TBP-like protein (TLP/TLF/TRF2) artificially recruited to a promoter stimulates basal transcription in vivo

Participation of the TBP-associated factors (TAFs) in cell differentiation

The understanding of the mechanisms that regulate gene expression to establish differentiation programs and determine cell lineages, is one of the major challenges in Developmental Biology. Besides the participation of tissue-specific transcription factors and epigenetic processes, the role of general transcription factors has been ignored. Only in recent years, there have been scarce studies that address this issue. Here, we review the studies on the biological activity of some TATA-box binding protein (TBP)-associated factors (TAFs) during the proliferation of stem/progenitor cells and their involvement in cell differentiation. Particularly, the accumulated evidence suggests that TAF4, TAF4b, TAF7L, TAF8, TAF9, and TAF10, among others, participate in nervous system development, adipogenesis, myogenesis, and epidermal differentiation; while TAF1, TAF7, TAF15 may be involved in the regulation of stem cell proliferative abilities and cell cycle progression. On the other hand, evidence suggests that TBP variants such as TBPL1 and TBPL2 might be regulating some developmental processes such as germ cell maturation and differentiation, myogenesis, or ventral specification during development. Our analysis shows that it is necessary to study in greater depth the biological function of these factors and its participation in the assembly of specific transcription complexes that contribute to the differential gene expression that gives rise to the great diversity of cell types existing in an organism. The understanding of TAFs' regulation might lead to the development of new therapies for patients which suffer from mutations, alterations, and dysregulation of these essential elements of the transcriptional machinery.

TBP-like protein (TLP) represses myogenesis via inhibition of the myogenin promoter

TBP-like protein (TLP) is one of the metazoan-restricted transcription factors participating in development and differentiation, though the molecular mechanism by which TLP regulates these processes remains unclear. In this study, we investigated the relationship between TLP and myogenesis of mouse C2C12 myoblasts. We found that TLP gene expression decreases during myogenic differentiation. Overexpression and knockdown of TLP revealed that the levels of muscle-specific myosin heavy chain and the myogenic transcription factor myogenin are downregulated by TLP. TLP inhibits the progression of morphological change from myoblasts to myotubes, thereby suppressing myogenesis. We further show that TLP represses the promoter activity of myogenin. The proximal AT-rich sequence of the myogenin promoter is responsible for TLP-mediated transcriptional repression. The results of this study suggest that TLP inhibits myogenesis through downregulation of the myogenin gene.

Interaction between transactivation domain of p53 and middle part of TBP-like protein (TLP) is involved in TLP-stimulated and p53-activated transcription from the p21 upstream promoter

TBP-like protein (TLP) is involved in transcriptional activation of an upstream promoter of the human p21 gene. TLP binds to p53 and facilitates p53-activated transcription from the upstream promoter. In this study, we clarified that in vitro affinity between TLP and p53 is about one-third of that between TBP and p53. Extensive mutation analyses revealed that the TLP-stimulated function resides in transcription activating domain 1 (TAD1) in the N-terminus of p53. Among the mutants, #22.23, which has two amino acid substitutions in TAD1, exhibited a typical mutant phenotype. Moreover, #22.23 exhibited the strongest mutant phenotype for TLP-binding ability. It is thus thought that TLP-stimulated and p53-dependent transcriptional activation is involved in TAD1 binding of TLP. #22.23 had a decreased transcriptional activation function, especially for the upstream promoter of the endogenous p21 gene, compared with wild-type p53. This mutant did not facilitate p53-dependent growth repression and etoposide-mediated cell-death as wild-type p53 does. Moreover, mutation analysis revealed that middle part of TLP, which is requited for p53 binding, is involved in TLP-stimulated and p53-dependent promoter activation and cell growth repression. These results suggest that activation of the p21 upstream promoter is mediated by interaction between specific regions of TLP and p53.

