The Oct-1 homoeodomain directs formation of a multiprotein-DNA complex with the HSV transactivator VP16

Direct and selective binding of an acidic transcriptional activation domain to the TATA-box factor TFIID

The potent transactivation domain of the herpes simplex virion protein VP16 was used as a column ligand for affinity chromatography. VP16 binds strongly and highly selectively to the human and yeast TATA box-binding factors. Our results imply that the principal target for acidic activation domains is the TATA-box factor TFIID.

The POU-specific domain of Pit-1 is essential for sequence-specific, high affinity DNA binding and DNA-dependent Pit-1-Pit-1 interactions

Pit-1 is a member of a family of transcription factors sharing two regions of homology: a highly conserved POU-specific (POUS) domain and a more divergent homeodomain (POUHD). Analysis of mutant Pit-1 proteins suggests that, while the POUHD is required and sufficient for low affinity DNA binding, the POUS domain is necessary for high affinity binding and accurate recognition of natural Pit-1 response elements. Pit-1 is monomeric in solution but associates as a dimer on its DNA response element, exhibiting DNA-dependent protein-protein interactions requiring the POUS domain. Analysis of alpha-helical domains and conserved structures in Pit-1 suggests that POU domain proteins interact with their DNA recognition sites differently than classic homeodomain proteins, with both the POUHD and the POUS domain contacting DNA. Transcriptional activity of Pit-1 on enhancer elements is conferred primarily by a Ser- and Thr-rich N-terminal region unrelated to other known transcription-activating motifs.

Assembly of alternative multiprotein complexes directs rRNA promoter selectivity

How can trans-activators with the same DNA binding specificity direct different transcriptional programs? The rRNA transcriptional apparatus offers a useful model system to address this question and to dissect the mechanisms that generate alternative transcription complexes. Here, we compare the mouse and human transcription factors that govern species-specific RNA polymerase I promoter recognition. We find that both human and mouse rRNA transcription is mediated by a specific multiprotein complex. One component of this complex is the DNA-binding transcription factor, UBF. Paradoxically, human and mouse UBF display identical DNA binding specificities even though transcription of rRNA is species specific. Promoter selectivity is conferred by a second essential factor, SL1, which, for humans, does not bind DNA independently but, instead, cooperates with UBF in the formation of high-affinity DNA-binding complexes. In contrast, mouse SL1 can selectively interact with DNA in the absence of UBF. Reconstituted transcription experiments establish that UBF and RNA polymerase I from the two species are functionally interchangeable, whereas mouse and human SL1 exhibit distinct DNA binding and transcription activities. Together, these results suggest a critical role for a specific multiprotein assembly in RNA polymerase I promoter recognition and reveal distinct mechanisms through which such complexes can generate functional diversity.

Adenovirus early region 3 promoter regulation by E1A/E1B is independent of alterations in DNA binding and gene activation of CREB/ATF and AP1

Transcription of the adenovirus early region 3 promoter is strongly induced by the adenovirus E1A protein. Previous DNase I footprinting has indicated that four regions in this promoter serve as binding sites for HeLa nuclear proteins. These include binding sites for NF-1 (site IV), AP1 (site III), CREB/activating transcription factor (ATF) (site II), and TATA (site I). To determine the relative importance of these sites in both the in vivo and in vitro transcriptional regulation of the E3 promoter, oligonucleotide-directed mutagenesis of these sites was performed. Each of these constructs was assayed by transfection onto HeLa cells in the presence of either dl434, an E1A/E1B deletion mutant, or wild-type adenovirus. Mutations of either the ATF- or AP1-binding sites but not the TATA- and NF1-binding sites resulted in severe decreases in both basal and E1A/E1B-induced transcriptional levels. These constructs were also assayed in in vitro transcription assays with cellular extracts prepared from dl434-infected or wild-type-adenovirus-infected HeLa cells. The wild-type E3 promoter was transcribed approximately 30 times more efficiently in extracts containing the E1A/E1B proteins compared with extracts lacking these proteins. Mutations of either the TATA element, the ATF site, or the AP1-binding site decreased both basal and E1A/E1B-induced transcriptional levels. Gel retardation analysis using these extracts indicated that the binding to ATF, AP1, or NF1 oligonucleotides was not altered in the presence of the E1A/E1B proteins compared with extracts lacking these proteins. Northern (RNA) blot analysis of c-jun and CREB RNA prepared from wild-type adenovirus and dl434-infected cells indicated that the levels of these RNAs were not altered by the E1A/E1B proteins. Immunoprecipitation of AP1 and CREB from both dl434- and wild-type-adenovirus-infected cells indicated that the amounts of these proteins were not significantly altered. These results suggest that E1A/E1B-induced activation of the E3 promoter does not involve activation of transcription factor genes nor a change in the DNA binding activity of important promoter-binding components. Our results are consistent with a model in which the E1A/E1B proteins either directly or indirectly alter the interactions of factors that bind to the basal E3 promoter transcription complex, thereby inducing transcription.

A DNA-binding homeodomain in histone H1

The structure of the globular domain of chicken histone H1 was compared here with that of the DNA-binding homeodomain in the Drosophila Antp protein, and they were observed to display considerable similarity. Both of them consist of three or four alpha-helices separated by well-defined turns. Charged residues in the aminoterminal end of alpha 3 are therefore suggested to be responsible for sequence-specific recognition of DNA by the histone. In addition, alpha 2 of H1, with a short leucine zipper in it, may be capable of protein-protein interaction in a similar manner to the other homeodomains.

