Biological properties of simian virus 40 host range mutants lacking the COOH-terminus of large T antigen

Three mutants of simian virus 40 (SV40), with deletions near the 3' end of the A gene, displayed a host range phenotype for growth and virus production in various African green monkey kidney cell lines. The mutants formed plaques in CV-1P cells at 40.5 degrees, in BSC-1 cells at 37 and 40.5 degrees, and in Vero cells at 32, 37, and 40.5 degrees. Virus yields in these three lines were cold sensitive: the burst size was greatest at 40.5 degrees and least at 32 degrees, but some progeny was produced under all conditions examined. Mutant yields never exceeded 10% of wild-type yields under the most permissive conditions (Vero cells at 37 or 40 degrees) and were less than 1% of wild type under the most restrictive conditions (CV-1P cells at 32 degrees). These mutants can be complemented by any SV40 mutant which produces a large T antigen containing a normal COOH-terminus. Mutants whose T antigens could not be transported to the nucleus were most efficient at complementation. Mutant virus production in a line of rhesus monkey kidney cells and in primary cultures of African green and rhesus monkey kidney cells was also substantially below wild type. These mutants were also completely defective for adenovirus helper function. Our data suggest that the host range property and adenovirus helper function represent the same activities of large T antigen.

Missense mutations in the VP1 gene of simian virus 40 that compensate for defects caused by deletions in the viral agnogene

Simian virus 40 mutants lacking sequences in the late leader region are viable but produce smaller plaques than does wild-type virus. Within three passages at low multiplicities of infection, virus stocks of several such mutants accumulated variants that synthesized an altered form of the major virion protein, VP1, having a slightly faster mobility in sodium dodecyl sulfate-polyacrylamide gels than did the wild-type protein. Because these variants overgrew the original virus stocks, we consider them to be second-site revertants. By construction and characterization of a series of recombinants, the second-site mutations were shown to map to at least two different regions of the VP1 gene. Nucleotide sequence analysis indicated that single-amino-acid changes were responsible for the rapid mobility of VP1. When combined in cis with either a wild-type or mutant leader region, these VP1 mutations sped up by 10 to 20 h the time course of accumulation of infectious progeny but not of viral DNA or VP1. LP1, the protein encoded by the agnogene, was shown previously to be necessary for the efficient transport of the virion proteins to the nucleus or for their efficient assembly with viral minichromosomes. The VP1 missense mutations reported here compensate for the lack of LP1 by facilitating this process. On the basis of these findings and findings reported previously by us and others, we hypothesize that LP1 facilitates the formation of infectious particles by inhibiting the polymerization of VP1 molecules until the time they interact with viral minichromosomes; the VP1 mutations reported here compensate for the loss of LP1 by lessening the potential of VP1 for self-polymerization.

Simian virus 40 infection is not mediated by lysosomal activation

The uptake of simian virus 40 (SV40) virions to the nucleus, the site of viral replication, proceeds via engulfment at the cytoplasmic membrane and transport in monopinocytotic vesicles through the cytoplasm to the nuclear membrane. In the case of Semliki Forest virus and poliovirus which undergo primary endocytosis in a similar manner, neutralization of the acid pH in these vesicles abolishes viral infectivity. We have examined the effects of the lysosomotropic agents chloroquine and ammonium chloride on the uptake of SV40 and find that neutralization of the acid pH in cellular organelles has no effect on the progress of SV40 infection. Although the initial endocytotic pathway appears similar for the viruses, the vesicular transport of SV40 to the nucleus proceeds, therefore, via an alternative endocytotic compartment which is not inhibited by increasing the endosomal pH.

Analysis of nonpermissivity in mouse cells overexpressing simian virus 40 T antigen

To analyze the nature of the nonpermissivity of mouse cells for simian virus 40 (SV40) DNA replication, we isolated mouse cells producing SV40 T antigen (Tag) at levels equal to or greater than that found in COS1 cells. These mouse cells were nonpermissive for the replication of exogenously added SV40 DNA, although purified Tag isolated from these cells was able to support SV40 DNA replication in vitro. Furthermore, when mouse cells expressing Tag were fused to monkey cells, SV40 DNA replication was observed. These results indicate that the mere production of large quantities of wild-type SV40 Tag does not overcome the block of nonpermissivity in mouse cells and that cellular factors must play a critical role.

