Cloning of murine TCF-1, a T cell-specific transcription factor interacting with functional motifs in the CD3-epsilon and T cell receptor alpha enhancers

Sox-4, an Sry-like HMG box protein, is a transcriptional activator in lymphocytes

Previous studies in lymphocytes have described two DNA-binding HMG box proteins, TCF-1 and LEF-1, with affinity for the A/TA/TCAAAG motif found in several T cell-specific enhancers. Evaluation of cotransfection experiments in non-T cells and the observed inactivity of an AACAAAG concatamer in the TCF-1/LEF-1-expressing T cell line BW5147, led us to conclude that these two proteins did not mediate the observed enhancer effect. We therefore searched for additional HMG box proteins. By a PCR-aided strategy, we cloned Sox-4, a gene with homology to the HMG box region of the sex determining gene SRY. Sox-4 was expressed in T and pre-B lymphocyte lines and in the murine thymus. Significantly, BW5147 T cells did not express Sox-4. Recombinant Sox-4 bound with high affinity (Kd 3 x 10(-11) M) to the minor groove of the AACAAAG motif, most likely contacting all seven base pairs. In contrast with observations on TCF-1 and LEF-1, cotransfection with Sox-4 unveiled a transactivating capacity, which mapped to its serine-rich C terminus. This region remained functional upon grafting onto a GAL4 DNA-binding domain. Sox-4 is thus the first 'classical' transcription factor in the Sox gene family with separable DNA-binding and transactivation domains. Our observations indicate that a detailed understanding of T cell-specific gene control must integrate the concerted activity of at least three tissue-specific HMG box genes.

Differential expression of the HMG box factors TCF-1 and LEF-1 during murine embryogenesis

The recent identification of a number of T lymphocyte-specific enhancers has allowed the cloning of several novel transcription factors. Two of these, TCF-1 and LEF-1, contain a virtually identical DNA-binding domain of the High Mobility Group (HMG-1) box type. TCF-1 and LEF-1 originate from a recent gene duplication event as evidenced by comparison with the chicken homologue, chTCF. We have now analyzed the differential expression of these two transcription factors. In a panel of lymphoid cell lines, TCF-1 was exclusively expressed in the T cell lineage. In contrast, LEF-1 mRNA was detected at equivalent levels in pro- and pre-B cells and in all T lineage cells. In situ hybridization on murine embryos revealed that TCF-1 and LEF-1 were widely expressed at day 7.5 of gestation. At later stages, the expression patterns were complex and only partially overlapping. The expression of TCF-1 and LEF-1 coincided until day 10.5, when mRNAs were detected in limb buds, neural crest, pharyngeal arches and nasal process. At later time points (day 13.5 to 14.5), sites of overlapping expression included lung, the urogenital system, tooth buds, thymus and choroid plexus. Unique expression sites for TCF-1 included Reichert's membrane and trophectoderm-derived cells, the ribs and thoracic prevertebrae, craniofacial structures, the adrenal gland and meninges. Unique LEF-1 expression was observed in the tail prevertebrae, brain and inner ear. Postnatally, expression of both genes could only be detected in lymphoid tissues. These observations suggest that TCF-1 and LEF-1 exert differential functions during murine embryogenesis.

Functional dissection of the lck proximal promoter

The lck gene encodes a protein tyrosine kinase that participates in lymphocyte-specific signal transduction pathways. Previous studies have established that lck transcription is regulated by two distinct promoter elements termed proximal (or 3') and distal (or 5'). The proximal promoter is active almost exclusively in thymocytes and becomes inactive later during T-cell maturation. To dissect the mechanisms responsible for lck gene regulation, we generated transgenic animals bearing 5' truncations in the proximal promoter element. Sequences between -584 and +37 with respect to the proximal promoter transcription start site act to direct tissue-specific and temporally correct transcription of either a tagged version of the lck gene itself or a heterologous reporter sequence (lacZ). This region contains binding sites for at least five distinct nuclear proteins, of which one is found only in cells that support proximal lck promoter activity and a second appears only in nonexpressing cells. Interestingly, the transcribed region of the lck gene contains positive control elements that can substantially boost expression from minimal (-130 bp) proximal promoter constructs. These results provide a basis for the biochemical dissection of transcriptional regulators that act at defined points during T-cell development.

