Sequences in the polyomavirus DNA regulatory region involved in viral DNA replication and early gene expression

The Major Capsid Protein, VP1, of the Mouse Polyomavirus Stimulates the Activity of Tubulin Acetyltransferase 1 by Microtubule Stabilization

Viruses have evolved mechanisms to manipulate microtubules (MTs) for the efficient realization of their replication programs. Studying the mechanisms of replication of mouse polyomavirus (MPyV), we observed previously that in the late phase of infection, a considerable amount of the main structural protein, VP1, remains in the cytoplasm associated with hyperacetylated microtubules. VP1–microtubule interactions resulted in blocking the cell cycle in the G2/M phase. We are interested in the mechanism leading to microtubule hyperacetylation and stabilization and the roles of tubulin acetyltransferase 1 (αTAT1) and deacetylase histone deacetylase 6 (HDAC6) and VP1 in this mechanism. Therefore, HDAC6 inhibition assays, αTAT1 knock out cell infections, in situ cell fractionation, and confocal and TIRF microscopy were used. The experiments revealed that the direct interaction of isolated microtubules and VP1 results in MT stabilization and a restriction of their dynamics. VP1 leads to an increase in polymerized tubulin in cells, thus favoring αTAT1 activity. The acetylation status of MTs did not affect MPyV infection. However, the stabilization of MTs by VP1 in the late phase of infection may compensate for the previously described cytoskeleton destabilization by MPyV early gene products and is important for the observed inhibition of the G2→M transition of infected cells to prolong the S phase.

VP1, the major capsid protein of the mouse polyomavirus, binds microtubules, promotes their acetylation and blocks the host cell cycle

VP1, the major structural protein of the mouse polyomavirus (MPyV), is the major architectural component of the viral capsid. Its pentamers are able to self-assemble into capsid-like particles and to non-specifically bind DNA. Surface loops of the protein interact with sialic acid of ganglioside receptors. Although the replication cycle of the virus, including virion morphogenesis, proceeds in the cell nucleus, a substantial fraction of the protein is detected in the cytoplasm of late-phase MPyV-infected cells. In this work, we detected VP1 mainly in the cytoplasm of mammalian cells transfected with plasmid expressing VP1. In the cytoplasm, VP1-bound microtubules, including the mitotic spindle, and the interaction of VP1 with microtubules resulted in cell cycle block at the G2/M phase. Furthermore, in the late phase of MPyV infection and in cells expressing VP1, microtubules were found to be hyperacetylated. We then sought to understand how VP1 interacts with microtubules. Dynein is not responsible for the VP1-microtubule association, as neither overexpression of p53/dynamitin nor treatment with ciliobrevin-D (an inhibitor of dynein activity) prevented binding of VP1 to microtubules. A pull-down assay for VP1-interacting proteins identified the heat shock protein 90 (Hsp90) chaperone, and Hsp90 was also detected in the VP1-microtubule complexes. Although Hsp90 is known to be associated with acetylated microtubules, it does not mediate the interaction between VP1 and microtubules. Our study provides insight into the role of the major structural protein in MPyV replication, indicating that VP1 is a multifunctional protein that participates in the regulation of cell cycle progression in MPyV-infected cells.

The encapsidation of polyomavirus is not defined by a sequence-specific encapsidation signal

Mouse polyomavirus (MPyV) is considered a potential tool for the application of gene therapy; however, the current knowledge of the encapsulation of DNA into virions is vague. We used a series of assays based on the encapsidation of a reporter vector into MPyV pseudovirions to identify putative cis-acting elements that are involved in DNA encapsidation. None of the sequences that were derived from MPyV have been shown to solely enhance the encapsidation of a reporter vector in the assay. The frequency of encapsidation strongly correlated with the total intracellular amount of the vector after transfection. The encapsidation of target DNA into the pseudovirions was shown to be non-specific, and the packaging of non-replicated DNA was observed. We propose that the actual concentration of target DNA at the sites of virion formation is the primary factor that determines its selection for encapsidation.

RNA processing in the polyoma virus life cycle

Not only is gene regulation in polyoma interesting, but it has also proven to be highly informative and illustrative of a number of novel concepts in gene regulation. Of special interest and importance are the mechanisms by which this virus switches from the expression of early gene products to late gene products after the onset of viral DNA replication. This switch is mediated at least in part by changes in transcription elongation and polyadenylation in the late region, and by the formation and editing of dsRNA in the nucleus. In this review we will summarize the regulation of RNA synthesis and processing during polyoma infection, and will point out in particular those aspects that have been most novel.

Use of monoclonal antibodies for expression cloning

This unit details the use of transient expression in mammalian cells to screen cDNA libraries with monoclonal antibodies (MAb) to isolate cDNA clones encoding cell-surface and intracellular proteins. The first protocol describes the cloning of cDNAs encoding cell-surface antigens. The second protocol is a modification that facilitates isolation of cDNAs encoding antigens that are expressed intracellularly. Both protocols are designed for use with the expression vector CDM8, which contains a polylinker for subcloning double-stranded cDNA.

Transient expression of proteins using COS cells

This unit describes the use of COS cells to efficiently produce a desired protein in a short period of time. These cells express high levels of the SV40 large tumor (T) antigen, which is necessary to initiate viral DNA replication at the SV40 origin. Each COS cell transfected with DNA encoding a cell-surface antigen (in the appropriate vector) or cytoplasmic protein will express several thousand to several hundred thousand copies of the protein 72 hr posttransfection. If the transfected DNA encodes a secreted protein, up to 10 mg of protein can be recovered from the supernatant of the transfected COS cells 1 week posttransfection. COS cell transient expression systems have also been used to screen cDNA libraries, to isolate cDNAs encoding cell-surface proteins, secreted proteins, and DNA binding proteins, and to test protein expression vectors rapidly prior to the preparation of stable cell lines.

Use of monoclonal antibodies for expression cloning

This unit details the use of transient expression in mammalian cells to screen cDNA libraries with monoclonal antibodies (MAb) to isolate cDNA clones encoding cell-surface and intracellular proteins. The first protocol in this unit describes the cloning of cDNAs encoding cell-surface antigens. Several steps in this protocol involve transfection procedures that are described in greater detail elsewhere in this volume. The second protocol is a modification that facilitates isolation of cDNAs encoding antigens that are expressed intracellularly. Both protocols are designed for use with the expression vector CDM8, which contains a polylinker for subcloning double-stranded cDNA.