Activation of purified simian virus 40 virions by free amino-group containing phospholipid liposomes

The infectivity of the simian virus 40 (SV40) virions purified after treatment with sodium deoxycholate is activated by mixing, prior to infection, the virions with the liposomes composed of phosphatidylserine or a mixture of phosphatidylethanolamine and phosphatidylcholine (H. Shimura, and G. Kimura (1985), Virology 144, 268-272). The sucrose-CsCl cushion sedimentation analysis of the virions mixed with the liposomes revealed that the density of the radiolabeled virions became lower and that of the radiolabeled liposomes became higher to give a similar range, suggesting the binding of virions with the liposomes. Electron microscopy revealed the side-to-side association of virions with liposomes. The efficiency of adsorption of the virions to monkey kidney BSC-1 cells varied depending on phospholipid types mixed with virions and did not always become high. In the case of phosphatidylethanolamine liposomes, the free amino group in the phospholipid molecule was essential for the activation of the virion infectivity, because mono- and di-methylated phosphatidylethanolamine failed to activate the infectivity. Fluid nature of phospholipids seemed to be necessary also for the infectivity activation, because dipalmitoyl and distearoyl phospholipids did not activate virion infectivity at 37 degrees, the temperature at which the liposomes of these phospholipids are supposed to be in a solid state. Presence of free amino groups and difference in acyl groups of the phospholipids did not influence the adsorption of the virions to cells. These results suggest that events which occur after adsorption of virions to cells are responsible for the activation of the SV40 virion infectivity by the liposomes composed of free amino-group containing phospholipids.

The SV40 enhancer is composed of multiple functional elements that can compensate for one another

We present evidence that the SV40 enhancer consists of three functional units, A, B, and C, each of which can cooperate with the others or with duplicates of itself to enhance transcription. We show that, when element C, containing the core consensus sequence, is inactivated by point mutations, revertants with restored enhancer function contain duplications of either one or both of the elements A and B. To search for additional elements, we isolated revertants of a mutant with the three elements mutated. These revertants do not identify any other elements; instead, enhancer function is effectively restored by "double duplications," in which the first duplication event either partially or entirely recreates one of the three elements A, B, and C and the second duplication then creates two copies of the newly created sequence.

trans Activation of the simian virus 40 enhancer

We describe experiments which demonstrated that the simian virus 40 (SV40) enhancer affects certain transcriptional units differently. We also found that a specific enhancer-transcriptional unit interaction can be regulated by trans-acting factors. Using transient assays, we examined the effects of the SV40 enhancer on herpesvirus thymidine kinase (tk) RNA levels when transcription was initiated either by the herpesvirus tk promoter or by an SV40 early promoter-tk fusion. We were unable to detect any effect of the enhancer on transcription from the tk promoter in CV-1 or HeLa cells. However, we found that the addition of T-antigen in trans allowed the enhancer to stimulate expression from the tk promoter. This induction by T-antigen did not require T-antigen-binding sites in cis and appeared to be an indirect effect. In contrast, tk expression from the SV40 early promoter fusion was greatly stimulated by the enhancer in CV-1 cells. Furthermore, in 293 cells the SV40 enhancer had only a marginal effect on the SV40 promoter-tk fusion, whereas it strongly stimulated tk expression from the tk promoter. Our results raise the possibility that the enhancer function may not show cell specificity per se; rather, the interaction between the enhancer and a specific gene may be responsible for cell specificity. We discuss these observations in terms of the SV40 early gene-to-late gene switch that occurs during SV40 lytic growth.

SV40 enhancer activation during retinoic acid-induced differentiation of F9 embryonal carcinoma cells

The transient expression vector pSV2CAT, which carries the bacterial chloramphenicol acetyl transferase (CAT) gene under the control of the SV40 early promoter, was used to transfect the murine embryonal carcinoma cell line F9 at various times during the retinoic acid-induced differentiation of these cells. Expression of the CAT gene under SV40 promoter control was found to increase markedly on F9 cell differentiation, measured relative to expression from the thymidine kinase promoter in the same cells. A series of constructs was prepared to identify the features of the SV40 early promoter required for transcription in differentiated and undifferentiated cells, as well as the factors limiting transcription in each case. The increased transcription seen on F9 cell differentiation was not observed when cells were transfected with molecules lacking a functional enhancer. It appears that as embryonal carcinoma cells differentiate, increased SV40 transcription results from enhancer sequence activation. In both differentiated and undifferentiated cell types the level of transcription was found to be limited by the availability and/or activity of cellular factors necessary for enhancer function.

