sustained level of expression in transduced T- and B-cell lines, and in primary T cells, than did the control vector lacking this element. Integration sites were diverse and a single copy of the proviral genome was present in all examined transduced cells. The mE3' failed to enhance transgene expression in most nonlymphoid cells, indicating it is relatively lineage-specific. Taken together, these results provide strong evidence that the mE3' functions as a locus control region (LCR) in conferring enhanced integration-site-independent expression of a retroviral transgene.

Combinatorial control of DNase I-hypersensitive site formation and erasure by immunoglobulin heavy chain enhancer-binding proteins

DNase I-hypersensitive sites in cellular chromatin are usually believed to be nucleosome-free regions generated by transcription factor binding. Using a cell-free system we show that hypersensitivity does not simply correlate with the number of DNA-bound proteins. Specifically, the leucine zipper containing basic helix-loop-helix protein TFE3 was sufficient to induce a DNase I-hypersensitive site at the immunoglobulin heavy chain micro enhancer in vitro. TFE3 enhanced binding of an ETS protein PU.1 to the enhancer. However, PU.1 binding erased the DNase I-hypersensitive site without abolishing TFE3 binding. Furthermore, TFE3 binding enhanced transcription in the presence and absence of a hypersensitive site, whereas endonuclease accessibility correlated strictly with DNase I hypersensitivity. We infer that chromatin constraints for transcription and nuclease sensitivity can differ.

Combination of 3′ and 5′ IgH regulatory elements mimics the B-specific endogenous expression pattern of IgH genes from pro-B cells to mature B cells in a transgenic mouse model

To ensure the B cell differentiation stage specificity of the intronic Emu element and of the locus control region (LCR) that lies downstream of the IgH chain locus, we generated transgenic mice harboring a V(H) promoter-GFP reporter gene linked to the 3'LCR region and the Emu element. By flow cytometry, GFP(+) lymphocytes were observed amongst pro-B cells (B220(+)CD43(+)CD117(+)) and at all stages of differentiation up to mature B cells (B220(+)IgM(+)IgD(+)). Expression was strictly confined to cells committed to the B lymphocyte lineage as judged by the lack of GFP(+)Thy1,2(+) cells (T lymphocytes) and GFP(+)B220(-)CD117(+)CD43(+) cells (uncommitted lymphohematopoietic progenitors). Therefore, the Emu-GFP-3'LCR transgene is not expressed by hematopoietic stem cells, begins its expression in pro-B cells and is specifically active at all stages of B cell maturation. The combination of 3' and 5' IgH regulatory elements thus appears as a potentially useful cassette in transgenes that require a stringent and early B lineage-specific expression.

LCR-regulated transgene expression levels depend on the Oct-1 site in the AT-rich region of beta -globin intron-2

Human beta-globin transgenes regulated by the locus control region (LCR) express at all integration sites in transgenic mice. For such LCR activity at ectopic sites, the 5'HS3 element requires the presence of the AT-rich region (ATR) in beta-globin intron-2. Here, we examine the dependence of 5'HS3 LCR activity on transcription factor binding sites in the ATR. In vitro DNaseI footprint analysis and electrophoretic mobility shift assays of the ATR identified an inverted double Gata-1 site composed of 2 noncanonical sequences (GATT and GATG) and an Oct-1 consensus site. Mutant Oct-1, Gata-1, or double mutant sites were created in the ATR of the BGT50 construct composed of a 5'HS3 beta/gamma-globin hybrid transgene. Transgenes with double mutant sites expressed at all sites of integration, but mean expression levels in transgenic mice were reduced from 64% per copy (BGT50) to 37% (P <.05). Mutation of the inverted double Gata-1 site had no effect at 61% per copy expression levels. In contrast, mutation of the Oct-1 site alone reduced per-copy expression levels to 31% (P <.05). We conclude that the ability of 5'HS3 to activate expression from all transgene integration sites is dependent on sequences in the ATR that are not bound at high affinity by transcription factors. In addition, the Oct-1 site in the ATR is required for high-level 5'HS3 beta/gamma-globin transgene expression and should be retained in LCRbeta-globin expression cassettes designed for gene therapy.

