Biological impact of natural COOH-terminal deletions of hepatitis B virus X protein in hepatocellular carcinoma tissues

The hepatitis B virus (HBV) X protein (HBx) is a transcriptional transactivator that has been implicated in the development of HBV-related hepatocellular carcinoma. Mutations in the HBx open reading frame have been reported, but their general impact on the biological function of HBx remains unknown. To address this issue, we comparatively analyzed the structures and biological functions of HBx sequences isolated from sera and from tumor and nontumor tissues of patients with a HBV-related hepatocellular carcinoma. In addition to the HBx sequences derived from free HBV genomes, HBx from HBV integrants was also obtained from the tumor tissues by use of a HBx-Alu PCR-based approach. Sequence analysis showed that the HBx sequences derived from tumor tissues (6 of 7), particularly those isolated from HBV integrants (4 of 4), contained a deletion in the distal COOH-terminal region. Interestingly, most of the COOH-terminally truncated HBx sequences obtained from tumor tissues, in contrast to the full-length HBx isolated from the sera and nontumor tissues, lost their transcriptional activity and their inhibitory effects on cell proliferation and transformation. Importantly, although full-length HBx suppressed the focus formation induced by the cooperation of ras and myc oncogenes in primary rat embryo fibroblasts, COOH-terminally truncated HBx enhanced the transforming ability of ras and myc. Finally, by analyzing the artificial mutants, we were able to more precisely map the functional domains located at the COOH-terminal of HBx. Taken together, our results suggest a key role for the HBx COOH-terminal end in controlling cell proliferation, viability, and transformation. This study further supports the hypothesis that natural HBx mutants might be selected in tumor tissues and play a role in hepatocarcinogenesis by modifying the biological functions of HBx.

Specific DNA binding and transactivation potential of recombinant, purified Stat5

The signal transducers and activators of transcriptions (Stats) are central mediators of cytokine responses especially in hematopoietic cells. The detailed molecular mechanisms of Stat activation, particularly the role of post-translational modifications and co-operation with cellular transcription factors are subject to intense investigation. The phosphorylation of a tyrosine residue in the carboxyl terminal domain is a common characteristic for the biologically active state of all known Stats. We studied the biological potential of purified recombinant murine Stat5a and Stat5b. These proteins were expressed in Sf9 insect cells upon infection with Stat5 encoding baculoviruses. We also obtained the tyrosine phosphorylated, activated forms of the Stat5 proteins by expressing the tyrosine kinase Janus kinase2 (Jak) in the same cells through co-infection with a kinase encoding virus. After purification, only the tyrosine phosphorylated form was able to bind specifically in vitro to the Stat5 DNA response element. This activated form of Stat5 is also able to support specific cell free in vitro transcription of a gene with a Stat5 response element in its promoter region. The recombinant purified Stat5 proteins were treated with the tyrosine specific protein phosphatase or with potato acidic phosphatase, which removes phosphate groups from serine and tyrosine residues. Phosphatase treatment resulted in the loss of specific DNA binding ability. This property could be restored by an in vitro reaction with recombinant, purified EGF or PDGF receptor kinases. Tyrosine rephosphorylation in vitro also restored the transactivation potential of Stat5. This modification is, therefore, a sufficient prerequisite for transcriptional induction by Stat5.

Mouse embryos lacking Smad1 signals display defects in extra-embryonic tissues and germ cell formation

