Inhibition of Simian virus 40 large T antigen helicase activity by fluoroquinolones

BACKGROUND

Fluoroquinolones represent a potent group of antibiotics that inhibit bacterial DNA replication by targeting the essential bacterial enzymes gyrase and topoisomerase IV. Inhibition of gyrase activity by quinolones involves the interaction of these drugs with the helicase component of bacterial gyrase. DNA tumour viruses also encode helicases that are essential for their DNA replication in the host.

METHODS

In this study we have evaluated the effect of fluoroquinolones on viral DNA replication using the DNA tumour virus Simian virus 40 (SV40) as our model. Four different fluoroquinolones, namely, levofloxacin, trovafloxacin, ciprofloxacin and ofloxacin, were tested for their ability to inhibit viral DNA replication.

RESULTS

We show here that all four quinolones tested were effective in the inhibition of SV40 plaque formation and DNA replication in CV1-P cells. In addition, we found that each of these quinolones was inhibitory to the helicase activity of SV40 large tumour antigen.

CONCLUSIONS

Fluoroquinolones and their derivates may therefore be useful in the treatment and/or prevention of infection by SV40-homologous human DNA viruses that encode helicase activity for their survival.

The carboxyl-terminal domain of large T antigen rescues SV40 host range activity in trans independent of acetylation

The host range activity of SV40 has been described as the inability of mutant viruses with deletions in the C terminal region of large T Ag to replicate in certain types of African green monkey kidney cells. We constructed new mutant viruses expressing truncated T Ag proteins and found that these mutant viruses exhibited the host range phenotype. The host range phenotype was independent of acetylation of T Ag at lysine 697. Co-expression of the C terminal domain of T Ag (aa 627-708) in trans increased both T Ag and VP1 mRNA as well as protein levels for host range mutant viruses in the restrictive cell type. In addition, the T Ag 627-708 fragment promoted the productive lytic infection of host range mutant viruses in the nonpermissive cell type. The carboxyl-terminal region of T Ag contains a biological function essential for the SV40 viral life cycle.

Distinct patterns of MCM protein binding in nuclei of S phase and rereplicating SV40-infected monkey kidney cells

BACKGROUND

Simian Virus 40 (SV40) infection of growth-arrested monkey kidney cells stimulates S phase entry and the continued synthesis of both viral and cellular DNA. Infected cells can attain total DNA contents as high as DNA Index, DI = 5.0-6.0 (10-12C), with host cell DNA representing 70-80% of the total. In this study, SV40-infected and uninfected control cells were compared to determine whether continued DNA replication beyond DI = 2.0 was associated with rebinding of the minichromosome maintenance (MCM) hexamer, the putative replicative helicase, to chromatin.

METHOD

Laser scanning cytometry was used to measure the total expression per cell and the chromatin/matrix-association of two MCM subunits in relation to DNA content.

RESULTS

MCM2 and MCM3 proteins that were associated with the chromatin/matrix fraction in G1 phase of both uninfected and SV40-infected cells were gradually released during progression through S phase. However, in SV40-infected cells that progressed beyond DI = 2.0, chromatin/matrix-associated MCM2 and MCM3 remained at the low levels observed at the end of S phase. Rereplication was not preceded by an obvious rebinding of MCM proteins to chromatin, as was observed in G1 phase.

CONCLUSIONS

The rereplication of host cell DNA in the absence of the reassociation of MCM proteins with chromatin indicates that SV40 infection induces a novel mechanism of licensing cellular DNA replication.

Insights into hRPA32 C-terminal domain--mediated assembly of the simian virus 40 replisome

Simian virus 40 (SV40) provides a model system for the study of eukaryotic DNA replication, in which the viral protein, large T antigen (Tag), marshals human proteins to replicate the viral minichromosome. SV40 replication requires interaction of Tag with the host single-stranded DNA-binding protein, replication protein A (hRPA). The C-terminal domain of the hRPA32 subunit (RPA32C) facilitates initiation of replication, but whether it interacts with Tag is not known. Affinity chromatography and NMR revealed physical interaction between hRPA32C and the Tag origin DNA-binding domain, and a structural model of the complex was determined. Point mutations were then designed to reverse charges in the binding sites, resulting in substantially reduced binding affinity. Corresponding mutations introduced into intact hRPA impaired initiation of replication and primosome activity, implying that this interaction has a critical role in assembly and progression of the SV40 replisome.

Pairs of Vp1 cysteine residues essential for simian virus 40 infection

Transient disulfide bonding occurs during the intracellular folding and pentamerization of simian virus 40 (SV40) major capsid protein Vp1 (P. P. Li, A. Nakanishi, S. W. Clark, and H. Kasamatsu, Proc. Natl. Acad. Sci. USA 99:1353-1358, 2002). We investigated the requirement for Vp1 cysteine pairs during SV40 infection. Our analysis identified three Vp1 double-cysteine mutant combinations that abolished viability as assayed by plaque formation. Mutating the Cys49-Cys87 pair or the Cys87-Cys254 pair led to ineffective nuclear localization and diminished accumulation of the mutant Vp1s, and the defect extended in a dominant-negative manner to the wild-type minor capsid proteins Vp2/3 and an affinity-tagged recombinant Vp1 expressed in the same cells. Mutating the Cys87-Cys207 pair preserved the nuclear localization and normal accumulation of the capsid proteins but diminished the production of virus-like particles. Our results are consistent with a role for Cys49, Cys87, and Cys254 in the folding and cytoplasmic-nuclear trafficking of Vp1 and with a role for Cys87 and Cys207 in the assembly of infectious particles. These findings suggest that transient disulfide bond formation between certain Vp1 cysteine residues functions at two stages of SV40 infection: during Vp1 folding and oligomerization in the cytoplasm and during virion assembly in the nucleus.

Molecular Analysis of SV-40-CAL, a New Slow Growing SV-40 Strain from the Kidney of a Caged New World Monkey with Fatal Renal Disease

A decline of the Callimico goeldii population in American zoos is presently occurring due to glomerulonephritis of unknown etiology. We hypothesized that this emerging idiopathic fatal renal disease (IFRD) was caused by a virus. We therefore attempted to isolate virus from the kidneys three C. goeldi in Illinois that had IFRD. Along with other viruses, Simian virus 40 (SV-40) strain CAL was isolated. SV-40-CAL is currently the slowest-growing natural isolate of SV-40 in CV-1 cells. Inefficient SV-40-CAL growth in CV-1 cells stems from two features: a suboptimal protoarchetypal regulatory region, and a Large tumor antigen gene sequence like that of SV-40 strain T302, previously considered the slow-growing natural isolate of SV-40. To our knowledge, this is the first documented isolation of SV-40 from a New World monkey outside of a laboratory setting. Though SV-40 is renaltropic, the role of SV-40-CAL in IFRD is uncertain. Transmission of SV-40 to C. goeldii through anthropogenic activity is suspected.

Factors Influencing the Production of Recombinant SV40 Vectors

Most gene therapy approaches employ viral vectors for gene delivery. Ideally, these vectors should be produced at high titer and purity with well-established protocols. Standardized methods to measure the quality of the vectors produced are imperative, as are techniques that allow reproducible quantitation of viral titer. We devised a series of protocols that achieve high-titer production and reproducible purification and provide for quality control and titering of recombinant simian virus 40 vectors (rSV40s). rSV40s are good candidate vehicles for gene transfer: they are easily modified to be nonreplicative and they are nonimmunogenic. Further, they infect a wide variety of cells and allow long-term transgene expression. We report here these protocols to produce rSV40 vectors in high yields, describe their purification, and characterize viral stocks using quality control techniques that monitor the presence of wild-type SV40 revertants and defective interfering particles. Several methods for reproducible titration of rSV40 viruses have been compared. We believe that these techniques can be widely applied to obtain high concentrations of high-quality rSV40 viruses reproducibly.