Characterization of the rat RALDH1 promoter. A functional CCAAT and octamer motif are critical for basal promoter activity

Retinal dehydrogenase type 1 (RALDH1) catalyzes the oxidation of retinal to retinoic acid (RA), a metabolite of vitamin A important for embryogenesis and tissue differentiation. Rat RALDH1 is expressed to high levels in developing kidney, and in stomach, intestine epithelia. To understand the mechanisms of the transcriptional regulation of rat RALDH1, we cloned a 1360-base pair (bp) 5'-flanking region of RALDH1 gene. Using luciferase reporter constructs transfected into HEK 293 and LLCPK (kidney-derived) cells, basal promoter activity was associated with sequences between -80 and +43. In this minimal promoter region, TATA and CCAAT cis-acting elements as well as SP1, AP1 and octamer (Oct)-binding sites were present. The CCAAT box and Oct-binding site, located between positions -72 and -68 and -56 and -49, respectively, were shown by deletion analysis and site-directed mutation to be critical for promoter activity. Nuclear extracts from kidney cells contain proteins specifically binding the Oct and CCAAT sequences, resulting in the formation of six complexes, while different patterns of complexes were observed with non-kidney cell extracts. Gel shift assays using either single or double mutations of the Oct and CCAAT sequences as well as super shift assays demonstrated single and double occupancy of these two sites by Oct-1 and CBF-A. In addition, unidentified proteins also bound the Oct motif specifically in the absence of CBF-A binding. These results demonstrate specific involvement of Oct and CCAAT-binding proteins in the regulation of RALDH1 gene.

POU/TBP cooperativity: a mechanism for enhancer action from a distance

Enhancers when functioning at a distance cannot effectively stimulate transcription from core promoters. We demonstrate that this is due to the inability of enhancer-bound activators to recruit TBP to a distal TATA box. Surprisingly, binding of a transcriptionally inert Oct-1 POU domain near a core promoter enables an enhancer to function from a distance. POU activity neither requires the coactivator OCA-B nor the interaction of TBP with TFIIA. Instead, the POU domain directly facilitates TBP recruitment to the promoter utilizing a bipartite interaction surface. These results establish that an interaction between the DNA binding domain of an activator and TBP can be used to stimulate transcription. Furthermore, they suggest a mechanism for long-range enhancer function in which a TBP complex is preassembled on a promoter via localized recruitment and then acted upon by distal activators.

A constitutively active dioxin/aryl hydrocarbon receptor induces stomach tumors

The dioxin/aryl hydrocarbon receptor (AhR) functions as a ligand-activated transcription factor regulating transcription of a battery of genes encoding xenobiotic metabolizing enzymes. Known receptor ligands are environmental pollutants including polycyclic aromatic hydrocarbons and polychlorinated dioxins. Loss-of-function (gene-disruption) studies in mice have demonstrated that the AhR is involved in toxic effects of dioxins but have not yielded unequivocal results concerning the physiological function of the receptor. Gain-of-function studies therefore were performed to unravel the biological functions of the AhR. A constitutively active AhR expressed in transgenic mice reduced the life span of the mice and induced tumors in the glandular part of the stomach, demonstrating the oncogenic potential of the AhR and implicating the receptor in regulation of cell proliferation.

I-kappa B kinase beta is critical for B cell proliferation and antibody response

The NF-kappaB proteins are critical in the regulation of the immune and inflammatory response. Stimulation of the NF-kappaB pathway leads to increases in I-kappaB kinase beta (IKKbeta) kinase activity to result in the enhanced phosphorylation and degradation of I-kappaB and the translocation of the NF-kappaB proteins from the cytoplasm to the nucleus. In this study, a dominant-negative IKKbeta mutant expressed from the IgH promoter was used to generate transgenic mice to address the role of IKKbeta on B cell function. Although these transgenic mice were defective in activating the NF-kappaB pathway in B cells, they exhibited no defects in B lymphocyte development or basal Ig levels. However, they exhibited defects in the cell cycle progression and proliferation of B cells in response to treatment with LPS, anti-CD40, and anti-IgM. Furthermore, selective defects in the production of specific Ig subclasses in response to both T-dependent and T-independent Ags were noted. These results suggest that IKKbeta is critical for the proliferation of B cells and the control of some aspects of the humoral response.

