Vol. 11, No. 1
MOLECULAR AND CELLULAR BIOLOGY, Jan. 1991, p. 75-83 0270-7306/91/010075-09$02.00/0 Copyright C 1991, American Society for Microbiology
Positive and Negative Regulation of Immunoglobulin Gene Expression by a Novel B-Cell-Specific Enhancer Element JIYANG WANG, MAKOTO OKETANI, AND TAKESHI WATANABE*
Medical Institute of Bioregulation, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812, Japan Received 14 August 1990/Accepted 29 September 1990
A new B-cell-specific enhancer element has been identified 3' of E4 and the octamerlike motifs in the human immunoglobulin heavy-chain gene enhancer. Tandem copies of this 67-bp Mnll-AluI fragment, when fused to the chloramphenicol acetyltransferase gene driven by the conalbumin promoter, stimulated transcription in B cells but not in Jurkat T cells or HeLa cells. Footprinting analysis revealed that the identical sequence CCGAAACTGAAAAGG, designated E6, was protected by nuclear extracts from B cells, T cells, or HeLa cels. Gel mobility shift assays using a synthetic E6 motif detected a B-cell-specific complex in addition to a ubiquitous band found also in T ceUls and HeLa cells. In agreement with the results of gel retardation assays, tandem copies of the E6 motif stimulated transcription in ARH77 and Raji cells but not in Jurkat or HeLa cells. Furthermore, a mutant E6 motif lost both in vitro binding activity and in vivo enhancer activity. In striking contrast to the mouse Ig heavy-chain enhancer, in which the octamer motif acts as a B-cell-specific enhancer element, the human enhancer contains an octamerlike sequence with one base substitution which bound octamer-binding proteins with only very low affinity and showed no enhancer activity of its own. Interestingly, the Mnll-AluI fragment could suppress the basal-level activity of the conalbumin promoter in both Jurkat and HeLa cells. Moreover, simian virus 40 enhancer activity was blocked by the MnII-AluI fragment in HeLa cells but not in B cells. Thus, the novel enhancer element identified in this study is probably a target site for both positive and negative factors.
these octamerlike sequences act as B-cell-specific enhancer elements and bind octamer-binding proteins. Our initial studies revealed that these octamerlike motifs have only very low affinity for the octamer-binding proteins in an in vitro binding assay, indicating that there could be other B-cell-specific elements instead of these octamerlike sequences. With the aim of identifying such B-cell-specific elements, we analyzed the human Ig gene enhancer in detail in a transient in vivo assay system. Short segments of the enhancer and polymers of them were tested for enhancer activity in various cell lines. In this study, we have identified a new B-cell-specific enhancer element located 3' of the E4 and the octamerlike sequence region. This element contains a novel motif named E6, which acts as an enhancer element in B cells but not in HeLa cells. Surprisingly, this element could also reduce the basal-level activity of the conalbumin promoter and even block simian virus (SV40) enhancer activity in HeLa cells. Thus, this element may be involved in both positive and negative regulation of Ig gene expression. Our data also revealed that the octamerlike sequence by itself showed no enhancer activity when polymerized but could increase the activity of the novel B-cell-specific element identified in this study.
Transcription of immunoglobulin (Ig) genes is regulated by at least two elements: promoters 5' of V regions and an enhancer within the J (joining)-C (constant) intron (3, 7, 11, 13, 25, 27, 28, 31). The J-C intron enhancer probably not only is involved in the regulation of transcription of rearranged Ig genes but also plays a crucial role in the regulation of Ig gene rearrangement during B-cell differentiation (8). The activity of the Ig gene enhancer is strictly regulated by multiple trans-acting factors that interact with each other and with cis-acting enhancer elements. Thus, identification and characterization of such factors, especially B-cell-specific ones, is important for elucidating the mechanism by which B cells differentiate from bone marrow stem cells. The mouse Ig heavy-chain gene enhancer has been investigated by many groups. Several enhancer motifs such as El to E5 and the octamer have been identified (6). These motifs bind trans-acting factors that act collectively to enhance transcription (15, 23, 34, 40). Among these, the octamer sequence ATT1lGCAT has been shown to be a B-cellspecific motif that binds both ubiquitous (octl) and B-cellspecific (oct2A and oct2B) factors (10, 22, 35, 36, 38). On the other hand, negative transcriptional control has also been suggested by several groups (17, 21). Two repressor elements, one located 5' of the E2 motif and the other found 3' of the E4 and octamer motifs, have been identified by deletion analysis (2, 16, 18, 32, 33, 41, 43). Therefore, both positive and negative regulation seem to be responsible for the B-cell specificity of the whole Ig enhancer. The human Ig heavy-chain enhancer does not contain the same complete octamer sequence as found in the mouse enhancer. Instead, it contains three octamerlike sequences, ATTTGCjT, ATGCAAAA, and ATTU_lCAT, each with one base substitution (24). It is necessary to clarify whether *
MATERIALS AND METHODS Construction of plasmids. The human Ig heavy-chain gene enhancer (958-bp MluI-HpaI fragment) was divided into six fragments. Various enhancer deletion mutants were cloned into the BamHI site either 5' upstream or 3' downstream of plasmid pconaCAT (20), which contains the conalbumin promoter driving the CAT (chloramphenicol acetyltransferase) gene, or into the 5' or 3' region of pIgCAT (containing the human Ig promoter). In some cases, two separate fragments were ligated to obtain an internal deletion mutant or to make appropriate combinations. These plasmids were
Corresponding author. 75