DA-125, a new antitumor agent, inhibits topoisomerase ii as topoisomerase poison and dna intercalator simultaneously

DA-125, a novel derivative of adriamycin, is known for its anti-cancer activity. In this study, the inhibitory mechanism of DA-125 on topoisomerase was investigated in the simian virus 40 (SV40) replicating CV-1 cell by studying the SV40 DNA replication intermediates and DNA-topoisomerase complexes. DNA-protein complexes that were formed in the drug-treated cells were quantitated by using a glass filter assay. SV40 DNA replication intermediates that were accumulated in the drug-treated CV-1 cell were analyzed in a high resolution gel. DA-125 did not accumulate B-dimers of SV40 DNA replication intermediates which were found in the adriamycin-treated CV-1 cells. DA-125 induced a dose-dependent formation of the DNA-protein complexes, while adriamycin did not. When adriamycin and etoposide (VP16) were added to the SV40-infected cells at the same time, adriamycin blocked the formation of the DNA-protein complexes induced by VP16 in a dose-dependent manner. However, DA-125 blocked the formation of the DNA-protein complexes induced by VP16 up to the maximum level of the DNA-protein complexes that were induced by DA-125 alone. Adriamycin and DA-125 did not inhibit the formation of the DNA-protein complexes that were caused by camptothecin, a known topoisomerase I poison. DA-125 is bifunctional in inhibiting topoisomerase II because it simultaneously has the properties of the topoisomerase II poison and the DNA intercalator. As a topoisomerase II poison, DA-125 alone induced dose-dependent formation of the DNA-protein complexes. However, as a DNA intercalator, it quantitatively inhibited the formation of the DNA-protein complexes induced by a strong topoisomerase II poison VP16. Furthermore considering that the levels of the DNA-protein complex induced by VP16 were decreased by DA-125 in terms of the topoisomerase II poison, we suggest that DA-125 has a higher affinity to the drug-binding sites of DNA than VP16 has.

The archetype enhancer of simian virus 40 DNA is duplicated during virus growth in human cells and rhesus monkey kidney cells but not in green monkey kidney cells

Archetype SV40, obtained directly from its natural host, is characterized by a single 72-bp enhancer element. In contrast, SV40 grown in cell culture almost invariably exhibits partial or complete duplication of the enhancer region. This distinction has been considered important in studies of human tumor material, since SV40-associated tumor isolates have been described having a single enhancer region, suggesting natural infection as opposed to possible contamination by laboratory strains of virus. However, the behavior of archetypal SV40 in cultured cells has never been methodically studied. In this study we reengineered nonarchetypal 776-SV40 to contain a single 72-bp enhancer region and used this reengineered archetypal DNA to transfect a number of simian and human cell lines. SV40 DNA recovered from these cells was analyzed by restriction endonuclease analysis, PCR, and DNA sequencing. Reengineered archetype SV40 propagated in green monkey TC-7 or BSC-1 kidney cells remained without enhancer region duplication even after extensive serial virus passage. Archetype SV40 grown in all but one of the rhesus or human cell lines initially appeared exclusively archetypal. However, when virus from these cell types was transferred to green monkey cells, variants with partial enhancer duplication appeared after as little as a single passage. These findings suggest (1) that virus with a single 72-bp enhancer may persist in cultured cells of simian and human origin; (2) that variants with partially duplicated enhancer regions may arise within cell lines in quantities below limits of detection; (3) that these variants may enjoy a selective advantage in cell types other than those from which they arose (e.g., green monkey kidney cells); and (4) that certain cell lines may support a selective growth advantage for the variants without supporting their formation. Our data indicate that enhancer duplication may also occur in human as well as rhesus kidney cells. Thus, detection of enhancer region duplication may not, a priori, indicate laboratory contamination, nor does detection of a single 72-bp enhancer exclude the possibility that contamination may have occurred. These findings may be of relevance to studies attempting to detect SV40 DNA in human tumors or other clinical specimens.

In vitro assessment of the cytotoxicity of nisin, pediocin, and selected colicins on simian virus 40-transfected human colon and Vero monkey kidney cells with trypan blue staining viability assays

Gram-positive bacterial bacteriocins (nisin and pediocin) and gram-negative bacterial bacteriocins (colicins [Col] E1, E3, E6, E7, and K) were evaluated for cytotoxicity against cultured simian virus 40-transfected human colon (SV40-HC) and Vero monkey kidney (Vero) cells. Bacteriocin-treated cells were assessed for viability by trypan blue staining. Monolayers of SV40-HC and Vero cells were cultured in tissue culture plates (35 degrees C, 10% CO2 in humidified air) with the use of Dulbecco's modified Eagle's medium supplemented with 10% (vol/vol) calf serum. Actively growing cells in the log phase (ca. 10(4) cells per ml) were treated with individual partially purified bacteriocin preparations at 170, 350, and 700 activity units per ml. Duplicate culture plates for each bacteriocin treatment and untreated controls were withdrawn after 16, 32, and 48 h of incubation. Cells were dissociated with trypsin and treated with trypan blue and were then counted in a hemocytometer with the use of a phase-contrast microscope. Viability assays indicated dose-dependent toxicity for some bacteriocins. Nisin, pediocin, and Col E6 were the most cytotoxic bacteriocins; SV40-HC cells demonstrated greater sensitivity than Vero cells did. Some bacteriocins can be toxic to mammalian cells; therefore, bacteriocins intended for use as biopreservatives must be evaluated for toxicity to mammalian cells and for other toxicities. Col E1, Col E3, Col E7, and Col K demonstrated little toxicity at the activities tested, indicating that they are safe and thus have potential for use as food biopreservatives.

The abundant nuclear enzyme PARP participates in the life cycle of simian virus 40 and is stimulated by minor capsid protein VP3

The abundant nuclear enzyme poly(ADP-ribose) polymerase (PARP) functions in DNA damage surveillance and repair and at the decision between apoptosis and necrosis. Here we show that PARP binds to simian virus 40 (SV40) capsid proteins VP1 and VP3. Furthermore, its enzymatic activity is stimulated by VP3 but not by VP1. Experiments with purified mutant proteins demonstrated that the PARP binding domain in VP3 is localized to the 35 carboxy-terminal amino acids, while a larger peptide of 49 amino acids was required for full stimulation of its activity. The addition of 3-aminobenzamide (3-AB), a known competitive inhibitor of PARP, demonstrated that PARP participates in the SV40 life cycle. The titer of SV40 propagated on CV-1 cells was reduced by 3-AB in a dose-dependent manner. Additional experiments showed that 3-AB did not affect viral DNA replication or capsid protein production. PARP did not modify the viral capsid proteins in in vitro poly(ADP-ribosylation) assays, implying that it does not affect SV40 infectivity. On the other hand, it greatly reduced the magnitude of the host cytopathic effects, a hallmark of SV40 infection. Additional experiments suggested that the stimulation of PARP activity by VP3 leads the infected cell to a necrotic pathway, characterized by the loss of membrane integrity, thus facilitating the release of mature SV40 virions from the cells. Our studies identified a novel function of the minor capsid protein VP3 in the recruitment of PARP for the SV40 lytic process.