Defective octamer-dependent transcription is responsible for silenced immunoglobulin transcription in Reed-Sternberg cells

The absence of immunoglobulin (Ig) expression in B-cell-derived Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin disease (cHD) was initially suggested to be caused by crippling mutations in the Ig promoter or coding region. More recent investigations have, however, challenged this concept. This study addressed the role of mutations in the Ig promoter region in HRS cells. Nine cases of cHD and 3 B-cell-derived HD lines were analyzed for mutations in the TATA box and octamer motif of the Ig promoter. Mutations in the octamer motif were found in only 1 of the 9 cases and in 1 of the 3 HD cell lines (L1236). Furthermore, in all cases either a complete lack or strong reduction in the expression of the Oct2 transcription factor and the BOB.1/OBF.1 coactivator were found. The relevance of the rare promoter mutations was investigated by assaying the activity of Ig promoter reporter constructs transfected into the HD cell line L1236, which harbors a mutated octamer motif. These Ig reporter constructs were completely inactive in L1236 cells; however, their activity could be reconstituted by the cotransfection of a BOB.1/OBF.1 expression vector. The additional transfection with an Oct2 expression vector did not further enhance the Ig promoter activity. The conclusions drawn from these results are that crippling mutations in the Ig promoter and coding region are not the sole cause for the lack of Ig expression in HRS cells and that defects in the transcription machinery such as absence of BOB.1/OBF.1 are more important for this phenomenon.

Transcription factors Oct-1 and NF-YA regulate the p53-independent induction of the GADD45 following DNA damage

The p53-regulated GADD45 gene is one of the important players in cellular response to DNA damage, and probably involved in the control of cell cycle checkpoint, apoptosis and DNA repair. There are both the p53-dependent and -independent pathways that regulate GADD45 induction. Following ionizing radiation, induction of the GADD45 gene is regulated by p53 through the p53-binding motif located in the third intron of the GADD45 gene. In contrast, GADD45 induction by methyl methanesulfonate (MMS), UV radiation (UV), and medium starvation is independent of p53 status although p53 may contribute to these responses. However, the regulatory elements that control the p53-independent induction of GADD45 remain uncertain. In this report, we have performed detailed analyses to characterize the responsive components that are required for the induction of the GADD45 promoter. We have found that the region between -107 and -62 of the GADD45 promoter is crucial for the induction. Sequence analysis indicates that there are two OCT-1 sites and one CAAT box located in this region. Site-directed mutations of both OCT-1 and CAAT motifs substantially abrogate the induction of the GADD45 promoter by DNA damage. In addition, both Oct-1 protein (binding to OCT-1 site) and NF-YA protein (binding to CAAT box) are induced after cell exposure to DNA damaging agents. Moreover, the Electrophoretic Mobility Shift Assay (EMSA) has demonstrated the direct bindings of Oct-1 and NF-YA proteins to their consensus sequences in the GADD45 promoter. Therefore, these results have presented the novel observation that transcription factors Oct-1 and NF-YA participate in the cellular response to DNA damage and are involved in the regulation of stress-inducible genes.

B cell development and immunoglobulin gene transcription in the absence of Oct-2 and OBF-1

Oct-2 and OBF-1 (also called OCA-B or Bob-1) are B cell-specific transcription factors that bind to the conserved octamer site of immunoglobulin promoters, yet their role in immunoglobulin transcription has remained unclear. We generated mice in which the lymphoid compartment was reconstituted with cells that lack both Oct-2 and OBF-1. Even in the absence of these two transcription factors, B cells develop normally to the membrane immunoglobulin M-positive (IgM+) stage and immunoglobulin gene transcription is essentially unaffected. These observations imply that the ubiquitous factor Oct-1 plays a previously unrecognized role in the control of immunoglobulin gene transcription and suggest the existence of another, as yet unidentified, cofactor. In addition, both factors are essential for germinal center formation, although OBF-1 is more important than Oct-2 for IgG production after immunization.

Matrix attachment region-dependent function of the immunoglobulin mu enhancer involves histone acetylation at a distance without changes in enhancer occupancy

Nuclear matrix attachment regions (MARs), which flank the immunoglobulin mu heavy-chain enhancer on either side, are required for the activation of the distal variable-region (V(H)) promoter in transgenic mice. Previously, we have shown that the MARs extend a local domain of chromatin accessibility at the mu enhancer to more distal sites. In this report, we examine the influence of MARs on the formation of a nucleoprotein complex at the enhancer and on the acetylation of histones, which have both been implicated in contributing to chromatin accessibility. By in vivo footprint analysis of transgenic mu gene constructs, we show that the occupancy of factor-binding sites at the mu enhancer is similar in transcriptionally active wild-type and transcriptionally inactive DeltaMAR genes. Chromatin immunoprecipitation experiments indicate, however, that the acetylation of histones at enhancer-distal nucleosomes is enhanced 10-fold in the presence of MARs, whereas the levels of histone acetylation at enhancer-proximal nucleosomes are similar for wild-type and DeltaMAR genes. Taken together, these data indicate that the function of MARs in mediating long-range chromatin accessibility and transcriptional activation of the V(H) promoter involves the generation of an extended domain of histone acetylation, independent of changes in the occupancy of the mu enhancer.

