Purification of transcription cofactor complex CRSP

Transcription of protein coding genes in metazoans involves the concerted action of enhancer binding proteins and the RNA polymerase II apparatus. The cross talk between these two classes of transcription factors is mediated by an elaborate set of cofactor complexes. For the activation of transcription by the promoter specificity protein 1 (Sp1), TATA binding protein-associated factors in the TFIID complex originally were identified as necessary coactivators, but the identity of additional cofactors required for activated transcription was unknown. Recently, we have reported the isolation and properties of a cofactor complex, CRSP (cofactor required for Sp1), which functions in conjunction with the TATA binding protein-associated factors to promote efficient activation of transcription by Sp1. CRSP contains unique subunits as well as polypeptides that are shared with other cofactor complexes. Here, we report a detailed purification protocol for the isolation of CRSP from human HeLa cells. Our purification strategy takes advantage of the ability of CRSP to bind Ni2+-nitrilotriacetic acid-agarose resin as well as other conventional chromatographic resins. We also describe a streamlined purification protocol that allows a more rapid and efficient means to isolate active CRSP.

Probing activation of the prokaryotic arginine transcriptional regulator using chimeric proteins

The major transcription factors controlling arginine metabolism in Escherichia coli and Bacillus subtilis, ArgR and AhrC, respectively, are homologous multimeric proteins that form l -arginine-dependent DNA-binding complexes capable of repressing transcription of the biosynthetic genes (both), activating transcription of catabolic genes (AhrC only) or facilitating plasmid dimer resolution (both). Multimerisation and l -arginine binding are associated with the C-terminal 70-80 residues; the N-terminal regions contain a winged helix-turn-helix DNA-binding domain. We have constructed chimeric genes in which the sequences for the N and C-terminal domains have been swapped. The resultant chimeric proteins and their corresponding native proteins have been analysed for their ability to multimerise and bind DNA operator sites in an L-arginine-dependent fashion. Gel filtration and equilibrium sedimentation analysis are consistent with the formation of hexamers by all four proteins in the presence of L-arginine and at high protein concentrations (>100 nM monomer). The hexamer sedimentation coefficients suggest that there is a reduction in molecular volume upon binding L-arginine, consistent with a conformational change accompanying an allosteric activation of DNA-binding. In the absence of L-arginine or at lower protein concentrations, the hexamers are clearly in rapid equilibrium with smaller subunits, whose dominant species appear to be based on trimers, as expected from the crystal structure of the ArgR C-terminal fragment, with the exception of the ArgR-C chimera, which apparently dissociates into dimers, suggesting that in the intact protein the DNA-binding domains may have a significant dimeric interaction. The hexamer-trimer Kdis in the micromolar range, suggesting that trimers are the principal species at in vivo concentrations.DNA binding by all four proteins has been probed by gel retardation and DNase I footprinting analysis using all three types of naturally occurring operators: biosynthetic sites encompassing two 18 bp ARG boxes separated by 2 bp; biosynthetic sites containing two such boxes and a third 18 bp ARG box at a distance of 100 bp downstream, i.e. within the structural gene; and finally a catabolic operator which contains a single ARG box site. The data show that all four proteins bind to the operators at the expected regions in an L-arginine-dependent fashion. From the apparent affinities of the chimeras for each target site, there is no obvious sequence-specificity associated with the N-terminal domains; rather the data can be interpreted in terms of differential allosteric activation, including DNA binding in the absence of L-arginine.Remarkably, the proteins show apparent "anti-competition" in the presence of excess, specific DNA fragments in gel retardation. This appears to be due to assembly of an activated form of the protein, probably hexamers, on the operator DNA. The data are discussed in terms of the current models for the mode of action of both native proteins.