Molecular identification of SV40 infection in human subjects and possible association with kidney disease

Simian virus 40 (SV40), a monkey polyomavirus that is believed to have entered the human population through contaminated vaccines, is known to be renotropic in simians. If indeed SV40 is endemic within the human population, the route of transmission is unknown. It was therefore hypothesized that SV40 might be renotropic in humans and be detected more frequently in samples obtained from patients with kidney diseases. This study found that typical polyomavirus cytopathic effects (CPE) were present and SV40 T antigen was detected in CV-1 cells cultured with peripheral blood mononuclear cells (PBMC) or urinary cells obtained from patients with kidney disease and healthy volunteers. DNA sequences homologous to the SV40 viral regulatory genome were detected by PCR in urinary cells from 15 (41%) of 36 patients with focal segmental glomerulosclerosis (FSGS), 2 (10%) of 20 patients with other kidney diseases, and 1 (4%) of 22 healthy volunteers (FSGS compared with other glomerular disease, P < 0.02; FSGS compared with healthy volunteers, P = 0.003). SV40 viral regulatory region genome was detected from PBMC at similar frequencies in patients with FSGS (35%), other glomerular diseases (20%), and healthy volunteers (22%). SV40 genome was detected by PCR in kidney tissues from 17 (56%) of 30 of patients with FSGS and 4 (20%) of 20 patients with minimal change disease and membranous nephropathy (P < 0.01). Considerable genetic heterogeneity of the viral regulatory region was detected, which argues against laboratory contamination. SV40 genome was localized to renal tubular epithelial cell nuclei in renal biopsies of patients with FSGS by in situ hybridization. This study demonstrates for the first time that human kidney can serve as a reservoir for SV40 replication and that SV40 may contribute to the pathogenesis of kidney disease, particularly FSGS.

Continuous porcine cell lines developed from alveolar macrophages: partial characterization and virus susceptibility

Porcine monomyeloid cell lines were established following transfection of primary porcine alveolar macrophage cultures with plasmid pSV3neo, carrying genes for neomycin resistance and SV40 large T antigen. The parental clone 3D4 exhibited a relatively rapid doubling time (25.5 h), high plating efficiency and mixed phenotype with respect to growth on a solid support. Single cell cloning of the 3D4 parent resulted in establishment of several cell lines; three of them designated 3D4/2, 3D4/21 and 3D4/31 were selected for further characterization. All three clones supported the replication of vesicular stomatitis virus (VSV), pseudorabies virus (PRV), classical swine fever virus (CSFV), swine vesicular disease virus (SVDV), swine poxvirus, African swine fever virus (ASFV), herpes simplex virus (HSV), parainfluenza virus, bovine adenovirus (BAV), vaccinia virus (VV), and porcine adenovirus (PAV). Under the conditions tested the cells did not support replication of porcine reproductive and respiratory syndrome virus (PRRSV). The swine myeloid character was confirmed for all three clones by fluorescence activated cell scanning (FACS) analysis using monoclonal antibodies 74-22-15 and DH59B, which recognize the pan-myeloid antigen cluster SWC3a. A subpopulation of each cell line was positive for nonspecific esterase activity and phagocytic activity to varying degrees depending on the media formulation. Cells from all three lines exhibited anchorage dependent growth when maintained in RPMI 1640 medium supplemented with 5-15% fetal bovine serum (FBS) and nonessential amino acids. Propagation in commercially formulated serum free media resulted in colony formation and growth in suspension. The addition of dimethyl sulfoxide (DMSO) or phorbol 12-myristate 13-acetate (PMA) to serum free media restored cell attachment. DMSO was also able to induce expression of CD14 monocyte marker in the 3D4/31 cell line maintained in FBS containing medium, as determined by FACS with monoclonal antibody CAM36A. Supplementation of RPMI medium with 10% porcine serum upregulated the expression of CD14 and induced expression of porcine macrophage markers recognized by antibodies 2B10 and 2G6 (Vet. Immunol. Immunopathol. 74 (2000) 163) in all three cell lines. The porcine myelomonocytic cell lines obtained may have a wide variety of applications in porcine virology and immunology.

Detection and growth kinetics of SV40 preparations with different enhancer element copy number

It has been reported that the rate of growth of SV40 in certain cell culture systems is affected by the number of copies of the enhancer element, and that this might in turn affect the ability of manufacturers of polio vaccines to detect SV40 contamination in tests for adventitious agents. Eleven strains of SV40 of which three were primary isolates, three were control strains used in different laboratories and five were well characterised were examined for their enhancer copy number. It was found that the three primary isolates contained a single copy while three preparations used as controls in different laboratories were mixtures of single and multiple copy strains. The remaining five strains consisted of one known double enhancer and four known single enhancer strains. All eleven strains were titrated in BSC-1 cells. There was no correlation between the number of copies of the enhancer element and the rate of growth or the final titre reached, although there was variation between the strains.

d(GA x TC)(n) microsatellite DNA sequences enhance homologous DNA recombination in SV40 minichromosomes

The genomic distribution of the abundant eukaryotic d(GA x TC)(n) DNA microsatellite suggests that it could contribute to DNA recombination. Here, it is shown that this type of microsatellite DNA sequence enhances DNA recombination in SV40 minichromosomes, the rate of homologous DNA recombination increasing by as much as two orders of magnitude in the presence of a d(GA x TC)(22) sequence. This effect depends on the region of the SV40 genome at which the d(GA x TC)(22) sequence is cloned. It is high when the sequence is located proximal to the SV40 control region but no effect is observed when located 3.5 kb away from the SV40 ori. These results indicate that the recombination potential of d(GA x TC)(n) sequences is likely linked to DNA replication and/or transcription. The potential contribution of the structural properties of d(GA x TC)(n) sequences to this effect is discussed.

Inhibitory effects of antisense oligonucleotides on the expression of procollagen type III gene in mouse hepatic stellate cells transformed by simian virus 40

The effects of phosphorothioate antisense oligonucleotides (ASO), complementary to the AUG start region, the junctional region of the intron and exon, and to exon of the procollagen type III gene, were investigated in a mouse hepatic stellate cell (HSC) line transformed by the simian virus 40 gene, SV68c-IS cells. ASO were transfected by lipofection. Immunohistochemistry, western and northern blotting showed inhibitory effects on procollagen type III gene expression by ASO that were complementary to the AUG start region and the junctional region of the intron and exon 2. However, ASO complementary to the exon 2 and 3, junctional region of the intron and exon 3, and sense oligonucleotides complementary to each ASO did not show any inhibitory effects. The effects of ASO complementary to the AUG start region were greater than those of ASO complementary to the junctional region. The effects of ASO were transient and a large amount of ASO was required to induce inhibitory effects without lipofection. ASO were effective in inhibiting the expression of the procollagen type III gene in the HSC which is well known to play a critical role in liver fibrosis.

Natural isolates of simian virus 40 from immunocompromised monkeys display extensive genetic heterogeneity: new implications for polyomavirus disease

Simian virus 40 (SV40) DNAs in brain tissue and peripheral blood mononuclear cells (PBMCs) of eight simian immunodeficiency virus-infected rhesus monkeys with SV40 brain disease were analyzed. We report the detection, cloning, and identification of five new SV40 strains following a quadruple testing-verification strategy. SV40 genomes with archetypal regulatory regions (containing a duplication within the G/C-rich regulatory region segment and a single 72-bp enhancer element) were recovered from seven animal brains, two tissues of which also contained viral genomes with nonarchetypal regulatory regions (containing a duplication within the G/C-rich regulatory region segment as well as a variable duplication within the enhancer region). In contrast, PBMC DNAs from five of six animals had viral genomes with both regulatory region types. It appeared, based on T-antigen variable-region sequences, that nonarchetypal virus variants arose de novo within each animal. The eighth animal exclusively yielded a new type of SV40 strain (SV40-K661), containing a protoarchetypal regulatory region (lacking a duplication within the G/C-rich segment of the regulatory region and containing one 72-bp element in the enhancer region), from both brain tissue and PBMCs. The presence of SV40 in PBMCs suggests that hematogenous spread of viral infection may occur. An archetypal version of a virus similar to SV40 reference strain 776 (a kidney isolate) was recovered from one brain, substantiating the idea that SV40 is neurotropic as well as kidney-tropic. Indirect evidence suggests that maternal-infant transmission of SV40 may have occurred in one animal. These findings provide new insights for human polyomavirus disease.