The Smad proteins are important intracellular mediators of the transforming growth factor β (TGFβ) family of secreted growth factors. Smad1 is an effector of signals provided by the bone morphogenetic protein (BMP) sub-group of TGFβ molecules. To understand the role of Smad1 in mouse development, we have generated a Smad1 loss-of-function allele using homologous recombination in ES cells. Smad1−/− embryos die by 10.5 dpc because they fail to connect to the placenta. Mutant embryos are first recognizable by 7.0 dpc, owing to a characteristic localized outpocketing of the visceral endoderm at the posterior embryonic/extra-embryonic junction, accompanied by a dramatic twisting of the epiblast and nascent mesoderm. Chimera analysis reveals that these two defects are attributable to a requirement for Smad1 in the extra-embryonic tissues. By 7.5 dpc, Smad1-deficient embryos show a marked impairment in allantois formation. By contrast, the chorion overproliferates, is erratically folded within the extra-embryonic space and is impeded in proximal migration. BMP signals are known to be essential for the specification and proliferation of primordial germ cells. We find a drastic reduction of primordial germ cells in Smad1-deficient embryos, suggesting an essential role for Smad1-dependent signals in primordial germ cell specification. Surprisingly, despite the key involvement of BMP signaling in tissues of the embryo proper, Smad1-deficient embryos develop remarkably normally. An examination of the expression domains of Smad1, Smad5 and Smad8 in early mouse embryos show that, while Smad1 is uniquely expressed in the visceral endoderm at 6.5 dpc, in other tissues Smad1 is co-expressed with Smad5 and/or Smad8. Collectively, these data have uncovered a unique function for Smad1 signaling in coordinating the growth of extra-embryonic structures necessary to support development within the uterine environment.

Environmental control of invasin expression in Yersinia pseudotuberculosis is mediated by regulation of RovA, a transcriptional activator of the SlyA/Hor family

Invasin is the primary invasive factor of Yersinia pseudotuberculosis that allows efficient internalization into eukaryotic cells. We investigated invasin expression and found that the inv gene is regulated in response to a variety of environmental signals, such as temperature, growth phase, nutrients, osmolarity and pH, and requires the product of rovA, a member of the SlyA/Hor transcriptional activator family. The rovA gene was found by a genetic complementation strategy that restores temperature regulation of an unexpressed inv-phoA fusion in Escherichia coli K-12. RovA plays a role in the invasion of Y. pseudotuberculosis into mammalian cells and mediates the regulation of invasin in response to all environmental signals analysed. Deletion analysis of the inv promoter region revealed a DNA segment extending 207 bp upstream of the transcriptional start site, which is required for maximal RovA-induced inv transcription. Gel retardation assays showed that RovA interacts preferentially with this promoter fragment and suggested two potential RovA binding sites. Studies with chromosomal gene fusions also demonstrated that rovA follows the same pattern of regulation as invasin, indicating that environmental control of inv expression is mainly mediated by the control of RovA synthesis. Furthermore, we showed that a rovA-lacZ fusion is only slightly expressed in a rovA mutant strain, indicating that a positive autoregulatory mechanism is also involved in rovA expression.

Sir2p exists in two nucleosome-binding complexes with distinct deacetylase activities

The absolute requirement for the histone deacetylase activity of Sir2p in silencing coupled with the conservation of Sir2p-like proteins in larger eukaryotes suggests that this molecule plays an important role in gene regulation in all organisms. Here we report the purification and characterization of two Sir2p-containing protein complexes; one of which contains Sir4p and the other Net1p. The Sir4p-containing complex has an NAD-dependent histone deacetylase activity, while the Net1p-containing complex possesses deacetylase activity but only weak NAD-dependent histone deacetylase activity. Finally, we demonstrate that the Sir2p-containing complexes bind nucleosomes efficiently and partially restrict accessibility of the linker DNA to enzymatic probes.

Sir3-dependent assembly of supramolecular chromatin structures in vitro

Baculovirus-expressed recombinant Sir3p (rSir3p) has been purified to near homogeneity, and its binding to naked DNA, mononucleosomes, and nucleosomal arrays has been characterized in vitro. At stoichiometric levels rSir3p interacts with intact nucleosomal arrays, mononucleosomes, and naked DNA, as evidenced by formation of supershifted species on native agarose gels. Proteolytic removal of the core histone tail domains inhibits but does not completely abolish rSir3p binding to nucleosomal arrays. The linker DNA in the supershifted complexes remains freely accessible to restriction endonuclease digestion, suggesting that both the tail domains and nucleosomal DNA contribute to rSir3p--chromatin interactions. Together these data indicate that rSir3p cross-links individual nucleosomal arrays into supramolecular assemblies whose physical properties transcend those of typical 10-nm and 30-nm fibers. Based on these data we hypothesize that Sir3p functions, at least in part, by mediating reorganization of the canonical chromatin fiber into functionally specialized higher order chromosomal domains.

