A Conserved Tissue-Specific Structure at a Human T-Cell Receptor β-Chain Core Promoter

Abstract

The T-cell receptor (TCR) ␤-chain promoters have been characterized as nonstructured basal promoters that carry a single conserved ubiquitous cyclic AMP-responsive element. Our investigation of the human TCR ␤ gene uncovers a surprisingly complex and tissue-specific structure at the TCR V␤ 8.1 promoter. The core of the promoter (positions ؊42 to ؉11) is recognized by the lymphoid cell-specific transcription factors Ets-1, LEF1, and AML1 as well as by CREB/ATF-1, as is demonstrated in gel shift and footprinting experiments. With the exception of LEF1, these factors activate transcription in T cells. Binding sites at the core region show little conservation with consensus sites. Nonetheless, CREB, Ets-1, and AML1 bind and activate cooperatively and very efficiently through the nonconsensus binding sites at the core promoter region. Moderate ubiquitous activation is further induced by CREB/ATF and Sp1 factors through proximal upstream elements. The tissue-specific core promoter structure is apparently conserved in other T-cell-specifically expressed genes such as the CD4 gene. Our observations suggest that both the enhancer and the promoter have a complex tissue-specific structure whose functional interplay potentiates T-cell-specific transcription.T-cell-specific expression of the T-cell receptor (TCR)-CD3 complex and the CD4-CD8 coreceptors depends on distal enhancer elements (8,18,22,29,31,38,46,64,78,80), which is reminiscent of the B-cell-specific regulation of transcription of the immunoglobulin genes (4). The enhancers contain multiple binding sites for ubiquitous, lymphoid cell-and T-cell-specific factors (reviewed in references 9 and 40). Many T-cell-and lymphoid cell-specific factors were initially identified because they bind to functional important elements of the TCR enhancers. Examples are members of the family of high-mobilitygroup (HMG) box-containing factors TCF1 and LEF1 (T-cell factor 1 and lymphoid enhancer binding factor 1) (74,75,77), the zinc finger protein Ikaros (17,49), and the runt domain factor PEBP2␣ (polyomavirus enhancer binding protein 2␣). The latter, also called CBF␣ (core binding factor ␣), is the murine homolog of human AML1 (52), which is involved in the induction of acute myeloid leukemia (reviewed in references 30 and 50). In addition, the lymphoid cell-specific factors Ets-1, Elf-1, and GATA3 as well as the ubiquitous CREB/ATF family bind to and activate the TCR enhancers (16,20,28,43,79,80).Expression of the TCR genes is dependent on distal enhancers (22,38,40,46). Like other tissue-specific and inducible enhancer elements (for reviews, see references 72 and 73), the TCR enhancers assemble the transcription factors in a complex multiprotein structure (19,20). The corresponding promoters are thought to contribute little to tissue-specific transcription of TCR genes (40). For example, sequence comparisons of the different ␤-chain promoters revealed only one conserved element (1, 59, 69). It resembles a cyclic AMPresponsive element (CRE) and is recognized by different members o...

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