Cryptic CTL epitope on a murine sarcoma Meth A generated by exon extension as a novel mechanism

Using the recently developed ELISPOT cloning methodology, we obtained cDNA clone S35 coding for the Ag epitope recognized by a murine sarcoma Meth A-specific CTL clone AT-1. Analysis of truncated S35 constructs and overlapping peptides revealed that the peptide epitope was LGAEAIFRL. AT-1 CTL lysed peptide-pulsed CMS8 cells at a nanomolar concentration, and the peptide strongly stimulated IFN-gamma production in AT-1 CTL. Sequence homology indicated that the S35 was derived from a mouse homologue of human retinoic acid-regulated nuclear matrix-associated protein (ramp). The ramp gene consisted of 15 exons. The majority of the ramp mRNA was the transcript normally spliced between exons 14 and 15, but a minor population of mRNA with an extended exon 14 was also present in Meth A cells. The epitope was derived from the newly created open reading frame, which resulted from extension of exon 14 after splicing of the adjacent intronic sequence.

Role of middle T-small T in the lytic cycle of polyomavirus: control of the early-to-late transcriptional switch and viral DNA replication

A comparative analysis of the lytic cycle of wild-type polyomavirus and middle T and small T defective mutants was carried out in the A2 genetic background. The results contrast with those obtained in comparisons between the hr-t type and their middle-T small-T-producing partners as previously described (20). The A2-derived mutants were found to share the maturation defect previously described for the hr-t mutants. However, their defect in DNA replication was more acute, resulting in a 5- to 100-fold decrease in the accumulation of viral genomes. Furthermore, their gene expression pattern was affected. A2-derived mutants displayed an early defect resulting in a 4- to 16-h delay in the expression of large T, and an alteration of the early-to-late transcriptional switch. In wild-type A2 infection, this switch is characterized by a large increase in the accumulation of early transcripts followed by late transcripts after the appearance of middle T and small T proteins and the onset of viral DNA replication (L. Chen and M. M. Fluck, J. Virol. 75: 8368-8379, 2001). In the mutant infection, increases in both classes of transcripts were delayed and reduced, but the effect on early transcripts was more pronounced. As has been described previously for the hr-t mutants (E. Goldman, J. Hattori, and T. Benjamin, Cell 13:505-513, 1979), the magnitude of these defects depended upon experimental conditions. Experiments using cytosine beta-arabinofuranoside to reduce genome amplification suggest that the effect of middle T-small T on the transcriptional switch is not solely mediated by the effect of these protein(s) on increasing the number of templates. These data provide the first direct demonstration of an effect of middle T and/or small T in the viral transcription pattern during viral infection. The results agree with previous results obtained with plasmid reporters and with our understanding that the downstream targets of the middle T signaling pathway include three transcription factors that have binding sites in the enhancer domain that play a key regulatory role in the expression of the viral genes.

Kinetic analysis of the steps of the polyomavirus lytic cycle

Kinetic studies of the accumulation of early and late transcripts, early and late proteins, genomes, and live virus, during the lytic cycle of murine polyomavirus wild-type A2, were carried out in synchronized NIH 3T3 cells released from G(0) by the addition of serum after infection. This first-time simultaneous analysis of all parameters of the virus life cycle led to new insights concerning the transcriptional control at the early-to-late transition. During the early phase, early transcripts were synthesized at very low levels, detectable only by reverse transcription-PCR, from 6 h postinfection (hpi). Large T protein could be detected by 8 hpi (while infected cells were in the G(1) phase). The level of expression of the middle T and small T proteins was lower than that of large T at all times, due, at least in part, to a splicing preference for the large-T 5' splice site at nucleotide 411. A large increase in the level of both early and late transcripts coincided closely with the detection in mid-S phase of viral genome amplification. Thereafter, both classes of transcripts continued to further accumulate up to the end of the experiments (48 hpi). In addition, during the late phase, "giant" multigenomic transcripts were synthesized from the early as well as the late promoter. Thus, a major type of transcriptional control appears to be applied similarly to the transcription of both early and late genes. This view differs from that in the literature, which highlights the enhancement of late transcription and the repression of early transcription. However, despite this parallel transcriptional control, additional regulations are applied which result in higher levels of late compared to early transcripts, as previously described. In the accompanying article, a key role for middle T and/or small T in this late-phase enhancement of early and late transcription is demonstrated (16). Other novel findings, e.g., the synthesis of a very abundant short early promoter proximal RNA, are also described.

Transplacental delivery of the Wnt antagonist Frzb1 inhibits development of caudal paraxial mesoderm and skeletal myogenesis in mouse embryos

Axial structures (neural tube/notochord) and surface ectoderm activate myogenesis in the mouse embryo; their action can be reproduced, at least in part, by several molecules such as Sonic hedgehog and Wnts. Recently, soluble Wnt antagonists have been identified. Among those examined only Frzb1 was found to be expressed in the presomitic mesoderm and newly formed somites and thus its possible role in regulating myogenesis was investigated in detail. When presomitic mesoderm or newly formed somites were cultured with axial structures and surface ectoderm on a feeder layer of C3H10T1/2 cells expressing Frzb1, myogenesis was abolished or severely reduced in presomitic mesoderm and the three most recently formed somites. In contrast, no effect was observed on more mature somites. Inhibition of myogenesis did not appear to be associated with increased cell death since the final number of cells in the explants grown in the presence of Frzb1 was only slightly reduced in comparison with controls. In order to examine the possible function of Frzb1 in vivo, we developed a method based on the overexpression of the soluble antagonist by transient transfection of WOP cells with a Frzb1 expression vector and injection of transfected cells into the placenta of pregnant females before the onset of maternofoetal circulation. Frzb1, secreted by WOP cells, accumulated in the embryo and caused a marked reduction in size of caudal structures. Myogenesis was strongly reduced and, in the most severe cases, abolished. This was not due to a generalized toxic effect since only several genes downstream of the Wnt signaling pathway such as En1, Noggin and Myf5 were downregulated; in contrast, Pax3 and Mox1 expression levels were not affected even in embryos exhibiting the most severe phenotypes. Taken together, these results suggest that Wnt signals may act by regulating both myogenic commitment and expansion of committed cells in the mouse mesoderm.