The role of the 34-kDa subunit of human replication protein A in simian virus 40 DNA replication in vitro

Human replication protein A (RPA) is a three subunit protein complex involved in DNA replication, repair, and recombination. We investigated the role of the 34-kDa subunit (p34) of RPA in DNA replication by generating a series of p34 mutants. While deletion of the N-terminal domain of p34 prevented its phosphorylation by both cyclin-dependent kinase (Cdk) and DNA-dependent kinase, a double point mutant that lacks the major phosphorylation sites for Cdk could be phosphorylated by DNA-dependent kinase. In simian virus 40 (SV40) DNA replication, RPA containing either of these mutants functioned as efficiently as wild-type RPA. However, mutant RPA containing C-terminally deleted p34 was only marginally active. This indicates that the C-terminal region, but not the phosphorylation domain of p34, is necessary for RPA function in DNA replication. Furthermore, RPA containing the C-terminally deleted p34 mutant could stimulate DNA polymerase alpha, and bind to single-stranded DNAs but was limited in its ability to unwind DNA or interact with SV40 large T antigen (T Ag). These results suggest that RPA p34 interacts with SV40 T Ag during the initiation of SV40 DNA replication and may be necessary for DNA unwinding.

Synthesis of simian virus 40 C-family catenated dimers in vivo in the presence of ICRF-193

The effect of ICRF-193, a non-cleavable, complex-stabilizing type of topoisomerase II inhibitor, on SV40 DNA replication in vivo was examined. As analyzed by one and two-dimensional gel electrophoresis, C-family catenated dimers, each composed of two intertwined, covalently closed SV40 DNAs, were mainly synthesized in the presence of the drug. On removal of the drug these C-family dimers were segregated into monomers. These results indicate that topoisomerase II is required for the segregation of replicated daughter molecules, but it is not absolutely required for the replication of DNA molecules up to the C-family dimers.

SV40 VP1 assembles into disulfide-linked postpentameric complexes in cell-free lysates

The simian virus 40 (SV40) capsid is composed of pentameric capsomeres of the major structural protein, Vp1. The chemical nature of Vp1-Vp1 interactions, as well as the role of the minor structural proteins, Vp2 and Vp3, in SV40 assembly is not clear. We show here that Vp1 molecules synthesized in rabbit reticulocyte lysates self-assembled into postpentameric 12S complexes in the absence of other viral structural proteins and in a time and concentration dependent manner. The 12S complexes were resistant to perturbants of noncovalent interactions but were sensitive to reduction by dithiothretiol. Nonreducing SDS-PAGE analysis revealed disulfide-linked VP1 complexes of > 400 kDa. Our results are consistent with crystallography studies of SV40 which suggest involvement of disulfide bonds at a postcapsomeric stage of viral assembly.

Artificial modification of the viral regulatory region improves tissue culture growth of SV40 strain 776

We describe here changes in the regulatory region of SV40 that influence its growth potential in cultured cells. Laboratory strains of papovaviruses BK and JC differ in the sequence of their regulatory regions from archetypes that have not been passaged in cell culture. These archetypes lack sequence repeats in the regulatory region; duplications that occur upon passage in cell culture confer selective growth advantage. Changes within the enhancer-promoter region of the well-characterized 776 strain of papovavirus SV40 that might affect its growth in tissue culture cells have not been documented. We measured the effect upon the growth of SV40 (776 strain) in CV-1 cells either of adding an additional 72-basepair (bp) enhancer element or of duplicating the entire 21-bp repeat region. SV40 growth in tissue culture was improved by reiteration of enhancer elements, whereas no growth advantage was conferred by tandem duplication of the 21-bp repeats. Viral DNA infectivity in CV-1 cells was directly proportional to the number of 72-bp elements but was unaffected by tandemly repeated 21-bp repeat elements. This study suggests that the 776 strain of SV40 is an evolutionary intermediate and that tissue-culture-adapted strains of SV40 do not accurately reflect the replication potential of natural isolates from primate hosts.

The hsc70 gene which is slightly induced by heat is the main virus inducible member of the hsp70 gene family

We have found that SV40 infection of CV1 cells induces the synthesis of a 72 kDa protein that upon molecular cloning was shown to be the product of the hsc70 gene. The above gene product was found to be mainly virus inducible, in contrast to the hsp70 gene product which was mainly heat inducible. The two genes were found to be cell cycle regulated in a distinctively different manner.

SV40 large T antigen functions at two distinct steps in virion assembly

The SV40 large T antigen mutant 5002 has two amino acid substitutions (L19-F; P28-S) and is defective for productive viral infection as demonstrated by its small plaques that arise very late and by a 100-fold reduced yield of infectious progeny. 5002 replicates viral DNA at the same time postinfection as wild-type SV40, and the production of progeny DNA molecules is only marginally reduced. Furthermore, the viral capsid proteins accumulate to near normal levels following infection with 5002. In this manuscript we report evidence that 5002 infection is blocked at a specific stage of viral assembly. The SV40 viral assembly pathway involves conversion of 75S chromatin complexes to 240S virions. Unlike mutants within the T antigen host range (HR) domain, that are also defective for viral assembly and accumulate 75S particles (Spence and Pipas, 1994), 5002 particles are blocked as 150S previrions containing viral DNA and capsid proteins. We have previously shown that 5002 and HR mutants cooperate to produce viable progeny in trans complementation tests. Thus, by two criteria, SV40 large T antigen encodes two distinct activities that function at different steps in virion assembly.

Dynamics of the nucleoprotein structure of simian virus 40 regulatory region during viral development

The regulatory region of SV40 is composed of multiple elements, including the origin of replication (ori), the encapsidation signal (ses) and the enhancer. Here, the structure of the chromatin and nucleoprotein complexes in a region encompassing ses and part of the enhancer was investigated in detail by in situ probing with DNase I. We have used a model experimental system based on plasmids which carry parts of the SV40 regulatory region. The results demonstrate that a specific nucleoprotein structure at the region is formed early after transfection. The overall structure is maintained throughout the viral life cycle. The observed DNase digestion pattern is consistent with the presence of a mixed population of viral minichromosomes with various, but not random, nucleosomal arrangements in that region. Specific modulations, which are associated with the various stages of the viral life cycle, are superimposed on the general structure. The most dramatic changes occur at nucleotides 34 and 113, located at both ends of ses and flanking the GC-box region. Some of the changes depend on the presence of viral gene product(s), probably a late (capsid) protein. The results further suggest that the condensed minichromosome within the viral particle assumes a highly specific configuration in this region. The nucleoprotein structure is sensitive to modifications of the primary nucleotide sequence and to flanking DNA elements. There is good correlation between distortions in the nucleoprotein structure and the inability of mutant plasmids to be packaged, substantiating the requirement for proper chromatin condensation in viral packaging.

