Redundant role of tissue-selective TAF(II)105 in B lymphocytes

RNA polymerases in plasma cells trav-ELL2 the beat of a different drum

There is a major transformation in gene expression between mature B cells (including follicular, marginal zone, and germinal center cells) and antibody secreting cells (ASCs), i.e., ASCs, (including plasma blasts, splenic plasma cells, and long-lived bone marrow plasma cells). This significant change-over occurs to accommodate the massive amount of secretory-specific immunoglobulin that ASCs make and the export processes itself. It is well known that there is an up-regulation of a small number of ASC-specific transcription factors Prdm1 (B-lymphocyte-induced maturation protein 1), interferon regulatory factor 4, and Xbp1, and the reciprocal down-regulation of Pax5, Bcl6 and Bach2, which maintain the B cell program. Less well appreciated are the major alterations in transcription elongation and RNA processing occurring between B cells and ASCs. The three ELL family members ELL1, 2 and 3 have different protein sequences and potentially distinct cellular roles in transcription elongation. ELL1 is involved in DNA repair and small RNAs while ELL3 was previously described as either testis or stem-cell specific. After B cell stimulation to ASCs, ELL3 levels fall precipitously while ELL1 falls off slightly. ELL2 is induced at least 10-fold in ASCs relative to B cells. All of these changes cause the RNA Polymerase II in ASCs to acquire different properties, leading to differences in RNA processing and histone modifications.

Shifting players and paradigms in cell-specific transcription

Historically, developmental-stage- and tissue-specific patterns of gene expression were assumed to be determined primarily by DNA regulatory sequences and their associated activators, while the general transcription machinery including core promoter recognition complexes, coactivators, and chromatin modifiers was held to be invariant. New evidence suggests that significant changes in these general transcription factors including TFIID, BAF, and Mediator may facilitate global changes in cell-type-specific transcription.

Molecular evolution of the testis TAFs of Drosophila

The basal transcription machinery is responsible for initiating transcription at core promoters. During metazoan evolution, its components have expanded in number and diversified to increase the complexity of transcriptional regulation in tissues and developmental stages. To explore the evolutionary events and forces underlying this diversification, we analyzed the evolution of the Drosophila testis TAFs (TBP-associated factors), paralogs of TAFs from the basal transcription factor TFIID that are essential for normal transcription during spermatogenesis of a large set of specific genes involved in terminal differentiation of male gametes. There are five testis-specific TAFs in Drosophila, each expressed only in primary spermatocytes and each a paralog of a different generally expressed TFIID subunit. An examination of the presence of paralogs across taxa as well as molecular clock dating indicates that all five testis TAFs likely arose within a span of approximately 38 My 63-250 Ma by independent duplication events from their generally expressed paralogs. Furthermore, the evolution of the testis TAFs has been rapid, with apparent further accelerations in multiple Drosophila lineages. Analysis of between-species divergence and intraspecies polymorphism indicates that the major forces of evolution on these genes have been reduced purifying selection, pervasive positive selection, and coevolution. Other genes that exhibit similar patterns of evolution in the Drosophila lineages are also characterized by enriched expression in the testis, suggesting that the pervasive positive selection acting on the tTAFs is likely to be related to their expression in the testis.

MYBBP1a is a Novel Repressor of NF-κB

NF-kappaB is an inducible transcription factor activated in many different cell types by inflammatory and stress signals. The transcription of a wide variety of NF-kappaB genes is regulated by the coordinated action of transcription co-activators and co-repressors. Previously we identified Myb binding protein 1a (MYBBP1a) as an interaction partner of the transcription activation domain of RelA/p65. MYBBP1a has been shown by others to regulate various transcription factors, through largely unknown mechanisms. Here we present evidence that MYBBP1a is a novel co-repressor of NF-kappaB. MYBBP1a interacted directly with RelA/p65 and expression of MYBBP1a in cells repressed NF-kappaB dependent reporter expression but did affect neither RelA/p65 nuclear translocation nor its DNA binding activity. In vitro, MYBBP1a inhibited transcription from chromatinized templates at a step before pre-initiation complex formation. MYBBP1a was found to compete with the histone acetyl transferase co-activator, p300, for interaction with the transcription activation domain of RelA/p65. Expression levels of MYBBP1a are dependent on the cell type, and are particularly high in Jurkat T cells. These results indicate that MYBBP1a is a novel NF-kappaB co-repressor of transcription that competes with p300 and may function to regulate cell type specific genes.