Production and characterization of monoclonal antibodies to the steroid receptor coactivator AIB1

The steroid coactivator AIB1 (Ampified In Breast cancer 1) is a member of the SRC-1 family of transactivation coactivators that is amplified in about 7% of breast tumor samples. Hybridomas were established that secreted monoclonal antibodies (MAbs) to a recombinant glutathione-S-transferase (GST) fusion protein expressing the steroid receptor interacting domain of AIB1 (GST-AIB.T1:a.a. 605-1294). Four MAbs from these hybridomas were characterized and designated AX15.1, AX15.2, AX15.3, and AX15.4. The MAbs were shown to be specific to AIB1 and did not cross-react with two similar coactivators SRC-1 and TIF2 as shown in Western blot analysis and enzyme-linked immunoadsorbent assay (ELISA). Western blot analysis using the four MAbs showed specific recognition of AIB1 protein as a 160 kDa band in lysates from cell lines containing AIB1 gene-amplification. The MAbs immunoprecipitated in vitro-translated AIB1 and cellular AIB1 from metabolically labeled cells. The results show that these newly described MAbs are useful in studies of AIB1 function and expression.

Pseudorabies virus early protein 0 trans-activates the TATA-associated promoter by stimulating the transcription initiation

Pseudorabies virus (PRV) early protein 0 (EP0) is a transactivator containing a RING finger domain. To assess the transactivation mechanism of PRV EP0, we performed the in vitro transcription by combining HeLa nuclear extract, purified recombinant EP0 and simple promoter constructs, and evaluated the results by primer extension. The data showed that EP0 could significantly activate the TATA-containing synthetic promoters. Moreover, EP0 activated transcription by stabilizing the formation of transcription initiation complex instead of enhancing the elongation rate. To further understand the role of EP0 on assembling the transcription initiation complex, we performed the pull-down assay using affinity precipitation of proteins from HeLa nuclear extracts and bacterially expressed glutathione-S-transferase EP0 RING finger fusion. The data showed that at least six nuclear proteins physically interacted with the EP0 RING finger. Overall, the transactivation of PRV EP0 is accomplished by enhancing the transcription initiation and is associated with at least six nuclear proteins.

CBP associates with the p42/p44 MAPK enzymes and is phosphorylated following NGF treatment

The ability of the CBP (CREB binding protein) coactivator to stimulate transcription has previously been shown to be stimulated by treatment of neuronal cells with nerve growth factor (NGF). This effect is dependent upon activation of the p42/p44 MAPK (mitogen activated protein kinase) pathway. Here we show that both CBP and the related p300 protein directly associate with the p42/p44 MAPK enzymes both prior to and following their activation by NGF and that CBP is phosphorylated following NGF treatment. These results indicate that phosphorylation of CBP itself by the p42/p44 MAPK pathway is likely to be critical for its role in NGF-mediated stimulation of gene expression.

Spi-C, a novel Ets protein that is temporally regulated during B lymphocyte development

A novel Ets protein was isolated by yeast one-hybrid screening of a cDNA library made from lipopolysaccharide-stimulated mouse splenic B cells, using the SP6 kappa promoter kappaY element as a bait. The novel Ets protein was most closely related to PU.1 and Spi-B within the DNA binding Ets domain and was therefore named Spi-C. However, Spi-C may represent a novel subgroup within the Ets protein family, as it differed significantly from Spi-B and PU.1 within helix 1 of the Ets domain. Spi-C was encoded by a single-copy gene that was mapped to chromosome 10, region C. Spi-C interacted with DNA similarly to PU.1 as judged by methylation interference, band-shift and site selection analysis, and activated transcription of a kappaY element reporter gene upon co-transfection of HeLa cells. Spi-C RNA was expressed in mature B lymphocytes and at lower levels in macrophages. Furthermore, pre-B cell and plasma cell lines were Spi-C-negative, suggesting that Spi-C might be a regulatory molecule during a specific phase of B lymphoid development.