TATA-binding protein (TBP)-like protein is required for p53-dependent transcriptional activation of upstream promoter of p21Waf1/Cip1 gene

TATA-binding protein-like protein (TLP) is involved in development, checkpoint, and apoptosis through potentiation of gene expression. TLP-overexpressing human cells, especially p53-containing cells, exhibited a decreased growth rate and increased proportion of G(1) phase cells. TLP stimulated expression of several growth-related genes including p21 (p21(Waf1/Cip1)). TLP-mediated activation of the p21 upstream promoter in cells was shown by a promoter-luciferase reporter assay. The p53-binding sequence located in the p21 upstream promoter and p53 itself are required for TLP-mediated transcriptional activation. TLP and p53 bound to each other and synergistically enhanced activity of the upstream promoter. TLP specifically activated transcription from the endogenous upstream promoter, and p53 was required for this activation. Etoposide treatment also resulted in activation of the upstream promoter as well as nuclear accumulation of TLP and p53. Moreover, the upstream promoter was associated with endogenous p53 and TLP, and the p53 recruitment was enhanced by TLP. The results of the present study suggest that TLP mediates p53-governed transcriptional activation of the p21 upstream promoter.

Variations in intracellular levels of TATA binding protein can affect specific genes by different mechanisms

We previously showed that reduced intracellular levels of the TATA binding protein (TBP), brought about by tbp heterozygosity in DT40 cells, resulted in a mitotic delay reflecting reduced expression of the mitotic regulator cdc25B but did not significantly affect overall transcription. Here we extend these findings in several ways. We first provide evidence that the decrease in cdc25B expression reflects reduced activity of the cdc25B core promoter in the heterozygous (TBP-het) cells. Strikingly, mutations in a previously described repressor element that overlaps the TATA box restored promoter activity in TBP-het cells, supporting the idea that the sensitivity of this promoter to TBP levels reflects a competition between TBP and the repressor for DNA binding. To determine whether cells might have mechanisms to compensate for fluctuations in TBP levels, we next examined expression of the two known vertebrate TBP homologues, TLP and TBP2. Significantly, mRNAs encoding both were significantly overexpressed relative to levels observed in wild-type cells. In the case of TLP, this was shown to reflect regulation of the core promoter by both TBP and TLP. Together, our results indicate that variations in TBP levels can affect the transcription of specific promoters in distinct ways, but overall transcription may be buffered by corresponding alterations in the expression of TBP homologues.

Two isoforms of Drosophila TRF2 are involved in embryonic development, premeiotic chromatin condensation, and proper differentiation of germ cells of both sexes

The Drosophila TATA box-binding protein (TBP)-related factor 2 (TRF2 or TLF) was shown to control a subset of genes different from that controlled by TBP. Here, we have investigated the structure and functions of the trf2 gene. We demonstrate that it encodes two protein isoforms: the previously described 75-kDa TRF2 and a newly identified 175-kDa version in which the same sequence is preceded by a long N-terminal domain with coiled-coil motifs. Chromatography of Drosophila embryo extracts revealed that the long TRF2 is part of a multiprotein complex also containing ISWI. Both TRF2 forms are detected at the same sites on polytene chromosomes and have the same expression patterns, suggesting that they fulfill similar functions. A study of the manifestations of the trf2 mutation suggests an essential role of TRF2 during embryonic Drosophila development. The trf2 gene is strongly expressed in germ line cells of adult flies. High levels of TRF2 are found in nuclei of primary spermatocytes and trophocytes with intense transcription. In ovaries, TRF2 is present both in actively transcribing nurse cells and in the transcriptionally inactive oocyte nuclei. Moreover, TRF2 is essential for premeiotic chromatin condensation and proper differentiation of germ cells of both sexes.

Transcriptional repression of the mouse wee1 gene by TBP-related factor 2

TBP-related factor 2 (TRF2), one of the TBP family proteins, is involved in various cellular functions through its transcription stimulation activity. We previously reported that TRF2 is involved in reduction of wee1 mRNA in genotoxin-treated chicken cells. In this study, we investigated the role of TRF2 in wee1 gene expression. It was found that wee1 mRNA was decreased in hydroxyurea-treated NIH3T3 cells. Mouse wee1 promoter activity was repressed by TRF2 in mouse and chicken cells. Chromatin immunoprecipitation and plasmid immunoprecipitation analyses revealed that TRF2 is recruited to the wee1 promoter in accordance with the transcriptional repression. A mutant TRF2 that lacks TFIIA-binding capacity lost its repressive function. This mutant was less recruited to the wee1 promoter than was the wild-type one, and provided a decline in promoter-recruited TFIIA. Data in this study suggest that transcription repressive activity of TRF2 to wee1 promoter needs association with the promoter and TFIIA.