Post-translational folding of the influenza C virus glycoprotein HEF: defective processing in cells expressing the cloned gene

The post-translational processing of the influenza C virus glycoprotein HEF was analysed. In cells infected with influenza C virus, HEF protein is synthesized as a glycosylated 80K polypeptide. A post-translational conformational rearrangement involving the formation of intramolecular disulphide bonds results in a decrease in its electrophoretic mobility. Therefore, SDS-PAGE under non-reducing conditions suggests an Mr of about 100K, whereas under reducing conditions an 80K protein is observed which is in accordance with the sequence data. The 100K form was detected 10 min after synthesis of HEF, and transport to the cell surface took about 60 min. This result indicates that the conformational change presumably occurs in the endoplasmic reticulum. A difference in post-translational processing was observed when the HEF gene was expressed in the absence of other influenza C virus genes. In cells infected with recombinant simian virus 40, the 80K precursor was synthesized, but this protein was neither converted to the 100K form nor transported to the cell surface. Deletion of the short cytoplasmic tail of HEF (Arg-Thr-Lys) or replacement of the two basic amino acids by hydrophobic (Ile) or acidic residues (Glu) resulted in HEF protein which was partially converted to the 100K form. Influenza C virus glycoprotein obtained after transient expression of the HEF gene using the vaccinia virus system was completely converted to the 100K form. However, in neither expression system was HEF transported to the cell surface. The possibility is discussed that the interaction of HEF with another viral protein is required for the post-translational folding and transport of this glycoprotein. The M protein of influenza C virus is suggested as a candidate for the chaperone which might interact with the cytoplasmic tail of HEF.

Simian virus 40 small-t antigen stimulates viral DNA replication in permissive monkey cells

The simian virus 40 (SV40) large-T antigen is essential for SV40 DNA replication and for late viral gene expression, but the role of the SV40 small-t antigen in these processes is still unclear. We have previously demonstrated that small t inhibits SV40 DNA replication in vitro. In this study, we investigated the effect of small t on SV40 replication in cultured cells. CV1 monkey cell infection experiments indicated that mutant viruses that lack small t replicate less efficiently than the wild-type virus. We next microinjected CV1 cells with SV40 DNA with and without purified small-t protein and analyzed viral DNA replication efficiency by Southern blotting. Replication of either wild-type SV40 or small-t deletion mutant DNA was increased three- to fivefold in cells coinjected with purified small t. Thus, in contrast to our in vitro observation, small t stimulated viral DNA replication in vivo. This result suggests that small t has cellular effects that are not detectable in a reconstituted in vitro replication system. We also found that small t stimulated progression of permissive monkey cells--but not of nonpermissive rodent cells--from G0-G1 to the S phase of the cell cycle, possibly leading to an optimal intracellular environment for viral replication.

Simian virus 40 prevents activation of M-phase-promoting factor during lytic infection

Simian virus 40 (SV40) infection stimulates confluent cultures of monkey kidney cells into successive rounds of cellular DNA synthesis without intervening mitosis. As an initial step in defining the mechanisms responsible for viral inhibition of mitosis, M-phase-promoting factor (MPF) was examined in SV40-infected CV-1 cells passing from G2 phase into a second S phase. MPF is a serine-threonine protein kinase that is essential for mitosis in eukaryotic cells. In SV40-infected cells exiting G2 phase, there was a reduced amount of MPF-associated H1 kinase activity relative to that of uninfected cells passing through mitosis. Both subunits of MPF, cyclin B and the p34cdc2 catalytic subunit, were present and in a complex in infected cells. In uninfected cultures, passage through mitosis was associated with the dephosphorylation of the p34cdc2 subunit, which is characteristic of MPF activation. In contrast, the p34cdc2 subunit remained in the tyrosine-phosphorylated, inactive form in SV40-infected cells passing from G2 phase into a second S phase. These results suggest that although the MPF complex is assembled and modified normally, SV40 interferes with pathways leading to MPF activation.

Reversion of a simian virus 40 enhancer mutant depends on the growth conditions

The simian virus 40 (SV40) enhancer contains three 8-bp purine-pyrimidine (R:Y) alternating sequences (Z-motifs) which are known to adopt the left-handed Z-DNA conformation in vitro. Mutations at these three Z-motifs seriously impair enhancer function. Reversion of one of these mutants (dpm12) is studied in this paper. The results indicate that, depending on growth conditions, recovery of the enhancer function is achieved through different mechanisms. Mutant viruses growing in solid-agar medium do not revert. On the other hand, revertants obtained in liquid medium contain a duplication of the enhancer sequences, showing no recovery of the original Z-motifs.

The genomic instability associated with integrated simian virus 40 DNA is dependent on the origin of replication and early control region

DNA rearrangements in the form of deletions and duplications are found within and near integrated simian virus 40 (SV40) DNA in nonpermissive cell lines. We have found that rearrangements also occur frequently with integrated pSV2neo plasmid DNA. pSV2neo contains the entire SV40 control region, including the origin of replication, both promoters, and the enhancer sequences. Linearized plasmid DNA was electroporated into X1, an SV40-transformed mouse cell line that expresses SV40 large T antigen (T Ag) and shows very frequent rearrangements at the SV40 locus, and into LMtk-, a spontaneously transformed mouse cell line that contains no SV40 DNA. Stability was analyzed by subcloning G-418-resistant clones and examining specific DNA fragments for alterations in size. Five independent X1 clones containing pSV2neo DNA were unstable at both the neo locus and the T Ag locus. By contrast, four X1 clones containing mutants of pSV2neo with small deletions in the SV40 core origin and three X1 clones containing a different neo plasmid lacking SV40 sequences were stable at the neo locus, although they were still unstable at the T Ag locus. Surprisingly, five independent LMtk- clones containing pSV2neo DNA were unstable at the neo locus. LMtk- clones containing origin deletion mutants were more stable but were not as stable as the X1 clones containing the same plasmid DNA. We conclude that the SV40 origin of replication and early control region are sufficient viral components for the genomic instability at sites of SV40 integration and that SV40 T Ag is not required.

SV40 virus expression vectors

SV40 late-replacement vectors provide an excellent means for expressing large amounts of proteins from cDNAs of less than 2400 bp. The amount of protein made 28 hr after infection is sufficient for extremely brief pulse-chase protocols. At periods postinfection when cells are still healthy, sufficient protein has been made for techniques in which only a small fraction of the protein is detected, including immunocytochemistry on cryosections. Because of the ease of subcloning DNA fragments into them, the ease of making virus stocks, and the ability to achieve comparable amounts of protein expression reproducibly with different infections, these vectors are superb tools for comparing series of mutants made by site-directed mutagenesis, especially if a normal cellular environment is important for the measurements to be made.