P53-transformation-related protein: kinetics of synthesis and accumulation in SV40-infected primary mouse kidney cell cultures

During abortive infection of Go/G1-arrested primary baby mouse kidney (BMK) cell cultures with simian virus 40 (SV40), expression of the viral large T antigen is followed by a mitotic host response including the stimulation of host macromolecular synthesis and induction into the cell cycle of Go/G1-arrested cells. We performed an extensive study of the sequential events taking place after SV40 infection of confluent BMK cell cultures. This study comprised a detailed kinetic analysis of transcription, synthesis, and accumulation of p53, in conjunction with the time course of large T antigen synthesis and SV40-induced cellular DNA replication. The monoclonal antibodies used for specifically recognizing mouse p53 were PAb 421, PAb 122, PAb 246, PAb 248, and RA3-2C2. Our results consistently show that under our experimental conditions, the stimulation of p53 synthesis and the accumulation of p53 occur well after the onset of T antigen-induced cellular DNA replication. This relatively late activation of p53 expression appears to be controlled at a level other than transcription. In conclusion, we suggest that, at least in certain cases, T antigen's mitogenic potential is not dependent on its interaction with p53.

Differential effect of ultraviolet light on the induction of simian virus 40 and a cellular mutator phenotype in transformed mammalian cells

UV irradiation of simian virus 40 (SV40)-transformed human and hamster cells induced them both to express a mutator phenotype and to produce SV40. The mutator could also be activated indirectly by transfecting unirradiated cells with UV-damaged calf thymus DNA. In contrast, UV-damaged exogenous DNA failed to rescue SV40 from unirradiated transformed cells. These results suggest that the expression of transforming viruses and of cellular mutator functions is regulated by at least partially independent mechanisms. Unlike the activation of a cellular mutator phenotype, the rescue of SV40 from virus-transformed mammalian cells by UV light might require that the integrated viral DNA and/or specific cellular sequences are directly damaged.

Survival and mutagenesis of ultraviolet irradiated simian virus 40 in foetal human fibroblasts

Survival and mutagenesis of UV-irradiated, temperature-sensitive simian virus 40 mutants (SV40) have been studied after infection of human fibroblasts. Survival of the viral progeny obtained after 6,8 or 10 days at permissive temperature decrease as a function of the UV-dose delivered to the virus. In cels which have been pretreated with 10 Jm-2 of UV 24 hours before infection, progeny survival was increased as compared to survival in control cells. The reactivation factor varies from one to ten, depending on the number of lytic cycles carried out at permissive temperature. The level of mutation frequency, as measured by the reversion from a temperature sensitive growth phenotype towards a wild type phenotype, increases with the dose of UV-irradiation given to the virus. Moreover, the mutation frequency is increased in the viral progeny produced in UV-irradiated human cells. Similar experiments carried out with SV40-transformed human fibroblasts, which constitutively express SV40 T antigen, gave comparable results. These experiments show that, as in monkey cells, a new error-prone recovery pathway can be induced by pretreating human cells with UV-light before infection.

Two classes of transformation-deficient, immortalization-positive simian virus 40 mutants constructed by making three-base insertions in the T antigen gene

We constructed two mutants of simian virus 40 (SV40) by introducing a three-base duplication at AvaII cutting sites within the large T antigen coding region, and we examined these mutants for their abilities to replicate in monkey GC7 cells, to transform rat cell line 3Y1 cells, and to transform and immortalize primary cells from newborn rats. Neither of the mutants could replicate in GC7 cells. One mutant with the duplication at 0.335 SV40 map units (m.u.) (inA942) could transform 3Y1 cells, but the other mutant with the duplication at 0.636 m.u. (inA941) could not. The two mutants could not transform primary rat cells but retained immortalization activity. The results suggest that transformation of primary cells by SV40 requires at least two distinct activities of the large T antigen, one of which can be replaced by a cellular function(s) expressed in immortalized 3Y1 cells.

A viable simian virus 40 variant that carries a newly generated sequence reiteration in place of the normal duplicated enhancer element

A segment comprising the transcriptional enhancer elements was deleted from a recombinant plasmid carrying the simian virus 40 genome. The mutated viral chromosome was excised from the plasmid and propagated through several cycles of growth in monkey kidney cells. A variant was obtained that carried reiterations of sequences that span both sides of the deleted enhancer region. The mutant virus, dup1495, displays a lag in its growth kinetics as compared to its parent, but it ultimately generates wild-type yields. The mutant virus expresses early mRNAs at near-normal levels, and the reiterated sequence functioned in cis to enhance transformation of mouse cells by the herpesvirus thymidine kinase gene. Thus dup1495 reiterated segments encode enhancer activity even though their primary sequence is radically different from that of the normal simian virus 40 enhancer.