Cell-specific nucleolar localization of TBP-related factor 2

TATA-binding protein (TBP)-related factor 2 (TRF2) is one of four closely related RNA polymerase II transcription factors. We compared the intracellular localizations of TBP and TRF2 during the cell cycle and mitosis in HeLa cells. We show that during interphase, endogenous or exogenously expressed TRF2 is located almost exclusively in the nucleolus in HeLa or Cos cells. TRF2 localization is not affected by stress or mitotic stimuli, but TRF2 is rapidly released from the nucleolus upon inhibition of pol I transcription or treatment by RNase. These results suggest that localization of HeLa TRF2 requires a nucleolar-associated RNA species. In contrast, in 3T3 fibroblast cells, exogenously expressed TRF2 localizes to the nucleoplasm. Constitutive expression of ectopic TRF2 in 3T3 cells leads to a prolonged S phase of the cell cycle and reduced proliferation. Together with previous data, our results highlight the cell-specific localization and functions of TRF2. Furthermore, we show that during cell division, HeLa TRF2 and TBP are localized in the mitotic cytoplasm and TRF2 relocalizes into the nascent nucleoli immediately after mitosis, whereas TBP reassociates with the chromatin. Although partially contradictory results have been reported, our data are consistent with a model where only small proportion of the cellular TBP remains associated with specific promoter loci during mitosis.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

Specific interaction with transcription factor IIA and localization of the mammalian TATA-binding protein-like protein (TLP/TRF2/TLF)

TBP-like protein (TLP) is structurally similar to the TATA-binding protein (TBP) and is thought to have a transcriptional regulation function. Although TLP has been found to form a complex with transcription factor IIA (TFIIA), the in vivo functions of TFIIA for TLP are not clear. In this study, we analyzed the interaction between TLP and TFIIA. We determined the biophysical properties for the interaction of TLP with TFIIA. Dissociation constants of TFIIA versus TLP and TFIIA versus TBP were 1.5 and 10 nm, respectively. Moreover, the dissociation rate constant of TLP and TFIIA (1.2 x 10(-4)/m.s was significantly lower than that of TBP (2.1 x 10(-3)/m.s). These results indicate that TLP has a higher affinity to TFIIA than does TBP and that the TLP-TFIIA complex is much more stable than is the TBP-TFIIA complex. We found that TLP forms a dimer and a trimer and that these multimerizations are inhibited by TFIIA. Moreover, TLP mutimers were more stable than a TBP dimer. We determined the amounts of TLPs in the nucleus and cytoplasm of NIH3T3 cells and found that the molecular number of TLP in the nucleus was only 4% of that in the cytoplasm. Immunostaining of cells also revealed cytoplasmic localization of TLP. We established cells that stably express mutant TLP lacking TFIIA binding ability and identified the amino acids of TLP required for TFIIA binding (Ala-32, Leu-33, Asn-37, Arg-52, Lys-53, Lys-78, and Arg-86). Interestingly, the level of TFIIA binding defective mutant TLPs in the nucleus was much higher than that of the wild-type TLP and TFIIA-interactable mutant TLPs. Immunostaining analyses showed consistent results. These results suggest that the TFIIA binding ability of TLP is required for characteristic cytoplasmic localization of TLP. TFIIA may regulate the intracellular molecular state and the function of TLP through its property of binding to TLP.

TATA-binding protein-like protein (TLP/TRF2/TLF) negatively regulates cell cycle progression and is required for the stress-mediated G(2) checkpoint