The lymphoid-specific cofactor OBF-1 is essential for the expression of a V(H) promoter/HS1,2 enhancer-linked transgene in late B cell development

Mice deficient for the lymphoid-specific cofactor OBF-1 display reduced levels of IgG, IgA and IgE. To examine whether the lowered immunoglobulin expression is linked to reduced activity of IgH cis-regulatory elements, OBF-1(-/-) mice were crossed with mice expressing transgenes driven by a V(H) or beta-globin promoter linked to the HS1,2 enhancer. Here we show that OBF-1 is essential for the induced expression of a V(H) promoter-linked transgene, in contrast to a beta-globin promoter-dependent transgene, in LPS/IL-4 or CD40-stimulated splenic B cells. Furthermore, impaired transgene expression is observed in OBF-1(-/-) peritoneal B cells. This deficiency may be linked to OBF-1, as peritoneal cells from normal mice express OBF-1 protein constitutively. Our data link OBF-1 to IgH gene expression in late B lymphoid development.

Role of OCA-B in 3'-IgH enhancer function

OCA-B (alternately called Bob1 and OBF-1) is a B cell-specific coactivator that interacts with the ubiquitously expressed Oct-1 and the B cell-restricted Oct-2 to activate transcription via the octamer site (5'-ATGCAAAT-3'). OCA-B-/- mice appear to undergo normal Ag-independent B cell maturation. However, Ag-dependent B cell differentiation, including germinal center formation, production of secondary Ig isotypes, and proliferation in response to surface Ig cross-linking, is greatly affected. We demonstrate that the observed reductions in expression of class-switched isotypes in OCA-B-/- mice may be due in part to deficiencies in the function of the 3'-IgH enhancer elements. Furthermore, we find that surface Ig cross-linking represses all the Ig enhancers and that this repression is absent in OCA-B-/- B cells. These results suggest an important role for OCA-B in Ig enhancer function in vivo.

Molecular mechanisms of extinction: old findings and new ideas

Fusion experiments between somatic cells have been used for a long time as a means to understand the regulation of gene expression. In hybrids between differentiated cells such as hepatocytes or lymphocytes and undifferentiated cells such as fibroblasts a phenomenon called extinction has been described. In such hybrids expression of cell-specific genes derived from the more differentiated parental cell is selectively turned off (extinguished), whereas genes expressed from both cells like housekeeping genes remain active after fusion. Study of the molecular basis of extinction of the liver-specifically expressed tyrosine aminotransferase gene and of the B-cell-specifically expressed immunoglobulin genes has revealed that in hybrids the transcriptional program of the differentiated cells is reset. This is accompanied by a loss of expression or activity of many of the regulatory molecules that were operating in the differentiated cells. In the light of new insights in eukaryotic gene regulation we speculate that molecular mechanisms such as chromatin remodelling, recruitment to heterochromatin or subnuclear localization could underly the extinction process.

Cutting edge: lack of peripheral B cells and severe agammaglobulinemia in mice simultaneously lacking Bruton's tyrosine kinase and the B cell-specific transcriptional coactivator OBF-1

OBF-1 is a B cell-restricted transcriptional coactivator that is recruited to octamer-containing promoters by interacting with the POU domain of Oct-1 or Oct-2. We have shown earlier that mice lacking OBF-1 were dramatically impaired in their ability to mount humoral immune responses and did not develop germinal centers in the spleen; however, they had a largely normal B cell development in the bone marrow. In this study, we demonstrate that OBF-1-deficient mice also have an early defect in B cell development and show that OBF-1-/- immature B cells are greatly impaired at the transition from the bone marrow to the spleen. In addition, when the OBF-1 mutation is combined to a mutation in the gene encoding Bruton's tyrosine kinase, a striking phenotype is observed. These double-deficient animals lack peripheral B cells and have virtually no serum Igs, thus closely resembling human X chromosome-linked agammaglobulinemia.