APCs Express DCIR, a Novel C-Type Lectin Surface Receptor Containing an Immunoreceptor Tyrosine-Based Inhibitory Motif

We have identified a novel member of the calcium-dependent (C-type) lectin family. This molecule, designated DCIR (for dendritic cell (DC) immunoreceptor), is a type II membrane glycoprotein of 237 aa with a single carbohydrate recognition domain (CRD), closest in homology to those of the macrophage lectin and hepatic asialoglycoprotein receptors. The intracellular domain of DCIR contains a consensus immunoreceptor tyrosine-based inhibitory motif. A mouse cDNA, encoding a homologous protein has been identified. Northern blot analysis showed DCIR mRNA to be predominantly transcribed in hematopoietic tissues. The gene encoding human DCIR was localized to chromosome 12p13, in a region close to the NK gene complex. Unlike members of this complex, DCIR displays a typical lectin CRD rather than an NK cell type extracellular domain, and was expressed on DC, monocytes, macrophages, B lymphocytes, and granulocytes, but not detected on NK and T cells. DCIR was strongly expressed by DC derived from blood monocytes cultured with GM-CSF and IL-4. DCIR was mostly expressed by monocyte-related rather than Langerhans cell related DC obtained from CD34+ progenitor cells. Finally, DCIR expression was down-regulated by signals inducing DC maturation such as CD40 ligand, LPS, or TNF-α. Thus, DCIR is differentially expressed on DC depending on their origin and stage of maturation/activation. DCIR represents a novel surface molecule expressed by Ag presenting cells, and of potential importance in regulation of DC function.

A Monoclonal Antibody Reactive with a 40-kDa Molecule on Fetal Thymocytes and Tumor Cells Blocks Proliferation and Stimulates Aggregation and Apoptosis

E710.2.3 is a murine thymic lymphoma cell line with an immature phenotype (CD4−CD8−) that proliferates in response to thymocytes or PMA when cultured at low density and proliferates spontaneously when grown at high density. To identify functional molecules on this cell line, we screened for mAbs that could block its proliferation. A hamster mAb, DMF10.62.3, inhibited the spontaneous, thymocyte-induced, and PMA-stimulated proliferation of E710.2.3 in vitro and induced these cells to undergo apoptosis. The mAb also caused homotypic aggregation of E710.2.3, which was inhibited by cytochalasin B, trifluoperazine, a combination of sodium azide and 2-deoxyglucose, EDTA, incubation at 4°C, or treatment with paraformaldehyde. The DMF10 62.3 mAb stained a number of immortalized murine and human cell lines and, where tested, blocked their proliferation and caused death to varying extents by apoptosis. The molecule recognized by the mAb DMF10.62.3 was expressed on day 14 fetal thymus Thy1.2-positive cells. However, it was not detected on adult murine thymocytes, splenocytes, or bone marrow cells or on splenic LPS-activated B cells or Con A-activated T cells. The Ab immunoprecipitated a 40-kDa molecule from E710.2.3 that was not glycosylphosphatidylinositol linked. The data suggest that the molecule recognized by DMF62.3 is a novel cell surface molecule that may be involved in cell proliferation and/or cell death.

Mechanisms of cell transformation induced by polyomavirus

Polyomavirus is a DNA tumor virus that induces a variety of tumors in mice. Its genome encodes three proteins, namely large T (LT), middle T (MT), and small T (ST) antigens, that have been implicated in cell transformation and tumorigenesis. LT is associated with cell immortalization, whereas MT plays an essential role in cell transformation by binding to and activating several cytoplasmic proteins that participate in growth factor-induced mitogenic signal transduction to the nucleus. The use of different MT mutants has led to the identification of MT-binding proteins as well as analysis of their importance during cell transformation. Studying the molecular mechanisms of cell transformation by MT has contributed to a better understanding of cell cycle regulation and growth control.

Expression cloning of mouse cDNA of CMP-NeuAc:Lactosylceramide alpha2,3-sialyltransferase, an enzyme that initiates the synthesis of gangliosides

Expression cloning of a cDNA for the alpha2,3-sialyltransferase (GM3 synthase) (EC 2.4.99.-) gene was performed using a GM3-lacking mouse fibroblast line L cell and anti-GM3 monoclonal antibody. Plasmids from a cDNA library generated with poly(A)+ RNA of a mouse fibrosarcoma line CMS5j and pdl3027 (polyoma T antigen) were co-transfected into L cells. The isolated cDNA clone pM3T-7 predicted a type II membrane protein with 13 amino acids of cytoplasmic domain, 17 amino acids of transmembrane region, and a large catalytic domain with 329 amino acids. Introduction of the cDNA clone into L cells resulted in the neo-synthesis of GM3 and high activity of alpha2,3-sialyltransferase. Among glycosphingolipids, only lactosylceramide showed significant activity as an acceptor, indicating that this gene product is a sialyltransferase specific for the synthesis of GM3. An amino acid sequence deduced from the cloned cDNA showed the typical sialyl motif with common features among alpha2,3-sialyltransferases. Among various mouse tissues, brain, liver, and testis showed relatively high expression of a 2.3-kilobase mRNA, whereas all tissues, more or less, expressed this gene.

Enhanced binding to origin DNA at low pH enables easy detection of polyomavirus large T antigen by gel mobility shift assay of unfixed complexes

Enhanced, stable binding by polyomavirus large T antigen to the viral DNA replication origin at pH 6 allowed the development of a gel mobility shift assay for the detection of large T antigen. Such assays were not possible at pH 7.6 without previous fixation, due to instability of the complexes. We demonstrated that the gel mobility shift assay at pH 6 is very sensitive, allowing the detection of as little as 5 ng large T antigen, and is highly specific for DNA containing G(A/G)GGC target sequences. This method was used to detect large T antigen in crude cell lysates from transformed yeast cell lines or nuclear extracts from infected insect cells. Large T antigen-DNA complexes remained at or near the loading well in 5% acrylamide or 1.5% agarose gels, indicating that these complexes are very large. Glycerol gradient analysis showed that protein-DNA complexes formed at pH 6 were massive, and that large T antigen also formed large complexes when incubated at low pH in the absence of DNA. These results show that pH has a major effect on binding of large T antigen to its multiple target sites in the viral origin of DNA replication, presumably by affecting protein-protein interactions that are important for the stability of large T antigen-DNA complexes.

Binding of hepatitis C virus to CD81

Chronic hepatitis C virus (HCV) infection occurs in about 3 percent of the world's population and is a major cause of liver disease. HCV infection is also associated with cryoglobulinemia, a B lymphocyte proliferative disorder. Virus tropism is controversial, and the mechanisms of cell entry remain unknown. The HCV envelope protein E2 binds human CD81, a tetraspanin expressed on various cell types including hepatocytes and B lymphocytes. Binding of E2 was mapped to the major extracellular loop of CD81. Recombinant molecules containing this loop bound HCV and antibodies that neutralize HCV infection in vivo inhibited virus binding to CD81 in vitro.