Extracellular signal-regulated kinase (ERK) interacts with signal transducer and activator of transcription (STAT) 5a

Serine phosphorylation of signal transducers and activators of transcription (STAT) 1 and 3 modulates their DNA-binding capacity and/or transcriptional activity. Earlier we suggested that STAT5a functional capacity could be influenced by the mitogen-activated protein kinase (MAPK) pathway. In the present study, we have analyzed the interactions between STAT5a and the MAPKs, extracellular signal-regulated kinases ERK1 and ERK2. GH treatment of Chinese hamster ovary cells stably transfected with the GH receptor (CHOA cells) led to rapid and transient activation of both STAT5a and ERK1 and ERK2. Pretreatment of cells with colchicine, which inhibits tubulin polymerization, did not inhibit STAT5a translocation to the nucleus and ERK1/2 activation. In vitro precipitation with a glutathione-S-transferase-fusion protein containing the C-terminal transactivation domain of STAT5a showed GH-regulated association of ERK1/2 with the fusion protein, while this was not seen when serine 780 in STAT5a was changed to alanine. In vitro phosphorylation of the glutathione-S-transferase-fusion proteins using active ERK only worked when the fusion protein contained wild-type STAT5a sequence. The same experiment, performed with full-length wild-type STAT5a and the corresponding S780A mutant, showed that serine 780 is the only substrate in full-length STAT5a for active ERK. In coimmunoprecipitation experiments, larger amounts of STAT5a-ERK1/2 complexes were detected in cytosol from untreated CHOA cells than in cytosol from GH-treated cells, suggesting the presence of preformed STAT5a-ERK1/2 complexes in unstimulated cells. Transfection experiments with COS cells showed that kinase-inactive ERK1 decreased GH stimulation of STAT5-regulated reporter gene expression. These observations show, for the first time, direct physical interaction between ERK and STAT5a and also clearly identify serine 780 as a target for ERK. Furthermore, it is also established that serine phosphorylation of STAT5a transactivation domain, via the MAPK pathway, is a means of modifying GH-induced transcriptional activation.

Binding site recognition by Rns, a virulence regulator in the AraC family

The expression of CS1 pili by enterotoxigenic strains of Escherichia coli is regulated at the transcriptional level and requires the virulence regulator Rns, a member of the AraC family of regulatory proteins. Rns binds at two separate sites upstream of Pcoo (the promoter of CS1 pilin genes), which were identified in vitro with an MBP::Rns fusion protein in gel mobility and DNase I footprinting assays. At each site, Rns recognizes asymmetric nucleotide sequences in two regions of the major groove and binds along one face of the DNA helix. Both binding sites are required for activation of Pcoo in vivo, because mutagenesis of either site significantly reduced the level of expression from this promoter. Thus, Rns regulates the expression of CS1 pilin genes directly, not via a regulatory cascade. Analysis of Rns-nucleotide interactions at each site suggests that binding sites for Rns and related virulence regulators are not easily identified because they do not bind palindromic or repeated sequences. A strategy to identify asymmetric binding sites is presented and applied to locate potential binding sites upstream of other genes that Rns can activate, including those encoding the CS2 and CFA/I pili of enterotoxigenic E. coli and the global regulator virB of Shigella flexneri.

Cloning and characterization of androgen receptor coactivator, ARA55, in human prostate

Androgen receptor (AR) is a hormone-activated transcriptional factor that can bind to androgen response elements and that regulates the transcription of target genes via a mechanism that presumably involves cofactors. We report here the cloning of a novel AR coactivator ARA55 using a yeast two-hybrid system. ARA55 consists of 444 amino acids with the predicted molecular mass of 55 kDa and its sequence shows very high homology to mouse hic5, a TGF-beta1-inducible gene. Yeast and mammalian two-hybrid systems and co-immunoprecipitation assays all prove ARA55 can bind to AR in a ligand-dependent manner. Transient transfection assay in prostate cancer DU145 cells further demonstrates that ARA55 can enhance AR transcriptional activity in the presence of 1 nM dihydrotestosterone or its antagonists such as 100 nM 17beta-estradiol or 1 microM hydroxyflutamide. Our data also suggest the C-terminal half of ARA55, which includes three LIM motifs, is sufficient to interact with AR. Northern blot and polymerase chain reaction quantitation showed ARA55 can be expressed differently in normal prostate and prostate tumor cells. Together, our data suggests that ARA55 may play very important roles in the progression of prostate cancer by the modulation of AR transactivation.