[Studies on cloning and expression of hepatitis B virus x gene]

OBJECTIVE

To express and purify the x gene of hepatitis B virus (HBV), and then the purity of the protein could specifically be analysed by the human antibodies against HBV x antigen (protein).

METHODS

HBV x gene was amplified from the complete genome by PCR, and cloned in the pGEX-2T fused expression vector. After transforming the plasmid into E.coli, the recombinant with HBx gene was obtained. The fusion protein was generated with E.coli fused expression system and purified with affinity chromatography.

RESULTS

The HBx protein could be applied to detecting specific antibody of hepatitis B virus patients.

CONCLUSIONS

The purity of the HBx protein can specifically be recognized by the human antibodies against HBV x antigen.

Bovine herpesvirus type 4: a special herpesvirus (review article)

This paper summarizes the history of and information on bovine herpesvirus type 4 (BoHV-4) from the first isolation to the most recent results. For almost twenty years BoHV-4 has been considered a typical herpes 'orphan' virus, which infects several species but causes no illness. The latest experiments revealed the close relationship of this virus with the immune system and other tissues. The virus was even considered as a possible candidate for a vector vaccine. BoHV-4 as a strange herpesvirus has several features which are not characteristic of other herpesviruses, such as several latency sites, persistence in serum, dividing cells necessary for virus replication, and the wide host range. In addition to describing the main features of the virion, replication, clinical signs, nomenclature problems, this review intends to concentrate on the new and strange results coming out from several laboratories worldwide. It is also suggested that because the virus combines several properties of various herpesvirus subfamilies and because of its close relationship with the immune system, it may deserve further attention as a representative of a potentially new genus within the Gammaherpesvirinae subfamily.

Phosphorylation of purified recombinant hepatitis B virus-X protein by mitogen-activated protein kinase and protein kinase C in vitro

The recombinant human hepatitis B virus-X protein (rhHBx) has been expressed as inclusion bodies in Escherichia coli and purified. By sequential dialysis of urea, rhHBx was folded into the native structure, which was demonstrated by both the efficacy of its transcriptional activation of the adenovirus major late promoter, fluorescence and circular dichroism (CD) analysis. The increase in CD values at 220 nm and a corresponding blue shift of the intrinsic fluorescence emission confirmed the ability of HBx to refold in lower concentrations of urea to produce the active protein. After purification and renaturation, the rhHBx protein was found to be phosphorylated by protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). In vivo phosphorylation of HBx was also demonstrated. Although PKC and MAPK enhance the HBx phosphorylation in vitro, neither protein kinase A nor caseine kinase II (CKII) phosphorylate HBx protein, though there are possible substrate residues of both kinases in HBx protein. Phosphoamino acid analysis of the total acid hydrolyzed HBx showed that serine residues can be phosphorylated by PKC or MAPK.

Mediator complexes and transcription

Over the past decade, various components of the transcription machinery have been identified as potential targets for activators. Recently, metazoan versions of yeast Mediator have been isolated and found to act as key coactivators to many transcription factors. Recent work has defined the composition, function and biology of metazoan mediator complexes, which has led us to propose a new nomenclature for the variously named versions of the mediator complex.

p300-mediated acetylation of human transcriptional coactivator PC4 is inhibited by phosphorylation

The human positive coactivator 4 (PC4) acts as a general coactivator for activator-dependent transcription, the activity of which is regulated negatively by phosphorylation. We report here that PC4 can be acetylated specifically by another coactivator, p300. Interestingly, phosphorylation of PC4 by casein kinase II inhibits the p300-mediated acetylation. Mass spectral analysis revealed that there are at least two lysine residues acetylated in PC4, as a result of which its DNA binding activity is stimulated.