Polyomavirus large T antigen binds cooperatively to its multiple binding sites in the viral origin of DNA replication

Polyomavirus large T antigen binds to multiple 5'-G(A/G)GGC-3' pentanucleotide sequences in sites 1/2, A, B, and C within and adjacent to the origin of viral DNA replication on the polyomavirus genome. We asked whether the binding of large T antigen to one of these sites could influence binding to other sites. We discovered that binding to origin DNA is substantially stronger at pH 6 to 7 than at pH 7.4 to 7.8, a range often used in DNA binding assays. Large T antigen-DNA complexes formed at pH 6 to 7 were stable, but a fraction of these complexes dissociated at pH 7.6 and above upon dilution or during electrophoresis. Increased binding at low pH is therefore due at least in part to increased stability of protein-DNA complexes, and binding at higher pH values is reversible. Binding to fragments of origin DNA in which one or more sites were deleted or inactivated by point mutations was measured by nitrocellulose filter binding and DNase I footprinting. The results showed that large T antigen binds cooperatively to its four binding sites in viral DNA, suggesting that the binding of this protein to one of these sites stabilizes its binding to other sites via protein-protein contacts. Sites A, B, and C may therefore augment DNA replication by facilitating the binding of large T antigen to site 1/2 at the replication origin. ATP stabilized large T antigen-DNA complexes against dissociation in the presence, but not the absence, of site 1/2, and ATP specifically enhanced protection against DNase I digestion in the central 10 to 12 bp of site 1/2, at which hexamers are believed to form and begin unwinding DNA. We propose that large T antigen molecules bound to these multiple sites on origin DNA interact with each other to form a compact protein-DNA complex and, furthermore, that ATP stimulates their assembly into hexamers at site 1/2 by a "handover" mechanism mediated by these protein-protein contacts.

CD40L activation of dendritic cells down-regulates DORA, a novel member of the immunoglobulin superfamily

Using a cDNA subtraction technique, a novel member of the immunoglobulin superfamily was isolated from human Dendritic cells (DC). This cDNA which we named DORA, for DOwn-Regulated by Activation encodes a protein belonging to the CD8 family of receptors containing a single V type loop domain with an associated J chain region, a transmembrane region containing an atypical tyrosine residue and a cytoplasmic domain containing three putative tyrosine phosphorylation sites. The hDORA gene has been localised to chromosome 16. From database searches a rat cDNA was identified that encoded a polypeptide with 63% identity to hDORA. The expression of the human cDNA was studied in detail. Northern blot analysis revealed 1.0 kb and 2.5 kb mRNAs in peripheral blood lymphocytes, spleen and lymph node, while low levels were observed in thymus, appendix, bone marrow and fetal liver. No signal was noted in non-immune system tissues. By RT-PCR analysis of hDORA revealed expression in cells committed to the myeloid lineage but not in CD34+ precursors or B cells and low expression in T cells. Expression was also observed in DC, purified ex vivo or generated in vitro from either monocytes or CD34+ progenitors. This was down-regulated following activation both by PMA and Ionomycin treatment and also by CD40L engagement. In situ hybridisation performed on tonsil sections showed the presence of hDORA in cells within Germinal Centers. This structure and expression suggests a function as a co-receptor, perhaps in an antigen uptake complex, or in homing or recirculation of DC.

HNMP-1: a novel hematopoietic and neural membrane protein differentially regulated in neural development and injury

The hnmp-1 (hematopoietic neural membrane protein) gene encodes a protein with striking similarity to the tetra-transmembrane-spanning protein encoded by pmp22. hnmp-1 was cloned from an elutriated human monocyte library and is expressed in various human hematopoietic and lymphoid lineages as well as adult mouse spleen and thymus. In the mouse nervous system, HNMP-1 mRNA is temporally expressed by Schwann cells during sciatic nerve myelination. Dorsal root ganglia sensory and spinal cord alpha-motoneurons acquire HNMP-1 protein selectively throughout development. In the fiber tracts of the spinal cord and in sciatic nerve, HNMP-1 protein is axon-associated. Additionally a rapid and sustained level of HNMP-1 expression is observed in response to acute PNS injury. HNMP-1 is constituitively induced in sciatic nerve of Trembler J mice, which are mutant for pmp22 and have a demyelinating/hypomyelinating phenotype. The expression pattern of HNMP-1 suggests a possible role for this molecule during active myelination.

HNMP-1: a novel hematopoietic and neural membrane protein differentially regulated in neural development and injury

The hnmp-1 (hematopoietic neural membrane protein) gene encodes a protein with striking similarity to the tetra-transmembrane-spanning protein encoded by pmp22. hnmp-1 was cloned from an elutriated human monocyte library and is expressed in various human hematopoietic and lymphoid lineages as well as adult mouse spleen and thymus. In the mouse nervous system, HNMP-1 mRNA is temporally expressed by Schwann cells during sciatic nerve myelination. Dorsal root ganglia sensory and spinal cord alpha-motoneurons acquire HNMP-1 protein selectively throughout development. In the fiber tracts of the spinal cord and in sciatic nerve, HNMP-1 protein is axon-associated. Additionally a rapid and sustained level of HNMP-1 expression is observed in response to acute PNS injury. HNMP-1 is constituitively induced in sciatic nerve of Trembler J mice, which are mutant for pmp22 and have a demyelinating/hypomyelinating phenotype. The expression pattern of HNMP-1 suggests a possible role for this molecule during active myelination.

Mutated mitogen-activated protein kinase: a tumor rejection antigen of mouse sarcoma

The molecular basis of the polymorphic tumor rejection antigens of chemically induced sarcomas of inbred mice remains a mystery, despite the discovery of these antigens over 40 years ago and their critical importance to the foundation of tumor immunology. In an analysis of a panel of BALB/c 3-methylcholanthrene-induced tumors, we identified one tumor, CMS5, that elicited a strong cytotoxic T cell response with exquisite specificity for CMS5. A stable cloned line of T cells with this specificity (C18) was used to screen a CMS5 cDNA expression library. The gene encoding the C18-defined antigen was identified as a mutated form of a mouse mitogen-activated protein kinase, ERK2, and a peptide incorporating the resulting amino acid substitution (lysine to glutamine) was efficiently recognized by C18. Vaccination with this peptide elicited specific resistance to CMS5 challenge. Extensive efforts to isolate antigen-loss variants of CMS5 were unsuccessful, suggesting that the mutated mitogen-activated protein kinase is essential for maintenance of the malignant phenotype.