Functional dissection of the lck proximal promoter

The lck gene encodes a protein tyrosine kinase that participates in lymphocyte-specific signal transduction pathways. Previous studies have established that lck transcription is regulated by two distinct promoter elements termed proximal (or 3') and distal (or 5'). The proximal promoter is active almost exclusively in thymocytes and becomes inactive later during T-cell maturation. To dissect the mechanisms responsible for lck gene regulation, we generated transgenic animals bearing 5' truncations in the proximal promoter element. Sequences between -584 and +37 with respect to the proximal promoter transcription start site act to direct tissue-specific and temporally correct transcription of either a tagged version of the lck gene itself or a heterologous reporter sequence (lacZ). This region contains binding sites for at least five distinct nuclear proteins, of which one is found only in cells that support proximal lck promoter activity and a second appears only in nonexpressing cells. Interestingly, the transcribed region of the lck gene contains positive control elements that can substantially boost expression from minimal (-130 bp) proximal promoter constructs. These results provide a basis for the biochemical dissection of transcriptional regulators that act at defined points during T-cell development.

A common ancestor of the mammalian transcription factors TCF-1 and TCF-1 alpha/LEF-1 expressed in chicken T cells

Several mammalian T cell-specific transcription factors have been cloned recently. Two of these, TCF-1 and TCF-1 alpha/LEF-1, display a moderate level of overall sequence similarity, and contain virtually identical versions of a novel type of DNA-binding domain, the HMG box. To study evolutionary aspects of the TCF transcription factors in relation to lymphoid differentiation, we have isolated chicken TCF clones from a spleen cDNA library. Low-stringency screening with human probes as well as a polymerase chain reaction-aided strategy resulted in the cloning of a single chicken TCF (chTCF) gene. Sequence comparison revealed that chTCF contained a TCF-1 alpha-like N terminus, and a TCF-1-like C terminus. Furthermore, TCF-1 and TCF-1 alpha were more homologous to chTCF than to each other. We postulate that chTCF is the direct descendant of a single ancestral gene, which has been duplicated in mammals to yield TCF-1 and TCF-1 alpha.

The HMG domain of lymphoid enhancer factor 1 bends DNA and facilitates assembly of functional nucleoprotein structures

The high mobility group (HMG) domain is a DNA-binding motif that is associated with several eukaryotic regulatory proteins, including the lymphoid enhancer-binding factor LEF-1 and the testis-determining factor SRY. Here, we provide evidence that DNA binding by the HMG domain of LEF-1 involves primarily minor groove contacts and induces a bend of approximately 130 degrees in the DNA helix. Bending was also found to accompany sequence-specific DNA binding by the SRY-HMG domain. Examining possible regulatory roles of HMG domain-induced DNA bends, we found that LEF-1 can function in a manner similar to bacterial integration host factor and facilitate communication between widely separated protein-binding sites in a recombination assay. Together with the previous observation that LEF-1 by itself is unable to augment basal promoter activity, these data suggest that HMG domain proteins can serve as "architectural" elements in the assembly of higher-order nucleoprotein structures.

DNA binding activity of recombinant SRY from normal males and XY females

The protein encoded by the human testis determining gene, SRY, contains a high mobility group (HMG) box related to that present in the T cell-specific, DNA-binding protein TCF-1. Recombinant SRY protein was able to bind to the same core sequence AACAAAG recognized by TCF-1 in a sequence dependent manner. In five XY females point mutations were found in the region encoding the HMG box. In four cases DNA binding activity of mutant SRY protein was negligible; in the fifth case DNA binding was reduced. These results imply that the DNA binding activity of SRY is required for sex determination.

Transcriptional regulation during T-cell development: the alpha TCR gene as a molecular model

The regulation of gene expression during lymphocyte differentiation is a complex process involving interactions between multiple positive and negative transcriptional regulatory elements. In this article, transcriptional regulation of the archetypal T-cell-specific gene, alpha TCR, is discussed. Major recent developments, including the identification of novel families of transcription factors that regulate multiple T-cell genes during thymocyte ontogeny and T-cell activation, are described.

An analysis of vertebrate mRNA sequences: intimations of translational control

Five structural features in mRNAs have been found to contribute to the fidelity and efficiency of initiation by eukaryotic ribosomes. Scrutiny of vertebrate cDNA sequences in light of these criteria reveals a set of transcripts--encoding oncoproteins, growth factors, transcription factors, and other regulatory proteins--that seem designed to be translated poorly. Thus, throttling at the level of translation may be a critical component of gene regulation in vertebrates. An alternative interpretation is that some (perhaps many) cDNAs with encumbered 5' noncoding sequences represent mRNA precursors, which would imply extensive regulation at a posttranscriptional step that precedes translation.