Nuclear matrix attachment regions antagonize methylation-dependent repression of long-range enhancer-promoter interactions

The immunoglobulin intragenic mu enhancer region acts as a locus control region that mediates transcriptional activation over large distances in germ line transformation assays. In transgenic mice, but not in transfected tissue culture cells, the activation of a variable region (V(H)) promoter by the mu enhancer is dependent on flanking nuclear matrix attachment regions (MARs). Here, we examine the effects of DNA methylation, which occurs in early mouse development, on the function of the mu enhancer and the MARs. We find that methylation of rearranged mu genes in vitro, before transfection, represses the ability of the mu enhancer to activate the V(H) promoter over the distance of 1.2 kb. However, methylation does not affect enhancer-mediated promoter activation over a distance of 150 bp. In methylated DNA templates, the mu enhancer alone induces only local chromatin remodeling, whereas in combination with MARs, the mu enhancer generates an extended domain of histone acetylation. These observations provide evidence that DNA methylation impairs the distance independence of enhancer function and thereby imposes a requirement for additional regulatory elements, such as MARs, which facilitate long-range chromatin remodeling.

Cutting Edge: Ig Heavy Chain 3′ HS1–4 Directs Correct Spatial Position-Independent Expression of a Linked Transgene to B Lineage Cells

The Ig H chain locus is regulated by a set of cis-acting elements. Hypersensitive sites (HS) located 3′ of the IgH, HS1–4, has been suggested to act as a locus control region (LCR) in cell lines. To assess the proposed role of HS1–4 acting as an LCR, we generated transgenic mice harboring a VH promoter-β-globin reporter gene linked to the Ig H chain HS1–4 3′regulatory sequences. Transgene expression is strictly confined to B lymphocytes, with no detectable expression outside the B cell lineage in all transgenic founder lines. Furthermore, reporter gene activity is integration independent but not copy number dependent. Thus, additional sequences are required to allow the HS1–4 regulatory region to act as a classical LCR in mice. Our data are discussed in the context of tissue-specific gene expression in B lineage cells.

Transcriptional regulation of the murine 3' IgH enhancer by OCT-2

Targeted disruption of either of the B cell-specific transcription factors Oct-2 or OCA-B/BOB-1/OBF-1 dramatically affects B cell terminal differentiation. The 3' enhancer of immunoglobulin heavy chain (IgH) locus is important for transcription of the locus in terminal plasma cells. Allele-specific suppression of mutant Oct-2 binding sites in this enhancer by a variant Oct-2 protein revealed that in a mature B cell line this enhancer was specifically dependent upon Oct-2, as contrasted to the closely related Oct-1 transcription factor. Phosphorylation of the Oct-2 protein was important for this activation and was synergistic for coactivation by the OCA-B factor. These results indicate that Oct-2 and OCA-B interact with the 3' enhancer in regulation of the IgH locus during B cell activation.

B-Cell coactivator OBF-1 exhibits unusual transcriptional properties and functions in a DNA-bound Oct-1-dependent fashion

Eukaryotic transcriptional activators generally comprise both a DNA-binding domain that recognizes specific cis-regulatory elements in the target genes and an activation domain which is essential for transcriptional stimulation. Activation domains typically behave as structurally and functionally autonomous modules that retain their intrinsic activities when directed to a promoter by a variety of heterologous DNA-binding domains. Here we report that OBF-1, a B-cell-specific coactivator for transcription factor Oct-1, challenges this traditional view in that it contains an atypical activation domain that exhibits two unexpected functional properties when tested in the yeast Saccharomyces cerevisiae. First, OBF-1 by itself has essentially no intrinsic activation potential, yet it strongly synergizes with other activation domains such as VP16 and Gal4. Second, OBF-1 exerts its effect in association with DNA-bound Oct-1 but is inactive when attached to a heterologous DNA-binding domain. These findings suggest that activation by OBF-1 is not obtained by simple recruitment of the coactivator to the promoter but requires interaction with DNA-bound Oct-1 to stimulate a step distinct from those regulated by classical activation domains.

Expression of an immunoglobulin heavy chain transgene in macrophage as well as lymphocyte lineages in vivo

A rearranged immunoglobulin heavy chain (IgH) transgene-encoded protein is expressed in macrophage lineage cells, in addition to B and T lineages, in transgenic mouse bone marrow. Peripheral macrophages also express transgenic IgH protein. Mature T cells express lower levels than immature thymocytes. Almost all B220+ cells in the bone marrow express transgenic IgH protein, and this early expression in the B lineage is accompanied by a reduction of cell frequency even in the early B220+ CD43+ BP-1- stages, although it is more prominent in BP-1+ pre-B cells. Thus, an IgH transgene can be expressed not only in lymphoid but also in myeloid cells, although its developmental effects are restricted to the B cell lineage.