Cell-type specific activation of the polyomavirus F9-1 regulatory region in transgenic mice

To characterize the activity of the polyomavirus regulatory region, two hybrid marker genes were constructed. In the first construct, the early promoter regulates expression of the CAT gene and the late promoter regulates expression of the lacZ gene. In the second construct, the lacZ gene was placed under the control of the early promoter. The fusion constructs were introduced into the mouse germline. Gene expression was analysed in the generated transgenic mice. A pronounced cell-type specific activation of the transcriptional control region was found in different tissues of the developing embryo and in the adult animal. The control region is recognized and activated in early preimplantation embryos. Around the time of implantation, sequential activation of the Py regulatory region was first observed in differentiating cells. Stage- and tissue-specific expression were noted later in embryonic development. Comparing reporter gene expression on the single-cell level, the different viral promoters display identical expression patterns throughout ontogenesis. Quantitative analysis revealed that marker gene expression from the late promoter was significantly higher than from the early promoter. Furthermore, the cell-type specificity of the control region is not altered in the presence of its regulatory protein, the LT.

Ultraviolet irradiation and c-jun over-expression regulates replication of polyoma sequences in WOP cells through a PEBP2 binding site

Mouse fibroblast cells (WOP) express permissive factors which support polyoma DNA replication. However, electroporation into WOP cells of a mammalian expression vector that encodes the c-jun cDNA results in repression of polyoma DNA replication in a dose-dependent manner. In previous studies we have shown that UV-irradiation is capable of mediating a similar effect on polyoma DNA replication. When c-jun over-expression was combined with ultraviolet (UV)-irradiation, polyoma DNA replication decreased further. The repression of replication mediated by c-jun appears to be mediated by factor(s) that bind to PEBP4/2 target sequences as oligomers bearing the PEBP2/4 target site were capable of restoring polyoma DNA replication when added to UV-treated or c-jun over-expressing cells. The binding to the PEBP2/4 is partially dependent on the availability of AP-1 proteins, since an AP-1 target sequence can efficiently compete one of the three complexes formed with the PEBP2 target site. PEPB2 sequences do not, however, affect binding to the AP1 site. The effect of PEBP2 on polyoma replication is not dependent on the adjacent AP-1 site since PEBP2 could restore replication of polyomavirus which is mutated at the AP-1 sequence. A similar replication pattern was noted in a deletion mutant of polyoma which lacks PEBP4, yet, contains an intact PEBP2 binding sequence, suggesting that PEBP2 is the principle target for mediating repression of polyoma DNA replication.

The dimerization/packaging sequence is dispensable for both the formation of high-molecular-weight RNA complexes within retroviral particles and the synthesis of proviruses of normal structure

Retroviral particles contain a dimer of two genomic RNA molecules, linked by noncovalent intermolecular bonds. Studies by electron microscopy of viral RNA extracted from virions as well as in vitro studies have implicated a sequence, designated the dimer linkage sequence (DLS), in the dimerization process. The DLS has been localized within a short region encompassing the psi packaging sequence, between nucleotides 212 and 563 for the Moloney murine leukemia retrovirus (MoMLV) RNA. In this report, we show that viral RNAs lacking both the DLS and psi packaging sequences--and even an RNA lacking the first 6,537 nucleotides of MoMLV--can assemble within retroviral particles as high-molecular-weight, slow-migrating, heat-sensitive complexes closely related to those observed for wild-type viral RNAs. Furthermore, we show that proviruses of normal structure are generated upon infection of test cells with retroviral particles which contain the DLS/psi-deleted viral RNAs. These observations demonstrate that the DLS and psi packaging sequences are not essential in cis to form a functional RNA complex for reverse transcription and integration.

PEBP2--a modulator of polyoma DNA replication

Previously, we have shown that integrated copies of polyoma DNA can be induced to replicate in rat fibroblasts (H3 cells) exposed to a DNA-damaging agent. In the current study, we demonstrate that UV-irradiation of mouse fibroblasts (WOP cells), transiently transfected with polyoma DNA, results in repression of polyoma replication. Cotransfection of oligomers representing wild-type but not mutated forms of the PEBP2 target sequence restored levels of viral replication indicating a role of PEBP2 binding proteins in mediating this effect. DNA-binding assays revealed that a different subset of complexes was formed with the PEBP2 target sequence when nuclear proteins from sham and UV-irradiated WOP and H3 cells were compared, suggesting that the activities of PEBP2 binding proteins are differentially regulated upon UV-irradiation in these two cell types. The ability of PEBP2 to modulate polyoma replication following UV-irradiation in WOP cells suggests a potential role of PEBP2 proteins in the cellular response to DNA damage.

Selection of DNA clones with enhancer sequences

A method is described for selection of DNA clones that contain enhancer sequences that activate gene expression. An Escherichia coli-rodent cell shuttle vector, pPyE0, was used that contains polyoma viral DNA without the polyoma enhancer region. Replication of pPyE0 DNA in mouse cells is markedly reduced due to deletion of the polyoma enhancer region. Insertion of mouse genomic DNA fragments that contain putative enhancer sequences into pPyE0 adjacent to the polyoma origin of replication restored, to varying extents, the ability of the recombinant plasmid DNA to replicate in mouse cells. Recombinant plasmids that replicate well in mouse cells, therefore, are amplified selectively. Transfection of mouse neuroblastoma or fibroblast cells that constitutively synthesize polyoma large tumor antigen with a library of mouse genomic DNA fragments inserted in pPyE0 yielded many recombinant plasmids. DNA inserts from each of the 16 clones that were examined stimulated the expression of an enhancerless chloramphenicol acetyltransferase reporter gene. The DNA inserts from 4 clones that were studied resulted in 4- to 13-fold increases in chloramphenicol acetyltransferase mRNA in transfected mouse cells. Nucleotide sequence analysis led to the identification of 5 genomic DNA clones that were obtained by selection. All of the homologies found were to regions of DNA that are thought to be involved in the regulation of gene expression.

Intramolecular recombination in polyomavirus DNA is controlled by promoter elements

We show here that intramolecular homologous recombination in polyomavirus (Py) DNA depends upon discrete sequence elements of the viral regulatory region which are believed to regulate transcription initiation and exert little or no cis-control over replication. Either deleting the viral early promoter (EP) or inverting the viral late promoter (LP) strongly impairs viral DNA recombination under conditions allowing viral DNA replication to proceed undisturbed. These findings suggest that bi-directional transcription proceeding from the intergenic region favors intramolecular recombination.