T-antigen kinase inhibits simian virus 40 DNA replication by phosphorylation of intact T antigen on serines 120 and 123

Simian virus 40 (SV40) DNA replication begins after two large T-antigen hexamers assemble on the viral minimal origin of replication and locally unwind the template DNA. The activity of T antigen in this reaction is regulated by its phosphorylation state. A form of casein kinase I purified from HeLa nuclear extracts (T-antigen kinase) phosphorylates T antigen on physiologic sites and inhibits its activity in the unwinding reaction (A. Cegielska and D. M. Virshup, Mol. Cell. Biol. 13:1202-1211, 1993). Using a series of mutant T antigens expressed by recombinant baculoviruses in Sf9 cells, we find that the origin unwinding activities of both TS677-->A and TS677,679-->A are inhibited by the T-antigen kinase, as is wild-type T antigen. In contrast, mutants TS120-->A and TS123,679-->A are resistant to inhibition by the kinase. Thus, phosphorylation of serines 120 and 123 is necessary for inhibition of T-antigen activity. Previous studies of casein kinase I substrate specificity have suggested that acidic residues or a phosphorylated amino acid amino terminal to the target residue are required to create a casein kinase I recognition site. However, we find that the T-antigen kinase can add more than 3 mol of Pi per mol to full-length bacterially produced T antigen and that it inhibits the unwinding activity of p34cdc2-activated bacterially produced T antigen. Since no prior phosphorylation is present in this bacterially produced T antigen, and no acidic residues are present immediately amino terminal to serines 120 and 123, other structural elements of T antigen must contribute to the recognition signals for T-antigen kinase. In support of this conclusion, we find that while T-antigen kinase phosphorylates amino-terminal residues in bacterially produced full-length T antigen, it cannot phosphorylate bacterially produced truncated T antigen containing amino acids 1 to 259, a 17-kDa amino-terminal tryptic fragment of T antigen, nor can it phosphorylate denatured T antigen. These findings strongly suggest that the carboxy-terminal domain of T antigen is an important modifier of the recognition signals for phosphorylation of the critical amino-terminal sites by the T-antigen kinase. This conclusion is consistent with previous studies suggesting close apposition of amino- and carboxy-terminal domains of T antigen in the native protein. The three-dimensional conformation of the substrate appears to make a significant contribution to T-antigen kinase substrate specificity.

Inhibition of heterologous DNA replication by the MVMp nonstructural NS-1 protein: identification of a target sequence

The nonstructural protein NS-1 of minute virus of mice (MVMp), an autonomous parvovirus, trans-inhibits the replication of a chimeric plasmid containing the SV40 origin of replication (ori) embedded in the MVMp genome. It appears that a 157-bp 5' proximal sequence of MVMp DNA is sufficient, in the presence of NS-1, to cause the inhibition of DNA replication driven by the SV40 ori placed on the same molecule. This effect is not dependent on the orientation of the MVMp target sequence and results from both a reduced level of utilization of SV40 ori and the blockage of progressing replication forks at the level of the target. Furthermore, replication driven by Epstein-Barr virus origin (oriP) is trans-inhibited by MVMp but this inhibition does not require the presence of parvoviral sequences in cis. On the basis of sequence homologies between EBV oriP and MVMp 5' terminal sequence, it is proposed that the direct or indirect interaction of NS-1 with parvovirus-like sequences present in heterologous viral and possibly also cellular genomes may result in an inhibition of DNA replication.

Human corneal epithelial primary cultures and cell lines with extended life span: in vitro model for ocular studies

PURPOSE

To develop an in vitro model of human corneal epithelium that can be propagated in serum-free medium that is tissue specific, species specific, and continuously available.

METHODS

Primary explant cultures from human cadaver donor corneas were generated and subsequently infected with Adeno 12-SV40 (Ad12-SV40) hybrid virus or transfected with plasmid RSV-T.

RESULTS

Several lines of human corneal epithelial cells with extended life span were developed and characterized. Propagation of both primary cultures and lines with extended life span, upon collagen membranes at an air-liquid interface, promoted multilayering, more closely approximating the morphology observed in situ.

CONCLUSIONS

In vitro models, using primary cultures of corneal epithelium and lines of corneal epithelial cells with extended life span, retain a variety of phenotypic characteristics and may be used as an adjunct to ocular toxicology studies and as a tool to investigate corneal epithelial cell biology.

Analysis of G418-selected Rat2 cells containing prototype, variant, mutant, and chimeric JC virus and SV40 genomes

The human polyomavirus JC virus (JCV) is highly tumorigenic in rodents, but transforms cells in culture inefficiently. To explore the basis for JCV's restricted transforming behavior, nonpermissive Rat2 cells were contransfected with pSV2-neo (encodes G418 resistance) and viral DNAs including prototype, variant, and mutant JCV genomes and two JCV-SV40 chimeras. By selecting cells displaying G418 resistance, lines were established that contain viral DNA and exhibit a wide range of transformed phenotypes. The G418-resistant lines were tested for their ability to grow under anchorage-independent conditions, to overgrow a monolayer of untransformed cells, and to form dense colonies on plastic. Expression of the viral T and t proteins and interaction of T protein with the cellular anti-oncoprotein p53 were measured. Also determined was the number of intact viral early coding regions integrated within the cellular DNA. The results of these studies suggested that most of the G418-resistant lines failed to express JCV T protein above a minimum threshold level required for their conversion to a fully transformed phenotype. In anchorage-independent growth assays, higher levels of a 17-kDa T-related peptide in JCV transformants appeared to compensate for decreased T antigen levels. Comparison of the T to p53 ratios in the cell lysates suggested that the quaternary structure of the JCV protein differed from that of its SV40 counterpart in the T-p53 complex. The presence of multiple vs single integrated copies of the viral genome in the cells did not correlate with elevated T antigen expression or an enhanced transformation status.

Opening and refolding of simian virus 40 and in vitro packaging of foreign DNA

Simian virus 40 (SV40) can be disassembled under mild conditions by reducing disulfide bonds in the capsid and removing calcium ions. The nucleoprotein complexes formed, analyzed by electron microscopy, were circular and made up of 59 +/- 4 subunits, each with a diameter of about 10 nm. The complexes contained the viral DNA, histones, and the viral capsid proteins. The complexes had much-reduced infectivities compared with intact SV40. Addition of calcium ions to the disrupted virus caused the nucleoprotein complexes to refold into virus-like structures which sedimented at the same rate as intact SV40 and regained infectivity. Treatment of the disrupted SV40 with a high concentration of salt dissociated the viral proteins from the DNA. Lowering stepwise the salt concentration, removing the reducing agent, and adding calcium ions allowed structures to be reformed, and these structures sedimented, like SV40, at 240S and were infectious. The plaque-forming ability of the reconstituted particles was between that of the dissociated components and that of intact SV40. The addition of purified DNA of polyomavirus to the dissociated SV40 before the lowering of the salt concentration showed that virus-like structures could be formed from SV40 proteins and a foreign DNA.

Expression of mouse furin in a Chinese hamster cell resistant to Pseudomonas exotoxin A and viruses complements the genetic lesion

RPE.40 is a strain of mutated CHO-K1 cells with elevated resistance to Pseudomonas exotoxin A, Sindbis virus, and Newcastle disease virus. Virus resistance is due to an inability to cleave precursor viral membrane glycoproteins and produce infectious virions. Transfection of RPE.40 cells with cDNA for mouse furin causes them to lose all resistance and become as sensitive as wild-type cells to the toxin and viruses. Transfection of RPE.40 cells with cDNA for the related yeast protease Kex2 reduces their resistance to the toxin and viruses, but does not completely eliminate it.

Wild-type p53 is not a negative regulator of simian virus 40 DNA replication in infected monkey cells

To analyze the proposed growth-inhibitory function of wild-type p53, we compared simian virus 40 (SV40) DNA replication in primary rhesus monkey kidney (PRK) cells, which express wild-type p53, and in the established rhesus monkey kidney cell line LLC-MK2, which expresses a mutated p53 that does not complex with large T antigen. SV40 DNA replication proceeded identically in both cell types during the course of infection. Endogenously expressed wild-type p53 thus does not negatively modulate SV40 DNA replication in vivo. We suggest that inhibition of SV40 DNA replication by wild-type p53 in in vitro replication assays is due to grossly elevated ratios of p53 to large T antigen, thus depleting the replication-competent free large T antigen in the assay mixtures by complex formation. In contrast, the ratio of p53 to large T antigen in in vivo replication is low, leaving the majority of large T antigen in a free, replication-competent state.