GRAB proteins, novel members of the NAC domain family, isolated by their interaction with a geminivirus protein

Geminiviruses encode a few proteins and depend on cellular factors to complete their replicative cycle. As a way to understand geminivirus-host interactions, we have searched for cellular proteins which interact with viral proteins. By using the yeast two-hybrid technology and the wheat dwarf geminivirus (WDV) RepA protein as a bait, we have isolated a family of proteins which we termed GRAB (for Geminivirus Rep A-binding). We report here the molecular characterization of two members, GRAB1 and GRAB2. We have found that the 37 C-terminal amino acids of RepA are required for interaction with GRAB proteins. This region contains residues conserved in an equivalent region of the RepA proteins encoded by other viruses of the WDV subgroup. The N-terminal domain of GRAB proteins is necessary and sufficient to interact with WDV RepA. GRAB proteins contain an unique acidic C-terminal domain while their N-terminal domain, of ca. 170 amino acids, are highly conserved in all of them. Interestingly, this conserved N-terminal domain of GRAB proteins exhibits a significant amino acid homology to the NAC domain present in proteins involved in plant development and senescence. GRAB1 and GRAB2 mRNAs are present in cultured cells and roots but are barely detectable in leaves. GRAB expression inhibits WDV DNA replication in cultured wheat cells. Our studies highlight the importance that the pathway(s) mediated by GRAB proteins, as well as by other NAC domain-containing proteins, might have on geminivirus DNA replication in connection to plant growth, development and senescence pathways.

Polypyrimidine-tract binding protein (PTB) is necessary, but not sufficient, for efficient internal initiation of translation of human rhinovirus-2 RNA

Initiation of translation of the animal picornavirus RNAs is via a mechanism of direct internal ribosome entry, which requires a substantial segment of the viral 5'-untranslated region, generally known as the IRES (for "internal ribosome entry site"). Because, however, translation of the RNAs of members of the enterovirus, and more especially, the rhinovirus subgroups of the Picornaviridae is restricted in the reticulocyte lysate system, but is greatly stimulated by the addition of HeLa cell extracts, the implication is that, in these cases, internal initiation also requires cellular trans-acting factors that are more abundant in HeLa cell extracts than in rabbit reticulocytes. This was used as the basis of a functional assay for the purification of the HeLa cell factors required for translation dependent on the human rhinovirus-2 (HRV) IRES. There are two such HeLa cell factors separable by ion-exchange chromatography, each of which is individually active in the assay, although their combined effect is synergistic. One of these activities is shown to be polypyrimidine-tract binding protein (PTB) on the grounds that (1) the activity copurifies to homogeneity with PTB and (2) recombinant PTB expressed in Escherichia coli stimulates HRV IRES-dependent translation with a specific activity similar to that of the purified HeLa cell factor. Furthermore, it is shown that recombinant PTB also stimulates the translation of RNAs bearing the poliovirus type 1 (Mahoney) IRES.

Identification of human cytomegalovirus target sequences in the human immunodeficiency virus long terminal repeat. Potential role of IE2-86 binding to sequences between -120 and -20 in promoter transactivation

OBJECTIVE

Because of the important medical consequences of human cytomegalovirus (HCMV) infection in human immunodeficiency virus (HIV)-infected individuals, we wanted to understand the molecular interactions that occur during co-infection. Specifically, in this study, we wanted to identify the transactivating target sequences on the HIV long terminal repeat (LTR) that responded to HCMV infection.

STUDY DESIGN/METHODS

In this study, we transfected the HIV-LTR into human fibroblasts and then mapped the regulation of this promoter following HCMV infection and co-transfection with the HCMV immediate-early (IE) gene product IE2-86. In addition, we examined IE2-86 binding to specific sequences in the HIV-LTR by electrophoretic mobility shift assay.