[Isolation and identification of PACE-binding protein rpn4--a new transcription activator, participating in regulation of 26S proteosome and other genes]

A new upstream activating sequence (UAS) 5'-GGTGGCAAA-3' was detected in the promoters of 27 out of the 33 proteasomal genes and of several genes related to the ubiquitin-proteasomal system of Saccharomices cerevisiae. The sequence was termed proteasome-associated control element (PACE). Gel retardation assay revealed specific binding of PACE with an extracted protein. The protein (64K) was purified by affinity chromatography and was homogeneous by SDS-PAGE. Microsequencing showed that the protein is Rpn4p. The ability of Rpn4p to activate transcription was demonstrated with constructs containing fragments of the RPN5 and CDC48 gene promoters and reporter cat. Binding to PACE, Rpn4p may act as a common transcription factor on the proteasomal and proteasome-related genes.

Expression of deletion mutants of the hepatitis B virus protein HBx in E. coli and characterization of their RNA binding activities

The hepatitis B virus protein HBx has been implicated in the development of liver cancer. It has been shown that the HBx protein is able to bind to single-stranded DNA in a specific manner. This DNA binding activity might be relevant for HBx oncogene character. To study the HBx interaction with nucleic acids in more detail we expressed full-length HBx as well as several N- and C-terminally truncated HBx proteins as 6xHis and GST-fusions in E. coli. Using a gel shift assay, we were able to demonstrate that all of the truncated HBx proteins have the ability to bind to an AU-rich RNA. The affinity of GST-HBx #3 (residues 80-142) was an order of magnitude higher than that of GST-HBx #2 (residues 5-79), indicating that a high affinity RNA binding site is located in HBx C-terminal half. AUF1 is the protein ligand that binds to AU-rich RNA regions present in certain proto-oncogene mRNAs and causes their rapid degradation. By a competitive binding experiment of AUF1 and HBx to the AU-rich RNA oligonucleotide, we show that HBx is able to displace AUF1 from its binding site on the RNA oligonucleotide. This new aspect of HBx function is discussed in the context of cellular transformation.

Fission yeast nascent polypeptide-associated complex binds to four-way DNA junctions

The four-way DNA junction (X-junction) is both a central intermediate of recombination reactions and, in some cases, a controlling element in transcription and the initiation of DNA replication. Many different proteins have been found to bind to X-junctions in a structure-specific manner. In some cases, this ability only reflects the proteins' general predilection for distorted DNAs but in others the interaction is highly specific and usually signifies that the X-junction is the real target for the protein in vivo. Here we identify the Schizosaccharomyces pombe (Sp) nascent polypeptide associated complex (NAC) as a potent binder of X-junction DNA. NAC is highly conserved in eukaryotes and has reported functions in transcription and the targeting of proteins within the cytosol. NAC is composed of alpha and beta subunits. Each SpNAC subunit has the capacity to bind X-junction DNA, but optimal binding depends on a heterodimer of subunits. Competition assays and binding comparisons using a range of different DNA substrates reveal that SpNAC is highly selective for the X-junction structure. By comparative gel electrophoresis we show that the X-junction is held in its open square conformation when bound by SpNAC. Junction binding is inhibited by concentrations of magnesium ions that are sufficient to "stack" the X-junction, suggesting that SpNAC recognises only the open junction structure. Finally, SpNAC can bind to X-junctions that are already bound by a tetramer of the Escherichia coli RuvA protein, indicating that it interacts with only one face of the junction. The possible biological significance of X-junction binding by SpNAC is discussed.

Identification and characterization of a novel OCA-B isoform. implications for a role in B cell signaling pathways

OCA-B is a B lymphocyte-specific transcription coactivator that mediates tissue- and stage-restricted transcription of immunoglobulin genes. Earlier genetic studies revealed that OCA-B is essential for germinal center formation and production of secondary immunoglobulin isotypes. Biochemically purified OCA-B contains p35 and p34 isoforms, and a further analysis has now revealed that p35 is derived from a newly found isoform, p40. More importantly, it has been found that p35 is myristoylated in vivo and that this leads to dramatic changes (including localization to membrane compartments) in its properties. These results suggest that the p35 isoform of OCA-B has functions distinct from those of the nuclear p34 and that it might be a component of a signaling pathway that is required for late-stage B cell development.