Structural requirements for simian virus 40 replication and virion maturation

The nuclear matrix plays an important role in simian virus 40 (SV40) DNA replication in vivo, since functional replication complexes containing large T and replicating SV40 minichromosomes are anchored to this structure (R. Schirmbeck and W. Deppert, J. Virol. 65:2578-2588, 1991). In the present study, we have analyzed the course of events leading from nuclear matrix-associated replicating SV40 minichromosomes to fully replicated minichromosomes and, further, to their encapsidation into mature SV40 virions. Pulse-chase experiments revealed that newly replicated SV40 minichromosomes accumulated at the nuclear matrix and were directly encapsidated into DNase-resistant SV40 virions at this nuclear structure. Alternatively, a small fraction of newly replicated minichromosomes left the nuclear matrix to associate with the cellular chromatin. During the course of infection, progeny virions continuously were released from the nuclear matrix to the cellular chromatin and into the cytoplasm-nucleoplasm. The bulk of SV40 progeny virions, however, remained at the nuclear matrix until virus-induced cell lysis.

Role of nuclear pore complex in simian virus 40 nuclear targeting

Cytoplasmically injected simian virus 40 (SV40) virions enter the nucleus through nuclear pore complexes (NPCs) and can express large T antigen shortly thereafter (J. Clever, M. Yamada, and H. Kasamatsu, Proc. Natl. Acad. Sci. USA 88:7333-7337, 1991). The nuclear import of the protein components of introduced SV40 was reversibly arrested by chilling and energy depletion, corroborating our previous observation that the nuclear entry of injected SV40 is blocked in the presence of wheat germ agglutinin and an antinucleoporin monoclonal antibody (mAb414), general inhibitors of NPC-mediated import. The nuclear accumulation of virion protein components and large T antigen in nonpermissive NIH 3T3 cells was similar to that in the permissive host, indicating that the ability to use NPCs as a route of nuclear entry appears to be a general property of the injected virus. Injected virions were capable of completing their lytic cycle and forming plaques in permissive cells. During the early phase of SV40 infection, the cytoplasmic injection of mAb414 effectively blocked nuclear T-antigen accumulation for up to 8 h of infection but had very little effect after 12 h of infection. The time-dependent interference with nuclear T-antigen accumulation by the antinucleoporin antibody is consistent with the hypothesis that the infecting virions enter the nucleus through NPCs. The interference study also suggests that the early phase of infection consists of at least two steps: a step for virion cell entry and intracytoplasmic trafficking and a step for virion nuclear entry followed by large-T-antigen gene expression and subsequent nuclear localization of the gene product. Virions were visualized as electron-dense particles in ultrathin sections of samples in which transport was permitted or arrested. In the former cells, electron-dense particles were predominantly observed in the nucleus. The virions were distributed randomly and nonuniformly in the nucleoplasm but were not observed in heterochromatin or in nucleoli. In the latter cells, the electron-dense particles were seen intersecting the nuclear envelope, near the inner nuclear membrane, and in NPCs. In tangential cross sections of NPCs, which appeared as donut-shaped structures, a spherical electron-dense particle was observed in the center of the structure. Immunoelectron microscopy revealed that NPCs were selectively decorated with 5-nm colloidal gold particles-anti-Vp1 immunoglobulin G at the cytoplasmic entrance to and in NPCs, confirming that the morphologically observed electron-dense particles in NPCs contain the viral structural protein. These results support the hypothesis that the nuclear import of SV40 is catalyzed through NPCs by an active transport mechanism that is similar to that of other karyophiles.

Replication of SV40 minichromosomes in vitro

We operationally define two forms of SV40 minichromosomes, a 75S-form, prepared at low salt concentration, referred to as native minichromosomes, and a 50S-form, obtained after treatment with 0.5 M potassium acetate, the salt-treated minichromosomes. Both preparations of minichromosomes serve well as templates for replication in vitro. Their respective replication products are strikingly different: replicated native minichromosomes contain a densely packed array of the maximal number of nucleosomes whereas replicated salt-treated minichromosomes carry, on average, half of the maximal number. We conclude that in both cases parental nucleosomes are transferred to progeny DNA, and, in addition, that an assembly of new nucleosomes occurs during the replication of native minichromosomes. This is apparently due to the presence of a nucleosome assembly factor as a constituent of native minichromosomes that dissociates upon treatment with salt. We further show that preparations of minichromosomes usually contain significant amounts of copurifying hnRNP particles and SV40 virion precursor particles. However, these structures do not detectably affect the replication and the chromatin assembly reactions.

Two synthetic Sp1-binding sites functionally substitute for the 21-base-pair repeat region to activate simian virus 40 growth in CV-1 cells

The 21-bp repeat region of simian virus 40 (SV40) activates viral transcription and DNA replication and contains binding sites for many cellular proteins, including Sp1, LSF, ETF, Ap2, Ap4, GT-1B, H16, and p53, and for the SV40 large tumor antigen. We have attempted to reduce the complexity of this region while maintaining its growth-promoting capacity. Deletion of the 21-bp repeat region from the SV40 genome delays the expression of viral early proteins and DNA replication and reduces virus production in CV-1 cells. Replacement of the 21-bp repeat region with two copies of DNA sequence motifs bound with high affinities by Sp1 promotes SV40 growth in CV-1 cells to nearly wild-type levels, but substitution by motifs bound less avidly by Sp1 or bound by other activator proteins does not restore growth. This indicates that Sp1 or a protein with similar sequence specificity is primarily responsible for the function of the 21-bp repeat region. We speculate about how Sp1 activates both SV40 transcription and DNA replication.

Host range analysis of a chimeric simian virus 40 genome containing the BKV capsid genes

Simian virus 40 (SV40) propagates poorly in cells from human embryonic kidney (HEK) and human fetal fibroblasts (HFF) while BK virus grows well in many human cell types. It has been suggested that sequences within the SV40 late region but not within the BKV late region may act to inhibit growth of virus in HEK and HFF cells. In order to test this and to identify a late region host range function, we have replaced the late region of wtSV40 DNA with the late region of RFV (a variant of BKV) to produce an intermolecular hybrid or chimera. The constructed SV40/RFV chimeric genome contained approx. 5900 base pairs, more than 650 base pairs greater than wtSV40. Nevertheless, when introduced by transfection the chimera appeared to be infectious. Three chimeric genomes were recovered from infected cells and all contained deletions of nearly 600 base pairs, exclusively at the region of the 3' terminal junction. Since all three chimeras propagated in human HFF and HEK cells, the RFV late region and not the RFV regulatory region possesses a host range function required for growth in human cells. Analysis of T-antigen gene expression suggests that the replacement of the SV40 late region with the BKV late region leads to full expression of the SV40 early region in human cells. Two chimeras exhibited a BKV-like host range and the third exhibited both a BKV and an SV40-like host range. We determined precisely which sequences were deleted in each chimera and we exchanged 3' terminal junction fragments containing these deletions, between two chimeras with different host ranges. From these experiments we demonstrated that: (1) The 3' terminus of the SV40 large T-antigen gene is required for growth of SV40/RFV in TC-7 and CV-1 simian cells but not for growth in human cells; (2) while the SV40 late region is refractory for growth in human cells, the RFV late region is not refractory for growth in simian cells; (3) the 3' terminus of the RFV T-antigen gene is not required for growth in human cells. The results of the 3' terminal junction exchanges and studies of early gene expression also demonstrate that BKV and SV40 can penetrate human and simian cells, even when they failed to grow in one cell type.