RESULTS

Our results documented that HCMV and IE2-86 could transactivate the HIV-LTR. In mapping the regions of the HIV-LTR that IE2-86 transactivates, we identified discrete target sequences between -120 and -20 that are the major transactivating regions for the IE2-86-mediated effects and determined that IE2-86 could specifically bind to several discrete sequences within this region of the HIV-LTR.

CONCLUSIONS

Our discovery of the binding of IE2-86 to the HIV-LTR, coupled with its ability to transactivate the HIV-LTR and induce cellular transcription factors, points to potential molecular mechanisms used by HCMV to upregulate the HIV life cycle and, consequently, exacerbate the conditions observed in individuals co-infected with HCMV and HIV.

Detection of intracellular phosphorylated STAT-1 by flow cytometry

We have applied flow cytometry to the investigation of interferon-gamma activation of human monocytes. This approach uses monoclonal antibodies that distinguish between the native and phosphorylated forms of STAT-1. It enables rapid and quantitative assessment of STAT-1 phosphorylation on a discrete cell basis and is both more sensitive and less time consuming than immunoblotting. Furthermore, it allows for discrimination between a mixture of cells that differ in their response to interferon-gamma. This approach should allow for the evaluation of different intracellular signaling pathways using a combination of monoclonal reagents that are specific for native and activation modified proteins. Application of this form of testing should prove valuable in screening for signaling defects in selected patients with recurrent infections. In addition, this technique should permit dissection of a full range of cellular signaling pathways at the protein level.

The transcriptional cofactor complex CRSP is required for activity of the enhancer-binding protein Sp1

Activation of gene transcription in metazoans is a multistep process that is triggered by factors that recognize transcriptional enhancer sites in DNA. These factors work with co-activators to direct transcriptional initiation by the RNA polymerase II apparatus. One class of co-activator, the TAF(II) subunits of transcription factor TFIID, can serve as targets of activators and as proteins that recognize core promoter sequences necessary for transcription initiation. Transcriptional activation by enhancer-binding factors such as Sp1 requires TFIID, but the identity of other necessary cofactors has remained unknown. Here we describe a new human factor, CRSP, that is required together with the TAF(II)s for transcriptional activation by Sp1. Purification of CRSP identifies a complex of approximate relative molecular mass 700,000 (M(r) approximately 700K) that contains nine subunits with M(r) values ranging from 33K to 200K. Cloning of genes encoding CRSP subunits reveals that CRSP33 is a homologue of the yeast mediator subunit Med7, whereas CRSP150 contains a domain conserved in yeast mediator subunit Rgr1. CRSP p200 is identical to the nuclear hormone-receptor co-activator subunit TRIP2/PBP. CRSPs 34, 77 and 130 are new proteins, but the amino terminus of CRSP70 is homologous to elongation factor TFIIS. Immunodepletion studies confirm that these subunits have an essential cofactor function. The presence of common subunits in distinct cofactor complexes suggests a combinatorial mechanism of co-activator assembly during transcriptional activation.

Microphthalmia-associated transcription factor interacts with PU.1 and c-Fos: determination of their subcellular localization

Marked osteopetrosis is observed only in mi/mi mutant mice although normal osteoclastgenesis is observed in other mutant mice including null mutants at the mi locus. Mutant microphthalmia-associated transcription factor (mi-MITF) has defective nuclear localization potential. We found normal MITF (+-MITF)-c-Fos and mi-MITF-c-Fos complexes in the cytoplasm by immunoblotting, and showed that PU.1 bound with both +-MITF and mi-MITF using an electrophoretic mobility shift assay. Furthermore, the nuclear localization of PU.1 and c-Fos was inhibited by over-expressed mi-MITF in WEHI-3 cells. These results indicate that mi-MITF expressing in osteoclasts specifically binds to c-Fos and PU.1 which are essential transcription factors of osteoclastgenesis and that mi-MITF blocks the nuclear localization of these other transcription factors, which may result in osteopetrosis in mi/mi mutant mice.