Isolation and characterization of a zebrafish homologue of the cone rod homeobox gene

PURPOSE

To isolate and characterize a zebrafish CRX: homologue. Mammalian CRX: genes are expressed specifically in photoreceptors and pinealocytes, regulate photoreceptor gene expression, are necessary for normal photoreceptor differentiation, and when mutated cause a variety of photoreceptor degenerations.

METHODS

A zebrafish retinal cDNA library was screened with a human CRX cDNA probe. Radiation hybrid mapping, Northern blot analysis, in situ hybridization, and transient transfection studies were performed using standard methods.

RESULTS

Based on amino acid sequence comparisons, zebrafish crx shows 50% identity with human CRX, and 85% identity in the homeodomain. A phylogenetic analysis indicates that zebrafish crx is most closely related to the mammalian Crx proteins, and more distantly related to the Otx proteins. Zebrafish crx maps between 49.6 and 54.5 cm from the top of linkage group LG05C, a map position consistent with the location of the mouse and human CRX genes. Northern blot analysis and in situ hybridization indicate that zebrafish crx is expressed in the retina and pineal gland. In adult zebrafish, crx is expressed by both rods and cones in the outer nuclear layer, and in cells in the outer zone of the inner nuclear layer, in the region occupied by bipolar cells. Similar to mammalian Crx, zebrafish crx interacts with neural retinal leucine zipper (Nrl) to activate, although weakly, rhodopsin promoter activity.

CONCLUSIONS

Based on molecular phylogeny, chromosomal location, expression pattern, and ability to activate rhodopsin promoter activity in transient transfection assays, zebrafish crx appears to be an orthologue and functional homologue of mammalian CRX:

Isolation, characterization, and expression analysis of zebrafish large Mafs

Large Maf proteins, which are members of the basic leucine zipper (b-Zip) superfamily, are involved in the determination and control of cellular differentiation. The expression patterns of various vertebrate large Maf mRNAs were described previously. Here, we report the cloning of a novel zebrafish large Maf cDNA, SMaf1 (Somite Maf1), and other zebrafish large Mafs, the N-terminus domains of which possess transactivational activity. We also analyzed the expression patterns of SMaf1 and SMaf2 (Somite Maf2)/Krml2 as well as MafB/Val and c-Maf during zebrafish embryogenesis. In particular, the robust expression of the novel SMaf1 mRNA, which overlapped that of MyoD, in somitic cells during somitogenesis was noteworthy. In addition, the expression patterns of SMaf2 and MafB in the blood-forming regions, and those of c-Maf and MafB in the lens cells showed spatial redundancy, although the temporal appearance of these genes at these sites differed. These data indicate that SMafs may play important roles in somitogenesis, and that Maf proteins may have overlapping and yet specific functions as to the determination and differentiation of cell lineages.

Recombinant expression and purification of smad proteins

Smads transduce intracellular signals initiated by members of the transforming growth factor beta (TGF beta) family, including activins, TGF betas, and bone morphogenetic proteins. Recently, various models concerning the mechanism of Smad action have been proposed; however, these models are basically qualitative. Quantitative verification of the validity of the models requires significant amounts of purified Smad proteins, but purification of full-length Smad protein has not been straightforward even using recombinant protein expression systems. Here, we report purification of Smad proteins expressed in E. coli as glutathione S-transferase-fused proteins. By glutathione-Sepharose affinity purification, ATP treatment, DEAE-Sepharose and hydroxylapatite columns, expressed Smads were purified to near homogeneity as judged by SDS-PAGE; protein recovery was ca. 1 mg/l culture for Smad2 and 100 microg/l culture for Smad4. The purified Smad proteins had three known in vitro activities: Smad2 phosphorylation by TGF beta receptor complexes immunoprecipitated from COS7 cells, Smad4 binding to Smad-binding DNA element, and Smad2 interaction with calmodulin. The data suggest that purified proteins could be useful for biochemical analyses to evaluate the current models quantitatively.