Mutation of large T-antigen-binding site A, but not site B or C, eliminates stalling by RNA polymerase II in the intergenic region of polyomavirus DNA

During transcription of the late strand of polyomavirus DNA, RNA polymerase II stalls and accumulates nearby the binding sites on viral DNA recognized by polyomavirus large T antigen. Stalling by RNA polymerases is eliminated when thermolabile large T antigen is inactivated by using a temperature-sensitive virus mutant (J. Bertin, N.-A. Sunstrom, P. Jain, and N. H. Acheson, Virology 189:715-724, 1992). To determine whether stalling by RNA polymerases is mediated through the interaction of large T antigen with one or more of its binding sites, viable polyomavirus mutants that contain altered large-T-antigen-binding sites were constructed. Point mutations were introduced by site-directed mutagenesis into the multiple, clustered G(A/G)GGC pentanucleotides known to be the target sequence for large T-antigen binding. Mutation of the G(A/G)GGC pentanucleotides in the first two binding sites encountered by RNA polymerases in the intergenic region (sites C and B) had no detectable effect on stalling as measured by transcriptional run-on analysis. However, mutation of the two GAGGC pentanucleotides in binding site A, which lies adjacent to the origin of viral DNA replication, eliminated stalling by RNA polymerases. We conclude that binding of large T antigen to site A blocks elongation by RNA polymerase II. Further characterization of virus containing mutated site A did not reveal any effects on early transcription levels or on virus DNA replication. However, the mutant virus gave rise to small plaques, suggesting impairment in some stage of virus growth. Stalling of RNA polymerases by large T antigen bound to the intergenic region of viral DNA may function to prevent transcription from displacing proteins whose binding is required for the normal growth of polyomavirus.

PG-M1: a new monoclonal antibody directed against a fixative-resistant epitope on the macrophage-restricted form of the CD68 molecule

A new anti-macrophage monoclonal antibody (PG-M1) was produced by immunizing BALB/c mice with fresh spleen cells from a patient with Gaucher's disease. PG-M1 reacts strongly with a fixative-resistant epitope of an intracytoplasmic molecule, selectively expressed by virtually all macrophages of the human body. Although attempts to immunoprecipitate the molecule recognized by PG-M1 have failed so far, the reactivity of the antibody with COS-1 and WOP cells transfected with a human complementary DNA clone encoding for the CD68 antigen suggests that PG-M1 is a new member of the CD68 cluster. However, unlike other CD68 antibodies (KP1, EBM11, etc.), which react with both macrophages and myeloid cells, PG-M1 detects a fixative-resistant epitope on the macrophage-restricted form of the CD68 antigen. In 957 routinely fixed, paraffin-embedded samples, PG-M1 showed a more restricted reactivity with elements of the monocyte/macrophage lineage than the previously described monoclonal antibodies MAC-387 (anti-calgranulins), KP1 (CD68) and Ki-M1P. Among hematological malignancies, PG-M1 only labels acute leukemias of M4 and M5 type and rare examples of malignant histiocytosis/true histiocytic sarcoma. In contrast, acute leukemias of the M1, M2, M3, M6, M7, and L1-L3 types, non-Hodgkin's lymphomas, and Hodgkin and Reed-Sternberg cells of Hodgkin's disease are consistently PG-M1-negative. In the daily diagnostic practice, PG-M1 seems to be particularly valuable for the diagnosis of myelomonocytic or monocytic leukemia and neoplasms of true histiocytic origin in routine paraffin sections.

Characterization of a new tissue-specific transcription factor binding to the simian virus 40 enhancer TC-II (NF-kappa B) element

We have biochemically and functionally characterized a new transcription factor, NP-TCII, which is present in nuclei from unstimulated T and B lymphocytes but is not found in nonhematopoietic cells. This factor has a DNA-binding specificity similar to that of NF-kappa B but is unrelated to this or other Rel proteins by functional and biochemical criteria. It can also be distinguished from other previously described lymphocyte-specific DNA-binding proteins.

Characterization of a new tissue-specific transcription factor binding to the simian virus 40 enhancer TC-II (NF-kappa B) element

We have biochemically and functionally characterized a new transcription factor, NP-TCII, which is present in nuclei from unstimulated T and B lymphocytes but is not found in nonhematopoietic cells. This factor has a DNA-binding specificity similar to that of NF-kappa B but is unrelated to this or other Rel proteins by functional and biochemical criteria. It can also be distinguished from other previously described lymphocyte-specific DNA-binding proteins.

Functional role of the ultraviolet light responsive element (URE; TGACAACA) in the transcription and replication of polyoma DNA

We have previously identified a novel 8 bp sequence (UV-responsive element, URE: TGACAACA) present in the regulatory region of polyoma DNA that interacts with protein factors induced in rat fibroblast cells by exposure to UV light. In the present study, we demonstrate through competitive binding assays that this sequence is distinct from the partially homologous AP1 and CRE target sequences. The proteins that bind to the URE appear to have transcriptional activity in UV-exposed rat fibroblasts. In addition, the URE appears to play a role in promoting the replication of polyoma DNA as determined through two different experimental approaches. Together, these findings suggest that the URE is a novel DNA binding element that interacts with proteins involved in the transcription and replication of polyoma sequences.

Ligand-induced redistribution of a human KDEL receptor from the Golgi complex to the endoplasmic reticulum

Resident luminal endoplasmic reticulum (ER) proteins carry a targeting signal (usually KDEL in animal cells) that allows their retrieval from later stages of the secretory pathway. In yeast, the receptor that promotes this selective retrograde transport has been identified as the product of the ERD2 gene. We describe here the properties of a human homolog of this protein (hERD2). Overproduction of hERD2 improves retention of a protein with a weakly recognized variant signal (DDEL). Moreover, overexpression of KDEL or DDEL ligands causes a redistribution of hERD2 from the Golgi apparatus to the ER. Mutation of hERD2 alters the ligand specificity of this effect, implying that it interacts directly with the retained proteins. Ligand control of receptor movement may limit retrograde flow and thus minimize fruitless recycling of secretory proteins.

