Glutamine-rich Activation Domains Research Articles

Oct-2 facilitates functional preinitiation complex assembly and is continuously required at the promoter for multiple rounds of transcription.

Octamer factor 2 (Oct-2, OTF-2, NF-A2) is an 'upstream' promoter factor that binds to the octamer motif (ATGCAAAT) implicated in control of immunoglobulin gene transcription in B-lymphocytes. We have studied the role of Oct-2 in the process of transcription initiation in vitro using both nuclear extracts and purified basal transcription factors. Oct-2 specifically stimulates transcription from octamer-containing promoters in both systems. Thus, Oct-2 is a 'true activator', rather than merely an 'anti-repressor' counteracting the effect of histones. In order-of-addition experiments, Oct-2 is required early, together with TFIID, to allow formation of a preinitiation complex. Oct-2 cannot functionally interact with cloned TATA binding protein (TBP) but rather requires 'coactivators' found in the TFIID fraction. In single-round transcription experiments, early competition for Oct-2 by an octamer oligonucleotide is deleterious, but no effect is seen after assembly of a complete preinitiation complex. However, for multiple rounds of transcription, Oct-2 is continuously required at the promoter; this result argues against a 'hit-and-run' mechanism whereby the activator becomes dispensible after organizing a TFIID-promoter complex. In agreement with our previous studies in vivo, the N-terminal glutamine-rich activation domain of Oct-2 is required for full activity in vitro, indicating that this domain directly interacts with basal transcription factors.

Molecular cloning and functional analysis of Drosophila TAF110 reveal properties expected of coactivators

The general transcription factor TFIID is a multiprotein complex containing the TATA-binding protein and several associated factors (TAFs), some of which may function as coactivators that are essential for activated, but not basal, transcription. Here we describe the isolation and characterization of the first gene encoding a TAF protein. The deduced amino acid sequence of TAF110 revealed the presence of several glutamine- and serine/threonine-rich regions reminiscent of the protein-protein interaction domains of the regulatory transcription factor Sp1 that are involved in transcription activation and multimerization. In both Drosophila cells and yeast, TAF110 specifically interacts with the glutamine-rich activation domains of Sp1. Moreover, purified Sp1 selectively binds recombinant TAF110 in vitro. These findings taken together suggest that TAF110 may function as a coactivator by serving as a site of protein-protein contact between activators like Sp1 and the TFIID complex.

Monoclonal antibodies to NF-Y define its function in MHC class II and albumin gene transcription.

NF-Y is a sequence-specific DNA-binding protein which, as a heterodimer, recognizes CCAAT motifs in a variety of transcriptional promoters. We have generated a panel of monoclonal and affinity-purified polyclonal antibodies directed against various epitopes of NF-Y. These reagents are highly specific for either of the A or B subunits; we have mapped the epitopes recognized by the monoclonal antibodies to the glutamine-rich activation domain of NF-YA. The antibodies inhibit in vitro transcription from the promoters of the albumin gene and of Ea, a class II gene of the major histocompatibility complex. These data definitively demonstrate the role of NF-Y in regulating the transcription of two tissue-specific genes whose expression patterns do not overlap. Interestingly, the antibodies cannot inhibit a formed pre-initiation complex, but do block reinitiation of subsequent rounds of transcription from the same templates.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity.

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity.

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

Intron-exon organization of the NF-Y genes. Tissue-specific splicing modifies an activation domain.

NF-Y is a highly conserved transcription factor which recognizes CCAAT motifs in a variety of genes. We report here the genomic organization of the genes encoding both subunits, which gives interesting clues about the functional organization of the proteins. We also report the existence of isoforms of NF-YA which result from differential splicing. These alternative splicing events map within a glutamine-rich activation domain and show a marked cell- and tissue-specific bias.

Different activation domains of Sp1 govern formation of multimers and mediate transcriptional synergism.

The process of transcriptional activation in eukaryotes by site-specific DNA-binding proteins is a key step in gene regulation. Here we have examined the properties of four distinct activator domains of the human transcription factor Sp1. In vivo transient cotransfection assays with Sp1 show that templates bearing multiple Sp1 sites activate transcription with a high degree of synergism. However, there is no evidence of cooperative binding of Sp1 to adjacent sites. Using deletion mutants of Sp1 we have determined that the glutamine-rich activation domains A and B and the previously uncharacterized carboxy-terminal domain D are all required for Sp1 to activate transcription synergistically. Gel-shift, DNase footprinting, and chemical cross-linking experiments reveal a strong correlation between the ability of Sp1 mutants to form homomultimeric complexes and their ability to activate transcription synergistically when bound to multiple sites. We have also examined the process of superactivation, in which a molecule of Sp1 tethered to DNA via its zinc fingers can be transcriptionally enhanced by interacting directly with fingerless Sp1 molecules. The domains involved in superactivation appear to be a subset of those necessary to achieve synergistic activation. These findings suggest that different domains of Sp1 carry out distinct functions and that the formation of multimeric complexes may direct synergism and superactivation.

Synergistic activation by the glutamine-rich domains of human transcription factor Sp1

We have examined the role of protein-protein interactions in modulating the activity of Sp1, a human transcription factor that utilizes glutamine-rich activation domains. These domains may represent a commonly used structural motif, since a nonhomologous glutamine-rich segment from the Drosophila Antennapedia protein is also a potent activator when fused to the Sp1 DNA binding domain. Sp1 is generally considered a proximal promoter factor that can only stimulate transcription when bound close to the initiation site. However, here we present evidence that distally and proximally bound Sp1 can stimulate transcription synergistically. In addition, a DNA binding-deficient mutant of Sp1 that retains glutamine-rich domains can interact with proximally bound Sp1 to superactivate transcription. Glutaraldehyde cross-linking provides direct evidence for an interaction between Sp1 monomers. Thus, Sp1-Sp1 interactions may play an important role in modulating promoter activity.