In vivo and in vitro activities of the Escherichia coli sigma54 transcription activator, PspF, and its DNA-binding mutant, PspFDeltaHTH

Transcription of the phage-shock protein (psp) operon in Escherichia coli is driven by a sigma54 promoter, stimulated by integration host factor and dependent on an upstream, cis-acting sequence and an activator protein, PspF. PspF belongs to the enhancer binding protein family but lacks an N-terminal regulatory domain. Purified PspF is not modified and has an ATPase activity that is increased twofold in the presence of DNA carrying the psp cis-acting sequence. Purified mutant His-tagged PspF that lacks the C-terminal DNA-binding motif has a DNA-independent ATPase activity when present at 30-fold the concentration of the wild-type protein. Both proteins oligomerize in solution in an ATP and DNA-independent manner. The wild-type activator protein, but not the DNA-binding mutant, binds specifically to the cis-acting sequence. Analysis of the sequence protected by PspF demonstrates the presence of two upstream binding sites within the sequence, UAS I and UAS II, which together constitute the psp enhancer. Protection at low protein concentrations is more pronounced and more extensive on a supercoiled DNA than on a linear template. Full expression of the psp operon upon hyperosmotic shock depends on wild-type PspF, but only partially requires the presence of the psp enhancer.

Site-directed mutagenesis of the AcMNPV p143 gene: effects on baculovirus DNA replication

Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) encodes a 143-kDa protein (P143) required for viral DNA synthesis and involved in host range determination. The predicted amino acid sequence of P143 contains seven motifs (I, Ia, II-VI) shared with a superfamily of helicases involved in the unwinding of duplex nucleic acids; a putative DNA binding motif; a putative nuclear localization signal (NLS); and a demonstrated host range motif. In this study, the functional significance of these conserved P143 motifs was examined by site-specific mutation resulting in amino acid substitutions of conserved residues within each of them. An in vivo complementation replication assay was developed and each mutated P143 protein expressed from a transfected plasmid was tested for its ability to complement the replication-negative ts8 baculovirus mutant for the amplification of an origin-containing plasmid. Mutations in the helicase motifs I, Ia, and II and in a potential helix-turn-helix motif abolished the ability of P143 to complement the ts8 defect in DNA replication, suggesting that these conserved amino acid residues may be essential for the replication function of the protein. In contrast, mutation of conserved amino acid residues in the helicase motifs IV, V, and VI did not affect the ability of the P143 proteins to complement the replication defect of ts8. A mutation in motif III caused a reduction in the replication function of P143. Deletion of Gly552 in the host range region eliminated the replication function of P143. Mutations within a putative NLS had no effect on the ability of P143 to support DNA replication, suggesting that these residues are nonessential and that the putative P143 NLS sequence may not be responsible for the nuclear localization of the protein. The transient complementation system used in this study provides a simple method for functional analysis of essential baculovirus genes in infected cell cultures.

Identification of a novel Stat3 recruitment and activation motif within the granulocyte colony-stimulating factor receptor

Stat3 is essential for early embryonic development and for myeloid differentiation induced by the cytokines granulocyte colony-stimulating factor (G-CSF) and interleukin-6 (IL-6). Two isoforms of Stat3 have been identified, (p92) and beta (p83), which have distinct transcriptional and biological functions. Activation of both Stat3 and Stat3beta requires the distal cytoplasmic domain of the G-CSFR, which contains four Tyr at positions 704, 729, 744, and 764. The studies reported here were undertaken to determine which, if any, of these tyrosine residues participated in Stat3/beta recruitment and activation. We showed that Stat3 and Stat3beta were affinity purified using phosphopeptides containing Y704 and Y744 but not by nonphosphorylated peptide analogues or by phosphopeptides containing Y729 and Y764. Complementary results were obtained in studies examining the ability of these peptides to destabilize and inhibit DNA binding of activated Stat3. Both Y704 and Y744 contributed to optimal activation of Stat3/beta in M1 murine myeloid leukemia cells containing wild-type and Y-to-F mutant G-CSFR constructs. Carboxy-terminal to Y704 at the +3 position is Gln; YXXQ represents a consensus Stat3 recruitment and activation motif. Y744 is followed at the +3 position by Cys (C); YXXC, represents a novel motif implicated in the recruitment and activation of Stat3. Modeling of the SH2 domain of Stat3 based on homologous SH2 domains of known structure revealed polar residues whose side chains contact the +3 position. This substitution may confer specificity for the Y704- and Y744-based ligands by allowing H-bond formation between the binding surface and the Gln or Cys found at the respective +3 position.