Genetic changes in mammalian cells reminiscent of an SOS response

Prior to the isolation of mammalian DNA repair genes and identification of their gene products, the comparison between the bacterial SOS response and various similar reactions in mammalian cells remains rather speculative. The increasing number of observed phenomena including enhanced DNA repair, virus induction, induced cellular differentiation, and neoplastic transformation, all following DNA damage or arrest of replication, are, however, suggestive of an SOS-like system of growth control and may form an entry into this fascinating area.

Identification of nuclear matrix proteins tightly bound to soluble simian virus 40 chromosomes

Nuclear matrix proteins (defined as the nuclear proteins which were highly enriched in an insoluble fraction following extraction of lipids, loosely bound proteins, and nucleic acids) from mock- and SV40-infected cells were identified by two-dimensional polyacrylamide gel electrophoresis, consisting of nonequilibrium pH gradients in the first dimension and sodium dodecyl sulfate gel electrophoresis in the second dimension. The proteins identified in the mock-infected nuclear matrix included M1 (molecular weight, 71K), M2 (69K) M3 (58K), M4 (50K), M5 (49K), M6 (36K), M7 (36K), and M8 (31K), while the nuclear matrix from SV40-infected cells included, in addition to all these proteins, VP-1 (45K), VP-1' (44K), VP-3 (25K), V1 (36K), V2 (35K), and V3 (35K). Except for M7 all of these proteins sedimented with SV40 chromosomes isolated and partially purified by glycerol gradient sedimentation at low ionic strength, and only M6 and M8 were removed from the SV40 chromosomes during more extensive purification of the SV40 chromosomes by subsequent sedimentation at high ionic strength (0.5 M NaCl). When the structures of the SV40 chromosomes were destroyed by digestion with DNAase I, these tightly bound proteins no longer sedimented to the position of SV40 chromosomes. Further subfractionation of SV40 chromosomes indicated that the proteins M1 to M4 were preferentially associated with the nonreplicating SV40 chromosomes, whereas M5 was associated with encapsidating SV40 chromosomes and virions.

Lack of enhanced reactivation of simian virus 40 irradiated with van de Graaff electrons in U.V.-irradiated CV-1 monkey cells

Simian virus 40 (SV40) irradiated with U.V. or van de Graaff electrons was assayed on CV-1 monkey cells irradiated with U.V. before virus infection. U.V.-irradiated cells enhanced the survival of U.V.-irradiated virus, while little or no enhancement was observed for electron-irradiated virus assayed on U.V.-irradiated cells. It is suggested that the U.V.-irradiated cells are able to increase the repair of U.V.-damaged viral DNA, but not of electron-damaged DNA.

Suppression of a VP1 mutant of simian virus 40 by missense mutations in serine codons of the viral agnogene

We isolated second-site revertants of a partially defective VP1 mutant of simian virus 40. The suppressing mutation in each of these pseudorevertants was mapped to the viral agnogene. Of six independently isolated pseudorevertants, all had a missense mutation in a serine codon, near the beginning of the agnogene, that would cause replacement of serine at position 7, 11, or 17 in the agnoprotein by a hydrophobic amino acid. Our results suggest that the agnoprotein interacts in a specific way with VP1 during the late stages of viral development.

The adenovirus type 2-simian virus 40 hybrid virus Ad2+ND4 requires deletion variants to grow in monkey cells