The TATA-box sequence in the basal promoter contributes to determining light-dependent gene expression in plants

A prototype 13-bp TATA-box sequence, TCACTATATATAG, was mutated at each nucleotide position and examined for its function in the core promoter. Specific nucleotides in the first TATA, the second TATA, as well as the flanking sequences influenced promoter function in transient transformation of tobacco (Nicotiana tabacum var Petit Havana) leaves. The effect of a given mutation on reporter gene expression in light versus dark was variable and sometimes contrasting. Some mutations, like T(7) or A(8)-->C or G, completely inactivated the expression of the minimal promoter in light but not in dark. In general, the sequence requirement for dark expression was less stringent than that for light expression. The selective effect of TATA-box mutations on light versus dark expression was exerted on core promoter function in the chromatin-integrated state also. Even in the presence of an upstream light response activator element, TATA-box mutations influenced modulation of the promoter by light. An A at the eighth position was specifically involved in the red light response of the promoter. Selectivity in gene expression was associated with a high level of transcript initiation from a site that was not active in the dark. Nuclear proteins from dark- and light-grown seedlings showed that the sequence variation within the TATA-box governs the formation of alternative transcriptional complexes. The experiments give direct evidence for the role of a core TATA-box sequence in determining the level as well as selectivity of gene expression in plants.

Cyclic AMP-dependent modification of gonad-selective TAF(II)105 in a human ovarian granulosa cell line

In response to gonadotropins, the elevated level of intracellular-cyclic AMP (cAMP) in ovarian granulosa cells triggers an ordered activation of multiple ovarian genes, which in turn promotes various ovarian functions including folliculogenesis and steroidogenesis. Identification and characterization of transcription factors that control ovarian gene expression are pivotal to the understanding of the molecular basis of the tissue-specific gene regulation programs. The recent discovery of the mouse TATA binding protein (TBP)-associated factor 105 (TAF(II)105) as a gonad-selective transcriptional co-activator strongly suggests that general transcription factors such as TFIID may play a key role in regulating tissue-specific gene expression. Here we show that the human TAF(II)105 protein is preferentially expressed in ovarian granulosa cells. We also identified a novel TAF(II)105 mRNA isoform that results from alternative exon inclusion and is predicted to encode a dominant negative mutant of TAF(II)105. Following stimulation by the adenylyl cyclase activator forskolin, TAF(II)105 in granulosa cells undergoes rapid and transient phosphorylation that is dependent upon protein kinase A (PKA). Thus, our work suggests that pre-mRNA processing and post-translational modification represent two important regulatory steps for the gonad-specific functions of human TAF(II)105.

TAF4b, a TBP associated factor, is required for oocyte development and function

Development of a fertilizable oocyte is a complex process that relies on the precise temporal and spatial expression of specific genes in germ cells and in surrounding somatic cells. Since female mice null for Taf4b, a TBP associated factor, are sterile, we sought to determine when during follicular development this phenotype was first observed. At postnatal day 3, ovaries of Taf4b null females contained fewer (P < 0.01) oocytes than ovaries of wild type and heterozygous Taf4b mice. However, expression of only one somatic cell marker Foxl2 was reduced in ovaries at day 15. Despite the reduced number of follicles, many proceed to the antral stage, multiple genes associated with granulosa cell differentiation and oocyte maturation were expressed in a normal pattern, and immature Taf4b null females could be hormonally primed to ovulate and mate. However, the ovulated cumulus oocyte complexes from the Taf4b null mice had fewer (P < 0.01) cumulus cells, and the oocytes were functionally abnormal. GVBD and polar body extrusion were reduced significantly (P < 0.01). The few oocytes that were fertilized failed to progress beyond the two-cell stage of development. Thus, infertility in Taf4b null female mice is associated with defects in early follicle formation, oocyte maturation, and zygotic cleavage following ovulation and fertilization.

Follicle-stimulating hormone induction of ovarian insulin-like growth factor-binding protein-3 transcription requires a TATA box-binding protein and the protein kinase A and phosphatidylinositol-3 kinase pathways

The current study was done to elucidate the mechanism of the FSH stimulation of IGF-binding protein 3 (IGFBP-3) expression and map the FSH response element on the pig IGFBP-3 promoter. Forskolin induced IGFBP-3 reporter activity in transiently transfected granulosa cells. The protein kinase A (PKA) inhibitor [N-[2-(p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, 2HCl] (and cotransfection with a PKA inhibitor expression vector), the phosphatidylinositol-3 kinase inhibitor [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one], and the ERK inhibitor [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene], all blocked FSH stimulation. Use of serial deletion constructs and site-directed mutagenesis show that a TATA box-binding protein site is required for FSH stimulation and that a specific protein 1 (Sp1) site is required for basal transcription. Gel shift assays of nuclear protein with a -61/-25 probe detected four protein-DNA complexes, with bands I and II having significantly higher intensities in FSH-treated cells than in controls. Mutation of the Sp1 site prevented formation of bands I and II whereas mutation of the TATA box-binding protein site prevented formation of band IV. Use of specific antibodies showed that Sp1 participates in formation of band I, Sp3 band II, and p300 in both I and II. Band III was nonspecifically competed out. We conclude that FSH stimulation of IGFBP-3 transcription is mediated by cAMP via the PKA pathway and requires the P1-3 kinase and likely the MAPK pathways.