Expression cloning systems

This review will cover the use of expression cloning in Xenopus oocytes, fission yeast, and mammalian cells. Of the systems covered herein, transient expression cloning systems in Xenopus oocytes and mammalian cells have proven to be the most effective and versatile, as demonstrated by the large number of cDNA clones isolated by these two methods in the past year. Of particular interest, are recent advances in the screening methodologies used in conjunction with transient expression in mammalian cells which have permitted the application of this system in the isolation of cDNAs encoding intracellular proteins.

PEA1 and PEA3 enhancer elements are primary components of the polyomavirus late transcription initiator element

The circular polyomavirus genome is transcribed from divergent promoter regions. Early mRNAs are initiated from a transcription complex formed at a TATA motif, the site of binding of transcription factor TFIID. Early transcription is promoted at a distance by the viral enhancer, which includes DNA motifs bound by cellular proteins of the PEA1 and PEA3 families of transcription activators. In contrast, the predominant viral late mRNAs are initiated within the viral enhancer, which lacks a TATA motif, near the PEA1 and PEA3 DNA motifs. Here, we demonstrate that these PEA1 and PEA3 binding sites are primary components of an autonomous transcription initiator element (Inr). They cause transcription of most polyomavirus late mRNAs and can direct the transcription of heterologous reporter genes. Alternative roles of these DNA motifs as activators of early mRNA transcription and as an initiator element for late mRNA transcription help explain how polyomavirus gene expression is regulated during lytic growth and provides a model for cellular transcription during development.

ATP induces the assembly of polyoma large tumor antigen into hexamers

Using zone velocity sedimentation and nondenaturing polyacrylamide gel electrophoresis, we have determined that purified polyoma large tumor antigen (Py T Ag) consists of discrete forms ranging from more abundant monomers and dimers to several higher but clearly distinguishable oligomeric species. Addition of ATP and MgCl2 to Py T Ag caused a dramatic increase in the appearance of Py T Ag hexamers, a form that, based on its SV40 T Ag counterpart, is likely to play a crucial role in its DNA replication functions. Other nucleotides in addition to ATP, as well as a nonhydrolyzable ATP derivative, were capable of inducing hexamer formation. This approach may further elucidate the role(s) of different forms of Py T Ag in viral regulatory processes.

Structural and functional analysis of a polyoma-related mammalian plasmid (L factor): the enhancer activity and plasmid establishment

L factor is a unique plasmid DNA which was originally discovered in a subclone (B822) of mouse L cells at a high copy number (more than 5,000 copies/cell). The presence of L factor caused no detectable abnormalities to the plasmid-bearing cells. We determined the total DNA sequence of the L factor I (and a part of L factor II) and compared it with that of polyoma DNA. Both DNA are common to the general construction of DNA frames such as early, late and noncoding regions, suggesting the two to be closely related. On the other hand, the L factor DNA sequences differ substantially from that of polyoma in the DNA sequences corresponding to the polyoma large T antigen, capsid proteins and a portion of the enhancer region. In order to investigate the mechanism of plasmid establishment of L factor, we compared the enhancer activity, capacity of DNA replication and efficiency of plasmid establishment of L factor with those of polyoma. The results indicate that L factor enhancer activity and DNA replication capacity were considerably lower than those of polyoma, suggesting that these altered (lowered) activities associated with L factor contribute to the plasmidal establishment and stable maintenance of L factor.

p lambda Zd39: a new type of cDNA expression vector for low background, high efficiency directional cloning

We have developed a new type of bacteriophage lambda vector which provides a strong biological selection against non-recombinants that is independent of the sequences immediately surrounding the cloning site. This system, which we call 'selective substitution', is ideally suited for cDNA expression vectors where it is necessary to flank the cDNA insert with sequence elements (promoters etc.) required to produce a biologically active mRNA in vivo. Selective substitution is a general method, which may be applied to many types of vectors. In this report, we have specifically applied selective substitution to the construction of a new mammalian retrovirus expression vector. The level of background obtained with this vector (that is, the number of plaques obtained when the vector is ligated in the absence of insert DNA) is 0.02% when compared to ligation with restriction fragments and 0.1% to 0.4% when compared to ligation with newly synthesized cDNA. These features have allowed us to easily and efficiently generate several large cDNA libraries using total and size selected cDNA.

The DNA-binding properties of polyomavirus large T antigen are altered by ATP and other nucleotides

We have examined the influence of ATP on the DNA-binding properties of polyomavirus large T antigen (Py TAg). Utilizing nitrocellulose filter binding, DNase I footprinting, and gel mobility shift assays, we observed that ATP increased Py TAg binding to DNA fragments containing either all Py TAg-binding sites (whole origin) or those sites within (core origin) or adjacent to (early) the origin of replication. Even nonspecific binding to DNA fragments lacking Py TAg-binding sites was increased somewhat by ATP. Binding to the core origin was increased to a greater extent than binding to other DNA fragments tested. Gel band mobility shift assays revealed that ATP increased the production of core origin-specific Py TAg-DNA complexes of high molecular weight. ATP stimulation depended on the presence of MgCl2. Other nucleotides and nonhydrolyzable ATP analogs also increased Py TAg binding to the core origin but to various degrees: ATP, dATP, 5'-adenylyl imidodiphosphate (AMPPNP) greater than 5'-adenylyl methylenediphosphate (AMPPCP) greater than dCTP greater than UTP greater than TTP. GTP and dGTP did not increase DNA binding by Py TAg. The rates of association and disassociation of Py TAg with all the DNA fragments were altered by the presence of ATP. DNase I footprinting showed that ATP extensively extended the region protected within the core origin and also produced a distinctive DNase I-hypersensitive site on the late strand at nucleotides 5255 to 5262 (TTACTATG).

Self-regulation of 70-kilodalton heat shock proteins in Saccharomyces cerevisiae

To determine whether the 70-kilodalton heat shock proteins of Saccharomyces cerevisiae play a role in regulating their own synthesis, we studied the effect of overexpressing the SSA1 protein on the activity of the SSA1 5'-regulatory region. The constitutive level of Ssa1p was increased by fusing the SSA1 structural gene to the GAL1 promoter. A reporter vector consisting of an SSA1-lacZ translational fusion was used to assess SSA1 promoter activity. In a strain producing approximately 10-fold the normal heat shock level of Ssa1p, induction of beta-galactosidase activity by heat shock was almost entirely blocked. Expression of a transcriptional fusion vector in which the CYC1 upstream activating sequence of a CYC1-lacZ chimera was replaced by a sequence containing a heat shock upstream activating sequence (heat shock element 2) from the 5'-regulatory region of SSA1 was inhibited by excess Ssa1p. The repression of an SSA1 upstream activating sequence by the SSA1 protein indicates that SSA1 self-regulation is at least partially mediated at the transcriptional level. The expression of another transcriptional fusion vector, containing heat shock element 2 and a lesser amount of flanking sequence, is not inhibited when Ssa1p is overexpressed. This suggests the existence of an element, proximal to or overlapping heat shock element 2, that confers sensitivity to the SSA1 protein.