An intact NF-kappa B signaling pathway is required for maintenance of mature B cell subsets

Members of the NF-kappaB/Rel transcription factor family are expressed constitutively during B cell development and are further induced by mitogen activation. Mice harboring germline disruptions in individual NF-kappaB subunits exhibit distinct defects in B lymphocyte activation and survival. However, the role of NF-kappaB in the production and maintenance of B cell subsets has been difficult to dissect in these knockout animals due to functional impairment of other immune cells. To directly address the cell autonomous requirements for NF-kappaB in humoral immune compartments, transgenic mice were generated that express a transdominant form of Ikappa-Balpha in B lineage cells. Whereas expression of the inhibitor had only modest effects on basal or LPS-induced levels of NF-kappaB, transgenic B cells were significantly impaired for cellular proliferation and NF-kappaB induction in response to B cell receptor (BCR) crosslinking. Furthermore, the trans-dominant inhibitor produced a dose-dependent reduction in the population of mature splenic B cells. This cellular defect was more pronounced in long-lived B lymphocyte subsets that recirculate to the adult bone marrow. Together, these results indicate that BCR-mediated signaling must maintain NF-kappaB levels above a stringent threshold for proper regulation of B cell homeostasis.

Transcriptional regulation during B cell development

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.

Transcriptional Repression of the IL-2 Gene in Th Cells by ZEB

Th1- and Th2-type cells mediate distinct effector functions via cytokine secretion in response to immunologic challenge. Precursor Th cells transcribe IFN-γ, IL-2, and IL-4 upon activation. Repeated stimulation of Th precursor cells in the presence of IL-4 leads to terminally differentiated Th2 cells that have lost the ability to transcribe the IL-2 gene. We provide evidence that repression of IL-2 gene expression in Th2 cells and partial repression in Th1 cells are mediated by ZEB, a zinc finger, E box-binding transcription factor. This factor binds to a negative regulatory element, NRE-A, in the IL-2 promoter, thereby acting as a potent repressor of IL-2 transcription.

Temporal control of IgH gene expression in developing B cells by the 3' locus control region

The suggested roles of the downstream 3' regions acting as a Locus Control Region (LCR), have allowed comparisons to be made between the regulation of the IgH locus with other model systems whose gene expression is governed by LCR activity. Here we summarize the importance of the IgH 3'LCR and its putative functional role in IgH gene expression and compare it with the 5'LCR regulatory region of the human beta-globin locus.

Using altered specificity Oct-1 and Oct-2 mutants to analyze the regulation of immunoglobulin gene transcription

Oct-1 and Oct-2 represent the prototypical example of related transcription factors with identical DNA recognition properties. They bind functionally critical octamer elements found in diverse regulatory sequences. It has not been possible to determine directly if these factors are functionally redundant or selective when interacting with different regulatory sequences in the appropriate cell type. An equivalent pair of altered DNA-binding specificity mutants of Oct-1 and Oct-2 are used to examine their function from varied regulatory contexts in B cells. These factors function as redundant activators of immunoglobulin (Ig) gene promoters (Vkappa and VH) and a histone H2B promoter. The structural basis of redundancy resides in the highly conserved DNA-binding POU domain, because this domain of either protein can activate transcription from both Ig and H2B promoters. We find that the nature of a distal enhancer dictates the relative potency of Oct-1 versus Oct-2 bound to a promoter. Oct-1 preferentially stimulates transcription from a VH or Vkappa promoter in combination with enhancers from the IgH or Igkappa locus, respectively. In this context, the more potent action of Oct-1 is dependent on a region external to the POU domain. These results suggest that Oct-1 may be the critical regulator of Ig gene transcription during B cell development and provide an explanation for selective Ig isotype expression defects in Oct-2 and OCA-B null mice.

The enhancer shift: a model to explain the developmental control of IgH gene expression in B-lineage cells

The roles attributed to the E mu enhancer and the downstream 3' regions acting as locus control regions (LCRs) have allowed comparisons to be drawn between regulation of the IgH locus in early and late B-cell development and other systems governed by LCR activity. Here, the importance of the IgH 3'LCR and its putative functional role in the control of IgH gene expression is evaluated and compared with the 5'LCR regulatory region of the human beta-globin locus.