A cis-acting DNA signal for encapsidation of simian virus 40

Encapsidation of simian virus 40 is a complex biological process involving DNA-protein and protein-protein interactions in the formation of a unique three-dimensional structure around the viral minichromosome. A pseudoviral system developed in our laboratory, in which the viral early and late gene products are supplied in trans (by helpers), was used to analyze the encapsidation process independent of viral gene expression. With this experimental system we have discovered a requirement for a specific DNA signal for encapsidation, ses (for simian virus 40 encapsidation signal).ses is present within a 200-bp DNA fragment, which includes, in addition to the viral origin of replication (ori), six GGGCGG repeats (GC boxes) and 26 bp of the enhancer element. Deletion of the GC boxes and the enhancer sequences almost abolished encapsidation, while DNA replication was only moderately decreased. The ability to encapsidate was not regained by reinserting a DNA fragment carrying ses in the sesdeleted plasmid 2 kbp away from the ori, suggesting that for encapsidation the two DNA elements have to be close to each other. These findings afford novel strategies for the investigation of viral encapsidation.

Efficient in vivo encapsidation of a shuttle vector into pseudo-simian virus 40 virions using a shuttle virus as helper

We have designed shuttle vectors containing the late region of simian virus 40 (SV40) DNA (coding for the capsid proteins) which could be encapsidated into pseudo-SV40 virions during passage in monkey cells. We describe here the use of these shuttle viruses as helpers for the encapsidation of another shuttle vector into viral particles. Following cotransfection into monkey cells, the efficiency of encapsidation was similar for the shuttle virus and the other plasmid. The amounts of pseudo-SV40 virions recovered from the two vectors reflected the amounts of their DNA present in monkey cells. Thus, the presence of the SV40 late region did not confer any significant advantage for encapsidation. The encapsidation of any shuttle vector into pseudo-SV40 virions is therefore possible and efficient, shuttle viruses constituting an interesting alternative to the use of SV40 as helper in this process.

Simian virus 40 mutants with amino-acid substitutions near the amino terminus of large T antigen

A series of amino-acid substitution mutants has been made with changes in the region of simian virus 40 large tumor antigen (T antigen) that is shared with the small tumor antigen (t antigen). Both single and multiple amino-acid replacements were obtained using the heteroduplex deletion loop method and sodium bisulfite as the mutagen. The mutants could be divided into five phenotypic classes on the basis of their biological properties: a) mutants whose changes did not affect their ability to propagate on permissive monkey cells, nor to transform nonpermissive rodent cells; b) mutants that were not viable, replicated their DNA to 5% or less of wild type, but were positive for transformation; c) mutants that were not viable, replicated their DNA to 5% or less of wild type, and were defective for transformation; and d) mutants that completely lost all three activities coordinately. In addition, one mutant with changes in this region, 5002, replicated its DNA to about 50% of wild type, had an impaired transformation activity, and produced virions at a level of about 4% that of wild type.

Mouse DNA primase plays the principal role in determination of permissiveness for polyomavirus DNA replication

We have investigated the species-specific replication of polyomavirus DNA in the cell-free system that was established previously (Y. Murakami, T. Eki, M. Yamada, C. Prives, and J. Hurwitz, Proc. Natl. Acad. Sci. USA 83:6347-6351, 1986). Extracts from various species of cells supported polyomavirus DNA replication in a species-specific manner that was consistent with the host range specificity of polyomavirus; extracts prepared from mouse and hamster cells were active, whereas extracts prepared from human, monkey, and insect cells were inactive. The addition of DNA polymerase alpha-primase purified from mouse cells induced the replication of polyomavirus DNA in a cell-free system containing polyomavirus large tumor antigen and nonpermissive cell extracts, such as human and insect cell extracts. Isolated mouse DNA primase alone also induced polyomavirus DNA replication in human cell extracts but not in insect cell extracts, indicating that mouse DNA primase plays the principal role in determining permissiveness for polyomavirus DNA replication.

Specific mutation of a regulatory site within the ATP-binding region of simian virus 40 large T antigen

In an attempt to distinguish simian virus 40 (SV40) large T antigen (T) binding to ATP from hydrolysis, specific mutations were made in the ATP-binding site of T according to our model for the site (M. K. Bradley, T. F. Smith, R. H. Lathrop, D. M. Livingston, and T. A. Webster, Proc. Natl. Acad. Sci. USA 84:4026-4030, 1987). Two acidic residues predicted to make contact with the magnesium phosphate were changed to alanines. The mutated T gene was completely defective for viral DNA synthesis and for virion production, and it was dominant defective for viral DNA replication. The defective T gene encoded a stable product (2905T) that oncogenically transformed mouse cell lines. 2905T, immunoprecipitated from transformed-cell extracts, bound SV40 origin DNA specifically and, surprisingly, it was active as an ATPase. A recombinant baculovirus was constructed for the production and purification of the mutant protein for detailed biochemical analyses. 2905T had only 10% of the ATPase and helicase of wild-type T. The Km of 2905T for ATP in ATPase assays was the same as the Km of wild-type T. ATP activated the ATPase activity of wild-type T, but not of 2905T. As tested by gel bandshift assay, 2905T bound to SV40 origin DNA and to individual sites I and II with affinities similar to that of the wild type. However, ATP did not modulate the DNA-binding activity of mutant T to site II. Therefore, this mutation in the ATP-binding site in T resulted in defects in the interaction between the protein and ATP that appeared to be responsible for the determination of the active state of T for DNA binding versus ATPase.

Phenotype-specific phosphorylation of simian virus 40 tsA mutant large T antigens in tsA N-type and A-type transformants

To identify molecular differences between simian virus 40 (SV40) tsA58 mutant large tumor antigen (large T) in cells of tsA58 N-type transformants [FR(tsA58)A cells], which revert to the normal phenotype after the cells are shifted to the nonpermissive growth temperature, and mutant large T in tsA58 A-type transformants [FR(tsA58)57 cells], which maintain their transformed phenotype after the temperature shift, we asked whether the biological activity of these mutant large T antigens at the nonpermissive growth temperature might correlate with phosphorylation at specific sites. At the permissive growth temperature, the phosphorylation patterns of the mutant large T proteins in FR(tsA58)A (N-type) cells and in FR(tsA58)57 (A-type) cells were largely indistinguishable from that of wild-type large T in FR(wt648) cells. After a shift to the nonpermissive growth temperature, no significant changes in the phosphorylation patterns of wild-type large T in FR(wt648) or of mutant large T in FR(tsA58)57 (A-type) cells were observed. In contrast, the phosphorylation pattern of mutant large T in FR(tsA58)A (N-type) cells changed in a characteristic manner, leading to an apparent underphosphorylation at specific sites. Phosphorylation of the cellular protein p53 was analyzed in parallel. Characteristic differences in the phosphorylation pattern of p53 were observed when cells of N-type and A-type transformants were kept at 39 degrees C as opposed to 32 degrees C. However, these differences did not relate to the different phenotypes of FR(tsA58)A (N-type) and FR(tsA58)57 (A-type) cells at the nonpermissive growth temperature. Our results, therefore, suggest that phosphorylation of large T at specific sites correlates with the transforming activity of tsA mutant large T in SV40 N-type and A-type transformants. This conclusion was substantiated by demonstrating that the biological properties as well as the phosphorylation patterns of SV40 tsA28 mutant large T in cells of SV40 tsA28 N-type and A-type transformants were similar to those in FR(tsA58)A (N-type) and in FR(tsA58)57 (A-type) cells, respectively. The phenotype-specific phosphorylation of tsA mutant large T in tsA A-type transformants probably is a cellular process induced during establishment of SV40 tsA A-type transformants, since tsA28 A-type transformant cells could be obtained by a large-T-dependent in vitro progression of cells of the tsA28 N-type transformant tsA28.3 (M. Osborn and K. Weber, J. Virol. 15:636-644, 1975).