Self-regulation of 70-kilodalton heat shock proteins in Saccharomyces cerevisiae

To determine whether the 70-kilodalton heat shock proteins of Saccharomyces cerevisiae play a role in regulating their own synthesis, we studied the effect of overexpressing the SSA1 protein on the activity of the SSA1 5'-regulatory region. The constitutive level of Ssa1p was increased by fusing the SSA1 structural gene to the GAL1 promoter. A reporter vector consisting of an SSA1-lacZ translational fusion was used to assess SSA1 promoter activity. In a strain producing approximately 10-fold the normal heat shock level of Ssa1p, induction of beta-galactosidase activity by heat shock was almost entirely blocked. Expression of a transcriptional fusion vector in which the CYC1 upstream activating sequence of a CYC1-lacZ chimera was replaced by a sequence containing a heat shock upstream activating sequence (heat shock element 2) from the 5'-regulatory region of SSA1 was inhibited by excess Ssa1p. The repression of an SSA1 upstream activating sequence by the SSA1 protein indicates that SSA1 self-regulation is at least partially mediated at the transcriptional level. The expression of another transcriptional fusion vector, containing heat shock element 2 and a lesser amount of flanking sequence, is not inhibited when Ssa1p is overexpressed. This suggests the existence of an element, proximal to or overlapping heat shock element 2, that confers sensitivity to the SSA1 protein.

Replication-dependent transactivation of the polyomavirus late promoter

When a plasmid containing the wild-type polyomavirus intergenic regulatory region fused to the bacterial cat gene was introduced into mouse NIH 3T3 cells along with a plasmid coding for the early viral proteins (T antigens), chloramphenicol transacetylase enzyme activity and mRNA levels were increased about 10-fold over levels observed in the absence of early proteins. To investigate this transactivation phenomenon further, 11 specific deletion mutant derivatives of the wild-type parent plasmid were constructed and studied. One mutant (NAL) with a minimal level of chloramphenicol transacetylase expression in the absence of T antigens was capable of being transactivated more than 40-fold. A number of other mutants, however, had little capacity for transactivation. Each of these mutants had in common a defect in large T-antigen-mediated DNA replication. Interestingly, one of the transactivation-defective mutants showed a basal late promoter activity fivefold higher than that of wild type and replicated in mouse cells in the absence of large T antigen. Subsequently, a small deletion abolishing viral DNA replication was introduced into those mutants capable of transactivation. The effect of the second deletion was to eliminate both replication and transactivation. Finally, wild-type and mutant constructs were transfected into Fisher rat F-111 cells in the presence or absence of early proteins. No transactivation or replication was ever observed in these cells. We concluded from these studies that the observed transactivation of the polyomavirus late promoter by one or more of the viral early proteins was due to either higher template concentration resulting from DNA replication or replication-associated changes in template conformation.

Inducible cellular responses to ultraviolet light irradiation and other mediators of DNA damage in mammalian cells

Both naturally occurring and carcinogen-induced tumors display not only point mutations in cellular oncogenes but also more complex changes in cellular oncogenes and other cellular genes. For this and other reasons, it seems likely that DNA damage in mammalian cells can induce alterations in gene expression that may have both short and long term consequences in the target cell. The purpose of this review is to summarize current available information on inducible responses to UV-irradiation and other mediators of DNA damage in mammalian cells, and to provide some working hypotheses. We have divided these responses into three time frames, immediate (0-12 hours), early (12-48) and late (beyond 48 hours). Immediate responses include the action of DNA repair enzymes, some of which are induced as a consequence of DNA damage, and transient inhibition of DNA synthesis. Within the past few years considerable evidence has accumulated that during this immediate period there is increased expression of certain cellular oncogenes, proteases and proteins whose functions remain to be identified. It is of interest that the expression of some of these genes is also induced by certain growth factors, tumor promoters and heat shock. Alterations in gene expression during the subsequent "early" period (12-48 hrs.) have not been studied in detail, but it is during this period that one can detect increased replication of several types of viruses in cells that harbor these viruses. We have examined in detail the induction of asynchronous polyoma DNA replication (APR) in a rat fibroblast cell line carrying integrated copies of this DNA. We have obtained evidence that UV-irradiation of these cells leads to the synthesis of a 40 kd protein, within the first 1-24 hrs after irradiation, that binds to a specific sequence TGACAACA in the regulatory region of polyoma DNA. We suggest that this protein acts together with other proteins to induce APR and that this serves as a useful model for understanding the mechanisms responsible for amplification of cellular genes, a phenomenon often seen in malignant tumors. Finally, we discuss how the events occurring during the immediate and early periods following DNA damage might lead to late effects in the target cell that are stable and contribute to the genotype and phenotype of some of the progeny of these cells that are destined to become tumor cells.

Constitutive expression of simian virus 40 large T antigen in monkey cells activates their capacity to support polyomavirus replication

Polyomavirus DNA replication is normally restricted to rodent cells, and simian virus 40 (SV40) DNA replication is restricted to primate cells. We demonstrate that DNAs containing the polyomavirus origin can be replicated in monkey cells which constitutively express SV40 large T antigen. Permissivity is most likely caused by SV40 T antigen modification of cellular protein(s) required to replicate the polyomavirus origin. A possible target for the T-antigen-induced modification is DNA polymerase alpha-DNA primase.

Control elements situated downstream of the major transcriptional start site are sufficient for highly efficient polyomavirus late transcription

In a transient expression assay in mouse fibroblasts in which neither replication nor T-antigen synthesis occurred, the polyomavirus late promoter functioned faithfully and even more efficiently than the simian virus 40 early promoter. Surprisingly, the DNA sequences upstream of the main transcriptional start sites were not required to obtain the high mRNA level observed. It appeared to result from the combined action of a basal promoter element within the A enhancer domain and of a more downstream element, located in the VP3 intron and abutting the late splice donor. We also show that although an enhancer region was required, enhancer function per se was not. Instead, it appeared that only a defined subset of the DNA-protein interactions necessary for enhancer function was involved in late promoter activity.