The TATA-binding protein (TBP) is a universal transcription factor required for all of the eukaryotic RNA polymerases. In addition to TBP, metazoans commonly express a distantly TBP-related protein referred to as TBP-like protein (TLP/TRF2/TLF). Although the function of TLP in transcriptional regulation is not clear, it is known that TLP is required for embryogenesis and spermiogenesis. In the present study, we investigated the cellular functions of TLP by using TLP knockout chicken DT40 cells. TLP was found to be dispensable for cell growth. Unexpectedly, TLP-null cells exhibited a 20% elevated cell cycle progression rate that was attributed to shortening of the G(2) phase. This indicates that TLP functions as a negative regulator of cell growth. Moreover, we found that TLP mainly existed in the cytoplasm and was translocated to the nucleus restrictedly at the G(2) phase. Ectopic expression of nuclear localization signal-carrying TLP resulted in an increase (1.5-fold) in the proportion of cells remaining in the G(2)/M phase and apoptotic state. Notably, TLP-null cells showed an insufficient G(2) checkpoint when the cells were exposed to stresses such as UV light and methyl methanesulfonate, and the population of apoptotic cells after stresses decreased to 40%. These phenomena in G(2) checkpoint regulation are suggested to be p53 independent because p53 does not function in DT40 cells. Moreover, TLP was transiently translocated to the nucleus shortly (15 min) after stress treatment. The expression of several stress response and cell cycle regulatory genes drifted in a both TLP- and stress-dependent manner. Nucleus-translocating TLP is therefore thought to work by checking cell integrity through its transcription regulatory ability. TLP is considered to be a signal-transducing transcription factor in cell cycle regulation and stress response.

Vertebrate TBP-like protein (TLP/TRF2/TLF) stimulates TATA-less terminal deoxynucleotidyl transferase promoters in a transient reporter assay, and TFIIA-binding capacity of TLP is required for this function

The TBP-like protein (TLP/TRF2/TLF), which belongs to the TBP family of proteins, is present in all metazoan organisms. Although the human TLP has been reported to interfere with transcription from TATA-containing promoters, the transcription activation potential of TLP in higher animals is obscure. We previously demonstrated that artificially promoter-recruited TLP behaves like an unconventional transcriptional activator. In this study, we investigated the effects of TLP on TATA-less promoters of mouse and human terminal deoxynucleotidyl transferase (TdT) genes by transient reporter assays. As expected, TLP repressed both basal and activator-augmented transcription from the TATA-containing adenovirus major late promoter (MLP) and E1B promoter. On the other hand, however, TLP significantly stimulated both basal and activated transcription from TdT promoters. We investigated the strength of the promoters in chicken DT40 cells that lack the TLP gene. The MLP showed higher activity but the TdT promoter showed lower activity in TLP-null cells than in the wild-type cells. Moreover, ectopic expression of mouse TLP in the TLP-null cells considerably stimulated the TdT promoter. Insertion of a TATA element upstream from the TdT core promoter resulted in a loss of TLP-mediated activation. The mouse TLP was demonstrated to bind specifically to TFIIA with greater strength than TBP. We constructed mutated TLPs having amino acid substitutions that impair TFIIA binding. A representative TLP mutant lacking TFIIA-binding ability could not stimulate transcription from the TdT promoter, whereas that mutation suppressed TLP-mediated transcription repression of TATA promoters. The results of the present study suggest that the vertebrate TLP potentiates exogenous TATA-less promoters and that TFIIA plays an important role in the TLP function.

Distinct functions of TBP and TLF/TRF2 during spermatogenesis: requirement of TLF for heterochromatic chromocenter formation in haploid round spermatids

TLF (TBP-like factor) is a protein commonly thought to belong to the general transcription initiation complex. TLF is evolutionarily conserved and has been shown to be essential for early development in C. elegans, zebrafish and Xenopus. In mammals however, TLF has a specialised function, as revealed by targeted mutation of the gene in the mouse germline. The TLF mutation elicits a complete arrest of late spermiogenesis and increased haploid cell apoptosis. We explored in more detail the molecular function that TLF plays in the differentiation program of male germ cells. A comparison of TBP and TLF reveals drastic differences, both in their temporal expression pattern and in their intracellular location. While TBP is ubiquitously expressed, TLF expression is strictly developmentally regulated, being very high in late pachytene spermatocytes, suggesting a function prior to the apoptosis of the haploid cells. A refined study of TLF-deficient mice reveals defective acrosome formation in early stage spermatids. Most importantly, our results uncover an unsuspected function of TLF in chromatin organisation. Indeed, early spermatids in TLF-deficient mice display a fragmentation of the chromocenter, a condensed structure formed by the association of centromeric heterochromatin and containing the HP1 proteins. This defect is likely to be the primary cause of spermatogenic failure in the TLF mutant mice.