The DNA binding protein Tfx from Methanobacterium thermoautotrophicum: structure, DNA binding properties and transcriptional regulation

In Methanobacterium thermoautotrophicum, the fmdECB operon encoding the molybdenum formyl-methanofuran dehydrogenase is directly preceded by an open reading frame tfx predicted to encode a DNA binding protein. The 16.1 kDa protein has an N-terminal basic domain with a helix-turn-helix motif for DNA binding and a C-terminal acidic domain possibly for transcriptional activation. We report here on the DNA binding properties of the Tfx protein heterologously overproduced in Escherichia coli. Tfx was found to bind specifically to a DNA sequence downstream of the promoter of the fmdECB operon, as shown by electrophoretic mobility shift assays and DNase I footprint analysis. Northern blot hybridizations revealed that transcription of tfx is repressed during the growth of M. thermoautotrophicum in the presence of tung-state. Based on its structure and properties, the DNA binding protein Tfx is proposed to be a transcriptional regulator composed of a basic DNA binding domain and an acidic activation domain.

Determination of the zebrafish forebrain: induction and patterning

We report an analysis of forebrain determination and patterning in the zebrafish Danio rerio. In order to study these events, we isolated zebrafish homologs of two neural markers, odd-paired-like (opl), which encodes a zinc finger protein, and fkh5, which encodes a forkhead domain protein. At mid-gastrula, expression of these genes defines a very early pattern in the presumptive neurectoderm, with opl later expressed in the telencephalon, and fkh5 in the diencephalon and more posterior neurectoderm. Using in vitro explant assays, we show that forebrain induction has occurred even earlier, by the onset of gastrulation (shield stage). Signaling from the early gastrula shield, previously shown to be an organizing center, is sufficient for activation of opl expression in vitro. In order to determine whether the organizer is required for opl regulation, we removed from late blastula stage embryos either the presumptive prechordal plate, marked by goosecoid (gsc) expression, or the entire organizer, marked by chordin (chd) expression. opl was correctly expressed after removal of the presumptive prechordal plate and consistently, opl was correctly expressed in one-eyed pinhead (oep) mutant embryos, where the prechordal plate fails to form. However, after removal of the entire organizer, no opl expression was observed, indicating that this region is crucial for forebrain induction. We further show that continued organizer function is required for forebrain induction, since beads of BMP4, which promotes ventral fates, also prevented opl expression when implanted during gastrulation. Our data show that forebrain specification begins early during gastrulation, and that a wide area of dorsal mesendoderm is required for its patterning.

NasR, a novel RNA-binding protein, mediates nitrate-responsive transcription antitermination of the Klebsiella oxytoca M5al nasF operon leader in vitro

