Elmaleh N, Matthias P, Schaffner W
Institut für Molekularbiologie II, Universität Zürich, Switzerland.
Eur J Biochem. 1990 Feb 14;187(3):507-13. doi: 10.1111/j.1432-1033.1990.tb15332.x.
The transcriptional enhancer located in the first intron of the immunoglobulin heavy chain constant region is a major determinant of B-cell-specific expression of immunoglobulin genes. Like other enhancers, the Ig heavy chain enhancer contains several short sequence motifs that bind specific transcription factors. Each binding site contributes to the overall activity of the enhancer, however no single element seems absolutely required for activity. For a better understanding of the Ig heavy chain enhancer components, we have cloned and analyzed individual sequence elements. We find that the factor that binds to the E3 enhancer motif, CATGTGGC, is a ubiquitous transcription factor. It is present in an active form in both B cells and non-B cells, where it can mediate transcriptional activation in vitro and in vivo. However, despite its ability to activate transcription of a transfected reporter gene, the factor is apparently unable to bind to the endogenous Ig heavy chain enhancer in non-lymphoid cells: In previous experiments by others, the characteristic in vivo footprint of this factor, designated NF-muE3, was detected in B cells but not in non-B cells. From this and other findings the picture emerges that there are at least three categories of factors which mediate cell-type-specific transcription in B lymphocytes: (a) cell-specific factors such as Oct-2A and Oct-2B that are not expressed in most other cell types: (b) ubiquitous factors such as NF-kappa B that are constitutively active in B cells but are sequestered in an inactive form in other cells; (c) ubiquitously active factors, exemplified by the one binding to the E3 sequence motif. This factor is present in an active form in a variety of cell types but is apparently unable to bind to the endogenous Ig heavy chain enhancer in non-B cells, perhaps due to a non-permissive chromatin structure of the Ig heavy chain locus.
位于免疫球蛋白重链恒定区第一个内含子中的转录增强子是免疫球蛋白基因B细胞特异性表达的主要决定因素。与其他增强子一样，Ig重链增强子包含几个结合特定转录因子的短序列基序。每个结合位点都对增强子的整体活性有贡献，然而似乎没有单个元件对于活性是绝对必需的。为了更好地理解Ig重链增强子的组成成分，我们克隆并分析了各个序列元件。我们发现与E3增强子基序CATGTGGC结合的因子是一种普遍存在的转录因子。它以活性形式存在于B细胞和非B细胞中，在体外和体内都能介导转录激活。然而，尽管它有能力激活转染报告基因的转录，但该因子显然无法在非淋巴细胞中与内源性Ig重链增强子结合：在其他人先前的实验中，这种被命名为NF-muE3的因子的特征性体内足迹在B细胞中被检测到，但在非B细胞中未被检测到。从这一发现和其他发现中可以看出，至少有三类因子介导B淋巴细胞中的细胞类型特异性转录：(a)细胞特异性因子，如Oct-2A和Oct-2B，在大多数其他细胞类型中不表达；(b)普遍存在的因子，如NF-κB，在B细胞中组成性激活，但在其他细胞中以无活性形式被隔离；(c)普遍活跃的因子，以与E3序列基序结合的因子为例。该因子以活性形式存在于多种细胞类型中，但显然无法在非B细胞中与内源性Ig重链增强子结合，这可能是由于Ig重链基因座的非许可染色质结构所致。