Interference with the assembly of a virus-host transcription complex by peptide competition

Induction of transcription of the immediate-early (IE) genes of herpes simplex virus (HSV) involves the assembly of a DNA-binding complex containing the cellular transcription factor Oct-1 and the virus regulatory protein Vmw65 (VP16). Complex assembly can be observed using deletion variants of Vmw65 which lack the acidic C-terminal activation domain and are therefore defective for IE transactivation. Similar variants of Vmw65 interfere with IE activation by the normal protein, and with HSV replication. It has therefore been suggested that dominant interfering products of viruses such as HSV and HIV could be used in a form of intracellular immunization against virus infection. Here we report that a small peptide overlapping a region of Vmw65 which is critical for complex assembly specifically inhibits assembly of the complex but has no observed effect on the DNA-binding activity of the cellular factor alone. Selective interference with the assembly of transcription complexes by short peptides corresponding to functionally critical regions of virus regulatory proteins may be more feasible than the use of defective polypeptides as an antiviral strategy based on competitive interference.

The MyoD DNA binding domain contains a recognition code for muscle-specific gene activation

A 60 amino acid domain of the myogenic determination gene MyoD is necessary and sufficient for sequence-specific DNA binding in vitro and myogenic conversion of transfected C3H10T1/2 cells. We show that a highly basic region, immediately upstream of the helix-loop-helix (HLH) oligomerization motif, is required for MyoD DNA binding in vitro. Replacing helix1, helix2, or the loop of MyoD with the analogous sequence of the Drosophila T4 achaete-scute protein (required for peripheral neurogenesis) has no substantial effect on DNA binding in vitro or muscle-specific gene activation in transfected C3H10T1/2 cells. However, replacing the basic region of MyoD with the analogous sequence of other HLH proteins (the immunoglobulin enhancer binding E12 protein or T4 achaete scute protein) allows DNA binding in vitro, yet abolishes muscle-specific gene activation. These findings suggest that a recognition code that determines muscle-specific gene activation lies within the MyoD basic region and that the capacity for specific DNA binding is insufficient to activate the muscle program.

Differential transcriptional activation by Oct-1 and Oct-2: interdependent activation domains induce Oct-2 phosphorylation

The ubiquitous Oct-1 and lymphoid Oct-2 POU homeodomain transcription factors bind to the same DNA sequence but differ in their activation potential. Oct-2 is a positive, negative, or neutral regulator of beta-globin transcription depending on the position and sequence of multimerized binding sites. To activate transcription, Oct-2 relies on two interdependent nonacidic domains, an N-terminal glutamine-rich region and a C-terminal serine-, threonine-, and proline-rich region. Oct-1 also contains a functional glutamine-rich region but fails to activate beta-globin transcription in our assay because the Oct-1 C-terminus is inactive, indicating that differential activation by Oct-1 and Oct-2 is determined by the combination of multiple activation domains. Oct-2 displays a unique phosphorylation pattern that is absent from molecules lacking one or the other activation domain, suggesting the activation domains have a role in inducing protein phosphorylation.

The b alleles of U. maydis, whose combinations program pathogenic development, code for polypeptides containing a homeodomain-related motif

U. maydis is a fungal pathogen of corn with two forms: one is yeast-like and nonpathogenic; the other is filamentous and pathogenic. The b locus, with 25 different alleles, regulates this dimorphism: any combination of two different alleles triggers pathogenic development, whereas the presence of identical alleles results in the yeast-like form. We have cloned four b alleles (b1, b2, b3, and b4) and show that the b locus contains a single open reading frame (ORF) of 410 amino acids with a variable N-terminal region and a highly conserved C-terminal region (60% and 93% identity, respectively). Mutational analysis confirms that this ORF is responsible for b activity. The b polypeptides appear to be DNA binding proteins because they contain a motif related to the homeodomain in their constant region. We propose that combinatorial interactions between b polypeptides generate regulatory proteins that determine the developmental program of the fungus.

A two-base change in a POU factor-binding site switches pituitary-specific to lymphoid-specific gene expression

The structurally related POU homeo domain proteins Pit-1 and Oct-2 activate pituitary- and lymphoid-specific transcription, respectively, by binding to similar AT-rich motifs in their target genes. In this study we identify bases critical for recognition and activation by Pit-1 and examine how small differences in Pit-1 and Oct-2-binding sites can impart differential transcriptional responses in pituitary and B-lymphoid cells. Scanning mutagenesis of Pit-1 response elements in both the rat prolactin and growth hormone genes reveals a critical binding motif recognized in an identical manner by the native Pit-1 protein and cloned Pit-1 gene product. This motif, ATTATTCCAT, differs by only two bases from the octamer element, ATTTGCAT, required for Oct-2-dependent activation of immunoglobulin genes. Cross recognition of Pit-1 and Oct-2 sites by both factors can be demonstrated in competitive binding assays, in which an oligometric Pit-1 site from the prolactin gene is converted to an Oct-2 site by a double point mutation. In contrast to the binding data, no cross activation of transcription is detectable in cultured cell lines. When inserted immediately 5' to a prolactin TATA box, the wild-type prolactin element enhances transcription strongly in pituitary cells but is inactive in B cells, whereas the octamer variant of the prolactin site activates expression in B cells but is silent in pituitary lines. Both elements are nonfunctional in heterologous cell lines that lack Pit-1 and Oct-2.(ABSTRACT TRUNCATED AT 250 WORDS)