In Klebsiella oxytoca (pneumoniae), enzymes required for nitrate assimilation are encoded by the nasFEDCBA operon. Previous genetic studies led to the conclusion that nitrate and nitrite induction of nasF operon expression is determined by a transcriptional antitermination mechanism. In the presence of nitrate or nitrite, the nasR gene product is hypothesized to inhibit transcription termination at the factor-independent terminator site located in the nasF operon leader region. To test this model in vitro, we first purified NasR as both a maltose binding protein fusion form (MBP-NasR) and a His6-tagged form (His6-NasR). Templates for in vitro transcription contained the nasF operon leader region, with a substitution of the sigma70-dependent tac promoter for the native sigmaN-dependent promoter. We found that in vitro transcription of the leader template terminated at the terminator site, and that MBP-NasR and His6-NasR proteins both caused transcription readthrough of this site in response to nitrate or nitrite. Half-maximal antitermination required nitrate or nitrite at moderate (1 to 10 microM) concentrations, and several other anions tested, including chlorate, were without effect. Previous in vivo analysis of leader deletions identified regions required for both negative regulation (the terminator) and for positive regulation. Results from in vitro transcription of these deletion templates correlated fully with the in vivo analysis. Finally, electrophoresis mobility shift analysis revealed that His6-NasR bound specifically to nasF leader RNA. This binding was independent of nitrate in vitro. These results strongly support the conclusions drawn from previous in vivo analysis, and establish that NasR mediates ligand-responsive transcription antitermination through interaction with nasF leader RNA.

Selection and characterization of mercury-independent activation mutants of the Tn501 transcriptional regulator, MerR

MerR is the transcriptional regulator of the mercury-resistance (mer) operon of transposon Tn501, acting at the mer promoter as both an activator in the presence of mercuric salts and a repressor in their absence. This paper reports a method for selection of constitutive activator mutants, which activate transcription in the absence of HgII, and the characterization of these MerRAC proteins. At least two mutations in the MerR protein were found necessary for strong mercury-independent activation, and these mutations lie in the C-terminal two-thirds of the MerR protein near the HgII-binding cysteines. A triple mutation was shown to increase activation over the corresponding double mutations. All mutant proteins caused further activation in the presence of HgII. The data support a mechanism in which a conformational change of one or both MerR subunits in the homodimer drives a distortion of DNA bound to a helix-turn-helix structure in the N-terminal region. A mutation in this putative helix-turn-helix region severely reduced both the repressor and activator functions of MerR.

The Pad1+ gene encodes a subunit of the 26 S proteasome in fission yeast

We have isolated a fission yeast mutant, mts5-1, in a screen for mutations that confer both methyl 2-benzimidazolecarbamate resistance (MBCR) and temperature sensitivity (ts) on Schizosaccharomyces pombe. This screen has previously isolated mutations in the 26 S proteasome subunits Mts2, Mts3, and Mts4. We show that the mutation in the mts5-1 strain occurs in the pad1(+) gene. pad1(+) was originally isolated on a multicopy plasmid that was capable of conferring staurosporine resistance on a wild type strain. mts5-1/pad1-1 has a similar phenotype to 26 S proteasome mutants previously isolated in the same screen and we show that Pad1 interacts genetically with two of these subunits, Mts3 and Mts4. In this study we describe the identification of Pad1 as a subunit of the 26 S proteasome in fission yeast.

Identity of the beta-globin locus control region binding protein HS2NF5 as the mammalian homolog of the notch-regulated transcription factor suppressor of hairless

Previously, we characterized a DNA-binding protein, HS2NF5, that bound tightly to a conserved region within hypersensitive site 2 (HS2) of the human beta-globin locus control region (LCR) (Lam, L. T. , and Bresnick, E. H. (1996) J. Biol. Chem. 271, 32421-32429). The beta-globin LCR controls the chromatin structure, transcription, and replication of the beta-globin genes. We have now purified HS2NF5 to near-homogeneity from fetal bovine thymus. Two polypeptides of 56 and 61 kDa copurified with the DNA binding activity. The two proteins bound to the LCR recognition site with an affinity (3.1 nM) and specificity similar to mouse erythroleukemia cell HS2NF5. The amino acid sequences of tryptic peptides of purified HS2NF5 revealed it to be identical to the murine homolog of the suppressor of hairless transcription factor, also known as recombination signal binding protein Jkappa or C promoter binding factor 1 (CBF1). The CBF1 site within HS2 resides near sites for hematopoietic regulators such as GATA-1, NF-E2, and TAL1. An additional conserved, high affinity CBF1 site was localized within HS4 of the LCR. As CBF1 is a downstream target of the Notch signaling pathway, we propose that Notch may modulate LCR activity during hematopoiesis.