Efficient transcription of an immunoglobulin kappa promoter requires specific sequence elements overlapping with and downstream of the transcriptional start site

The expression of immunoglobulin (Ig) genes depends on tissue-specific elements in the promoter and enhancer regions of light chain and heavy chain genes. In contrast to the complex modular character of Ig enhancers, the promoters appear to be simple, depending primarily on a conserved TATA box and octamer elements. We have analyzed the role of proximal sequences for Igkappa promoter function. Igkappa promoter transcription critically depends on initiator-like sequences and on a downstream element located at +24 to +39 relative to the start site. Replacement of these sequences resulted in strong reduction of promoter activity. In vitro, these elements were found to be more effective in extracts of lymphoid than of non-lymphoid origin. Deletion of the downstream and initiation site regions had a comparable effect on promoter activity to obliteration of the TATA box or octamer element. The downstream sequence was bound by two nuclear proteins, identical to the previously identified Ig-specific C5 and C6 complexes. Whereas C5 is found in HeLa cells and in lymphoid cells, C6 is lymphoid specific. Thus, further specific sequences in addition to the previously characterized elements, the octamer and the TATA box, are required for efficient kappa promoter expression in B lymphocytes.

Extension of chromatin accessibility by nuclear matrix attachment regions

Transcription of the variable region of the rearranged immunoglobulin mu gene is dependent on an enhancer sequence situated within one of the introns of the gene. Experiments with transgenic mice have shown that activation of the promoter controlling this transcription also requires the matrix-attachment regions (MARs) that flank the intronic enhancer. As this mu gene enhancer can establish local areas of accessible chromatin, we investigated whether the MARs can extend accessibility to more distal positions. We eliminated interactions between enhancer- and promoter-bound factors by linking mu enhancer/MAR fragments to the binding sites for bacteriophage RNA polymerases that were either close to or one kilobase distal to the enhancer. The mu enhancer alone mediated chromatin accessibility at the proximal site but required a flanking MAR to confer accessibility upon the distal promoter. This long-range accessibility correlates with extended demethylation of the gene construct but not with whether it is being actively transcribed. MARs thus collaborate with the mu enhancer to generate an extended domain of accessible chromatin.

Regulatory elements of the mb-1 gene encoding the Ig-alpha component of the human B-cell antigen receptor

The mb-1 gene encodes the Ig-alpha component of the B-cell antigen receptor. It is specifically expressed in pre-B and mature B cells but not in plasma cells losing membrane Ig (mIg) expression. We looked for transcriptional regulatory elements within a 12 kb genomic fragment. A strong promoter activity was found in a 591 bp fragment harboring consensus binding sites for known transcription factors including Ets, EBF/BlyF, LyF1/micro B and Spl. It was able to drive transcription of a reporter gene in the absence of any additional enhancer and was mostly active in B lymphocytes not in plasma cells or T cells. Although no fragment from the mb-1 gene displayed enhancer activity in combination with either the SV40, a Ig VH or a Ig VL promoter, a 1078 bp fragment corresponding to the 5' part of the gene behaved as a strong enhancer in either orientation in constructs driven by the mb-1 promoter itself. Deletions within this fragment allowed to delineate shorter sequences with enhancer activity upstream the first exon. The tissue-restricted, promoter-restricted and stage-specific activity of this 5' flanking region suggests that it is the main regulatory element of the mb-1 gene.

B-cell-specific coactivator OBF-1/OCA-B/Bob1 required for immune response and germinal centre formation

The B-lymphocyte-specific transcriptional factor called Oct binding factor (OBF)-1, OCA-B or Bob1 (refs 1-3) is thought to be involved in the transcription of immunoglobulin genes through recruitment to the highly conserved octamer site of immunoglobulin promoters, mediated by either Oct-1 or Oct-2. To define the in vivo role of OBF-1 we have used gene targeting in embryonic stem cells to generate mice lacking the coactivator OBF-1. Such OBF-1-/- mice are born normally, are fertile and seem healthy, and surprisingly, rearrangement and transcription of immunoglobulin genes are largely unaffected. However, mice deficient in OBF-1 have reduced numbers of mature B cells and a severe reduction in the number of recirculating B cells, but otherwise show normal B-cell differentiation. Serum IgA and particularly IgG levels are greatly reduced. If mutant mice are immunized with either a thymus-independent or a thymus-dependent antigen, their immune responses are dramatically weakened. Strikingly, germinal centres completely fail to develop after immunization with thymus-dependent antigen. Our results demonstrate that in vivo OBF-1 is not required for initial transcription of immunoglobulin genes or for B cell development, but instead is essential for the response of B cells to antigens, and is required for the formation of germinal centres.