The alpha and beta subunit of the nascent polypeptide-associated complex have distinct functions

Nascent polypeptide-associated complex (NAC) is probably the first cytosolic protein to contact nascent polypeptide chains emerging from ribosomes. In this way NAC prevents inappropriate interactions with other factors. Eventually other factors involved in targeting and folding, like the Signal Recognition Particle or cytosolic chaperones, must gain access to the nascent chain. All NAC preparations to date consist of two copurifying polypeptides. Here we rigorously show that these two polypeptides, termed alpha- and betaNAC, form a very stable complex in vivo and in vitro and that a functional complex can be reconstituted from the individual subunits. A dissection of the contributions of the individual subunits to NACs function revealed that both subunits are in direct contact with nascent polypeptide chains on the ribosome and that both contribute to the prevention of inappropriate interactions. However, betaNAC alone directly binds to the ribosome and is sufficient to prevent ribosome binding to the endoplasmic reticulum membrane.

Functions of the subunits in the FlhD(2)C(2) transcriptional master regulator of bacterial flagellum biogenesis and swarming

In enterobacteria like Salmonella, biogenesis of cell surface flagella needed for motility is dependent upon the master operon flhDC at the apex of the flagellar gene hierarchy. The operon products FlhD and FlhC act together in a FlhD(2)C(2 )heterotetramer to induce flagellar gene transcription, while FlhD also represses cell septation. The flhDC operon is pivotal to differentiation into elongated hyperflagellated swarm cells that undergo multicellular migration, most strikingly in Proteus. We set out to establish the mechanism of action of the FlhD(2)C(2) multimer. In Proteus swarm cell extracts, all the FlhC was assembled into the FlhD(2)C(2 )transcription activator, but FlhD additionally formed approximately equimolar amounts of a FlhD(2) homodimer. Both FlhD and FlhC subunits homodimerised in vivo and in vitro, suggesting that self-interactions stabilise the FlhD(2)C(2 )complex. The FlhC and FlhD subunit proteins were separately expressed and purified, and the FlhD(2)C(2)heterotetramer was reconstituted in vitro. Purified FlhC bound specifically and cooperatively to the promoter region of the flhDC-regulated flhB flagellar gene in the absence of FlhD. Purified FlhD was unable to bind this target DNA, but binding by the FlhD(2)C(2)complex was approximately tenfold greater than the FlhC subunit alone, suggesting that FlhD potentiated the FlhC/DNA interaction. In support of this possibility, pre-incubation of FlhC with FlhD reduced the apparent dissociation constant, K(D), for the FlhC/DNA complex from 100 nM to 13 nM. Furthermore, in competition assays, FlhD substantially increased the specificity of DNA recognition by FlhC, and also stabilised the resultant labile protein/DNA complex, prolonging its half-life from around two minutes to more than 40 minutes. FlhD(2)C(2)is therefore an atypical prokaryotic transcription activator in which interaction of the FlhC subunit with DNA target sequences is enhanced by the coexpressed helper subunit FlhD.

Functional uncoupling of the Janus kinase 3-Stat5 pathway in malignant growth of human T cell leukemia virus type 1-transformed human T cells