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MOLECULAR AND CELLULAR BIOLOGY, Jan. 1991, p. 75-83 0270-7306/91/010075-09$02.00/0 Copyright C 1991, American Society for Microbiology
Positive and Negative Regulation of Immunoglobulin Gene Expression by a Novel B-Cell-Specific Enhancer Element JIYANG WANG, MAKOTO OKETANI, AND TAKESHI WATANABE*
Medical Institute of Bioregulation, Kyushu University, Maidashi 3-1-1, Higashi-ku, Fukuoka 812, Japan Received 14 August 1990/Accepted 29 September 1990
A new B-cell-specific enhancer element has been identified 3' of E4 and the octamerlike motifs in the human immunoglobulin heavy-chain gene enhancer. Tandem copies of this 67-bp Mnll-AluI fragment, when fused to the chloramphenicol acetyltransferase gene driven by the conalbumin promoter, stimulated transcription in B cells but not in Jurkat T cells or HeLa cells. Footprinting analysis revealed that the identical sequence CCGAAACTGAAAAGG, designated E6, was protected by nuclear extracts from B cells, T cells, or HeLa cels. Gel mobility shift assays using a synthetic E6 motif detected a B-cell-specific complex in addition to a ubiquitous band found also in T ceUls and HeLa cells. In agreement with the results of gel retardation assays, tandem copies of the E6 motif stimulated transcription in ARH77 and Raji cells but not in Jurkat or HeLa cells. Furthermore, a mutant E6 motif lost both in vitro binding activity and in vivo enhancer activity. In striking contrast to the mouse Ig heavy-chain enhancer, in which the octamer motif acts as a B-cell-specific enhancer element, the human enhancer contains an octamerlike sequence with one base substitution which bound octamer-binding proteins with only very low affinity and showed no enhancer activity of its own. Interestingly, the Mnll-AluI fragment could suppress the basal-level activity of the conalbumin promoter in both Jurkat and HeLa cells. Moreover, simian virus 40 enhancer activity was blocked by the MnII-AluI fragment in HeLa cells but not in B cells. Thus, the novel enhancer element identified in this study is probably a target site for both positive and negative factors.
these octamerlike sequences act as B-cell-specific enhancer elements and bind octamer-binding proteins. Our initial studies revealed that these octamerlike motifs have only very low affinity for the octamer-binding proteins in an in vitro binding assay, indicating that there could be other B-cell-specific elements instead of these octamerlike sequences. With the aim of identifying such B-cell-specific elements, we analyzed the human Ig gene enhancer in detail in a transient in vivo assay system. Short segments of the enhancer and polymers of them were tested for enhancer activity in various cell lines. In this study, we have identified a new B-cell-specific enhancer element located 3' of the E4 and the octamerlike sequence region. This element contains a novel motif named E6, which acts as an enhancer element in B cells but not in HeLa cells. Surprisingly, this element could also reduce the basal-level activity of the conalbumin promoter and even block simian virus (SV40) enhancer activity in HeLa cells. Thus, this element may be involved in both positive and negative regulation of Ig gene expression. Our data also revealed that the octamerlike sequence by itself showed no enhancer activity when polymerized but could increase the activity of the novel B-cell-specific element identified in this study.
Transcription of immunoglobulin (Ig) genes is regulated by at least two elements: promoters 5' of V regions and an enhancer within the J (joining)-C (constant) intron (3, 7, 11, 13, 25, 27, 28, 31). The J-C intron enhancer probably not only is involved in the regulation of transcription of rearranged Ig genes but also plays a crucial role in the regulation of Ig gene rearrangement during B-cell differentiation (8). The activity of the Ig gene enhancer is strictly regulated by multiple trans-acting factors that interact with each other and with cis-acting enhancer elements. Thus, identification and characterization of such factors, especially B-cell-specific ones, is important for elucidating the mechanism by which B cells differentiate from bone marrow stem cells. The mouse Ig heavy-chain gene enhancer has been investigated by many groups. Several enhancer motifs such as El to E5 and the octamer have been identified (6). These motifs bind trans-acting factors that act collectively to enhance transcription (15, 23, 34, 40). Among these, the octamer sequence ATT1lGCAT has been shown to be a B-cellspecific motif that binds both ubiquitous (octl) and B-cellspecific (oct2A and oct2B) factors (10, 22, 35, 36, 38). On the other hand, negative transcriptional control has also been suggested by several groups (17, 21). Two repressor elements, one located 5' of the E2 motif and the other found 3' of the E4 and octamer motifs, have been identified by deletion analysis (2, 16, 18, 32, 33, 41, 43). Therefore, both positive and negative regulation seem to be responsible for the B-cell specificity of the whole Ig enhancer. The human Ig heavy-chain enhancer does not contain the same complete octamer sequence as found in the mouse enhancer. Instead, it contains three octamerlike sequences, ATTTGCjT, ATGCAAAA, and ATTU_lCAT, each with one base substitution (24). It is necessary to clarify whether *
MATERIALS AND METHODS Construction of plasmids. The human Ig heavy-chain gene enhancer (958-bp MluI-HpaI fragment) was divided into six fragments. Various enhancer deletion mutants were cloned into the BamHI site either 5' upstream or 3' downstream of plasmid pconaCAT (20), which contains the conalbumin promoter driving the CAT (chloramphenicol acetyltransferase) gene, or into the 5' or 3' region of pIgCAT (containing the human Ig promoter). In some cases, two separate fragments were ligated to obtain an internal deletion mutant or to make appropriate combinations. These plasmids were
Corresponding author. 75
76
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