Gene structure and characterization of the murine homologue of the B cell-specific transcriptional coactivator OBF-1

The B cell-specific activity of immunoglobulin (Ig) gene promoters is to a large extent mediated by the conserved octamer motif ATTTGCAT. This requires the DNA binding octamer factors Oct-1 and/or Oct-2, as well as an additional B cell-restricted non-DNA binding cofactor. We recently cloned such a coactivator specific for Oct-1 or Oct-2 from human B cells and called it OBF-1. Here we report the isolation and characterization of the murine homologue. Full-length cDNA clones as well as genomic clones were isolated and the gene structure was determined. The deduced protein sequence shows that the mouse protein has an identical length, is likewise proline rich and shows 89% overall identity to the human protein. The OBF-1 gene is expressed in a very highly B cell-specific manner and is transcribed in cells representative of all stages of B cell differentiation, including the earliest ones. We show that OBF-1 interacts in the absence of DNA with the POU domain of Oct-1 or Oct-2 and also with the general transcription factors TBP and TFIIB. Furthermore, we demonstrate that although OBF-1 efficiently activates promoter octamer sites, it does not activate enhancer octamer sites.

Cloning and characterization of the murine B-cell specific transcriptional coactivator Bob1

From a murine B-cell cDNA-library we have cloned a cDNA encoding the murine B-cell specific coactivator mBob1. The protein is the murine homologue to the recently described human coactivator Bob1 (hBob1), also referred to as OBF-1 or OCA-B. We have also characterized the genomic mBob1 clone. Analysis of its intron-exon structure has allowed identification of a C-terminal splice variant. mBob1 is B-cell restricted, and is found in all B-cell lines representing different stages of B-cell differentiation. mBob1 interacts with the octamer transcription factors Oct-1 and Oct-2 and stimulates transcription mediated by these factors.

Molecular analysis of the distal enhancer of the mouse alpha-fetoprotein gene

The mouse alpha-fetoprotein (AFP) gene is transcribed at high levels in the visceral endoderm of the yolk sac and fetal liver and at much lower rates in the endoderm of the fetal gut. Expression of the gene in vivo requires the presence of at least one of three enhancers which lie in its 5' flanking region. In this report, we establish that the most distal AFP enhancer directed consistent expression of a linked AFP minigene in all three endodermal tissues in transgenic mice. The enhancer is composed of three domains, each of which is essential for full enhancer function by transient transfection assays. DNase I footprinting identified three regions of the enhancer which are protected by human hepatoma nuclear extracts, one of which corresponded to a consensus site for HNF-3 binding. Site-directed mutations in this site caused a 10-fold reduction in enhancer function by transient transfection. In transgenic mice, however, the mutation resulted in sporadic expression of the transgene, dependent on the site of integration. A similar acquisition of position-dependent sporadic expression of the transgene was observed with a mutation in a second protein binding site, despite the fact that this mutation had very little effect on enhancer function as assessed by transient transfection. These studies underscore the value of examining the functions of specific protein binding sites in vivo.

OBF-1, a novel B cell-specific coactivator that stimulates immunoglobulin promoter activity through association with octamer-binding proteins

Recent biochemical and genetic studies indicate that in addition to the octamer-binding proteins Oct-1 and Oct-2, other B cell components are required for lymphoid-restricted, octamer site-mediated immunoglobulin gene promoter activity. Using a genetic screen in yeast, we have isolated B cell-derived cDNAs encoding Oct-binding factor 1 (OBF-1), a novel protein that specifically associates with Oct-1 and Oct-2. Biochemical studies demonstrate that OBF-1 has no intrinsic DNA-binding activity and recognizes the POU domains of Oct-1 and Oct-2, but not those of Oct-4 and Oct-6. The OBF-1 mRNA is expressed in a highly cell-specific manner, being most abundant in B cells and essentially absent in most of the other cells or tissues tested. Furthermore, expression of OBF-1 in HeLa cells selectively stimulates the activity of a natural immunoglobulin promoter in an octamer site-dependent manner. Thus, OBF-1 has all the properties expected for a B cell-specific transcriptional coactivator protein.