Human T cell leukemia virus type 1 (HTLV-1) transforms cytokine-dependent T lymphocytes and causes adult T cell leukemia. Janus tyrosine kinase (Jak)3 and transcription factors Stat5a and Stat5b are essential for the proliferation of normal T cells and are constitutively hyperactivated in both HTLV-1-transformed human T cell lines and lymphocytes isolated from HTLV-1-infected patients; therefore, a critical role for the Jak3-Stat5 pathway in the progression of this disease has been postulated. We recently reported that tyrphostin AG-490 selectively blocked IL-2 activation of Jak3/Stat5 and growth of murine T cell lines. Here we demonstrate that disruption of Jak3/Stat5a/b signaling with AG-490 (50 microM) blocked the proliferation of primary human T lymphocytes, but paradoxically failed to inhibit the proliferation of HTLV-1-transformed human T cell lines, HuT-102 and MT-2. Structural homologues of AG-490 also inhibited the proliferation of primary human T cells, but not HTLV-1-infected cells. Disruption of constitutive Jak3/Stat5 activation by AG-490 was demonstrated by inhibition of 1) tyrosine phosphorylation of Jak3, Stat5a (Tyr(694)), and Stat5b (Tyr(699)); 2) serine phosphorylation of Stat5a (Ser(726)) as determined by a novel phosphospecific Ab; and 3) Stat5a/b DNA binding to the Stat5-responsive beta-casein promoter. In contrast, AG-490 had no effect on DNA binding by p50/p65 components of NF-kappaB, a transcription factor activated by the HTLV-1-encoded phosphoprotein, Tax. Collectively, these data suggest that the Jak3-Stat5 pathway in HTLV-1-transformed T cells has become functionally redundant for proliferation. Reversal of this functional uncoupling may be required before Jak3/Stat5 inhibitors will be useful in the treatment of this malignancy.

Cloning and identification of regulatory gene UL76 of human cytomegalovirus

The major immediate-early promoter/enhancer (MIEP, -1139 to +52) of human cytomegalovirus (HCMV) is regulated by cell type-specific transcriptional factors, its own MIE proteins (IE2p40, IE1p55, IE1p72 and IE2p86) as well as viral proteins pUL69, pUL82 and pUL84. To investigate the hypothesis that the regulation of HCMV MIEP is modulated by additional viral genes, HCMV (AD169) genomic sublibraries were constructed and in vitro transient co-transfection assays were performed to assess the ability of these sublibraries to modulate MIEP expression. In this study, enhancement of MIEP expression was exhibited by a number of sublibraries, from one of which a genomic clone was selected for augmentation of expression. Subcloning the insert fragment led to the identification of the responsible locus, UL76. To generate a UL76-specific antibody for immunodetection, the UL76 ORF was constructed as a histidine-tagged fusion protein that was produced in prokaryotic cells. A polyclonal antibody raised against the UL76 fusion protein immunoreacts with a protein of 38 kDa (pUL76) in UL76 ORF-transfected cells. Additionally, pUL76 is present in HCMV-infected cells at the immediate-early to late stages of the reproductive cycle. Characterized by its highly basic composition (predicted pI 11.6), a free form of pUL76 tagged with green fluorescent protein was found to localize exclusively to the nucleus. In this report, pUL76 is defined as a novel regulatory protein that modulates both activation and repression of gene expression, depending on the promoter context and the ratio of transfected effector DNA.

An in vitro transcription system that recapitulates equine infectious anemia virus tat-mediated inhibition of human immunodeficiency virus type 1 Tat activity demonstrates a role for positive transcription elongation factor b and associated proteins in the mechanism of Tat activation

Equine infectious anemia virus (EIAV) activates transcription via a Tat protein, a TAR element, and the equine elongation factor positive transcription elongation factor b (P-TEFb). In human cells, EIAV Tat (eTat) can inhibit the ability of human immunodeficiency virus type 1 (HIV-1) Tat (hTat) to activate transcription from the HIV-1 long terminal repeat, demonstrating that EIAV Tat can interact nonproductively with human P-TEFb. To study the mechanism of EIAV Tat and HIV-1 Tat activation, we developed an in vitro elongation assay that recapitulates EIAV Tat-mediated inhibition of HIV-1 Tat trans-activation. We found that eTat specifically inhibits activation of elongation by HIV-1 Tat while having no effect on basal transcription elongation. The competitive inhibition of hTat activation was reversed by an activity present in HeLa cell nuclear extracts, most likely a form of P-TEFb. Recombinant P-TEFb (cyclin T1 and CDK9) overcame the inhibition of transcription by eTat but in a nonspecific manner. EIAV Tat affinity chromatography was used to purify the activity present in nuclear extract that was capable of reversing eTat inhibition. We characterized the protein components of this activity, which include cyclin T1, CDK9, Tat-SF1, and at least three unidentified proteins. These data suggest that additional factors are involved in the mechanism of Tat activation.