The Ad2+ND4 virus is an adenovirus type 2 (Ad2)-simian virus 40 (SV40) recombination. The Ad2 genome of this recombinant has a rearrangement within early region 3; Ad2 DNA sequences between map positions 81.3 and 85.5 have been deleted, and the SV40 DNA sequences between map positions 0.11 and 0.626 have been inserted into the deletion in an 81.3-0.626 orientation. Nonhybrid Ad2 is defective in monkey cells; however, the Ad2+ND4 virus can replicate in monkey cells due to the expression of the SV40-enhancing function encoded by the DNA insert. Stocks of the Ad2+ND4 hybrid were produced in primary monkey cells by using the progeny of a three-step plaque purification procedure and were considered to be homogeneous populations of Ad2+ND4 virions because they induced plaques in primary monkey cells by first-order kinetics. By studying the kinetics of plaque induction in continuous lines (BSC-1 and CV-1) of monkey cells, we have found that stocks (prepared with virions before and after plaque purification) of Ad2+ND4 are actually heterogeneous populations of Ad2+ND4 virions and Ad2+ND4 deletion variants that lack SV40 and frequently Ad2 DNA sequences at the left Ad2-SV40 junction. Due to the defectiveness of the Ad2+ND4 virus, the production of progeny in BSC-1 and CV-1 cells requires complementation between the Ad2+ND4 genome and the genome of an Ad2+ND4 deletion variant. Since the deletion variants that have been obtained from Ad2+ND4 stocks do not express the SV40-enhancing function in that they cannot produce progeny in monkey cells, we conclude that they are providing an Ad2 component that is essential for the production of Ad2+ND4 progeny. These data imply that the Ad2+ND4 virus is incapable of replicating in singly infected primary monkey cells without generating deletion variants that are missing various amounts of DNA around the left Ad2-SV40 junction in the hybrid genome. As the deletion variants that arise from the Ad2+ND4 virus are created by nonhomologous DNA recombination, the generation of deletion variants in monkey cells infected with Ad2+ND4 may be a useful model for studying this process.

An SV40 mammalian inductest for putative carcinogens

This paper describes an in vitro mammalian inductest for putative carcinogens. Several chemical agents were tested using this system which relies on the induction of Simian virus 40 from SV40-transformed hamster kidney cells as an indicator of potential carcinogenic hazard. Aflatoxin B1, sterigmatocystin and aflatoxin G1 were found to be the most efficient inducers in this system followed by the polycyclic hydrocarbons 9,12-dimethylbenzanthracene and benzo[a]pyrene. In principle, this test is similar to a bacterial inductest and the results obtained in the mammalian inductest are compared to those obtained for the same compounds in the bacterial inductest. In addition, SV40 induction is known to occur in response to ultraviolet radiation and ultraviolet radiation plus the photosensitizing drug, 8-methoxypsoralen.

Optimal conditions for titration of SV40 by the plaque assay method

The parameters of the Simian Virus 40 (SV40) plaque assay on African green monkey kidney cells were optimized for reproducibility and maximum plaquing efficiency. Plaques were visible as early as 8 days postinfection; maximum titers were obtained with a 10- to 11-day incubation period. Titers read 12-16 days postinfection were not significantly higher than those observed after 10-11 days. Adsorption volumes greater than 0.1 ml/60 mm Petri dish decreased plaque forming units (PFUs) detected. Times greater than 60 min for adsorption of virus to the cell monolayer did not significantly increase the titer; adsorption times less than 60 min resulted in decreased titers. Under standard conditions, 3 ml of overlay medium containing 0.8% agar was applied following virus adsorption and again on days 5 and 10. Concentrations of fetal calf serum (FCS) in the overlay medium of 2.5 to 7.5% gave equal plaque formation. FCS concentrations of 1 and 10% resulted in slightly decreased and increased plaquing efficiencies respectively. Of the reagents tested, agar or agarose containing overlay media produced plaques of maximum number and size. An overlay of methyl cellulose resulted in the same number of plaques, but their size was reduced by approximately 70% relative to those observed in agar; thus longer incubation times were required. Gum tragacanth overlay medium was actually inhibitory to plaque development. DEAE-dextran, dextran sulfate, or DMSO added to agar overlay medium did not enhance plaque number or size, nor did they shorten the incubation period required for their detection.

Simian virus 40 large tumor antigen on replicating viral chromatin: tight binding and localization on the viral genome

Pulse-labeled simian virus 40 (SV40) chromatin as well as uniformly labeled viral chromatin are immunoprecipitable by an SV40-specific tumor antiserum and therefore contain bound tumor antigen (T antigen). Single-stranded calf thymus DNA, immobilized on cellulose, competes effectively for T antigen binding with uniformly labeled nonreplicating, but not with pulse-labeled replicating, chromatin. Furthermore, T antigen dissociates in 0.5 M NaCl from nonreplicating chromatin and from purified SV40 DNA, whereas most T antigen remains associated with replicating chromatin even in the presence of 1.2 to 1.5 M NaCl. We used filtration through DNA-cellulose columns and treatment with high salt to prepare pulse-labeled immunoreactive viral chromatin. The viral DNA was digested before, and in other experiments after, immunoprecipitation with the restriction endonuclease HindIII. We found that SV40 DNA sequences, most probably representing the entire genome, remain in the immunoprecipitate after HindIII digestion, indicating an association of T antigen with origin-distal sections of replicating viral DNA. The results suggest that T antigen in replicating chromatin may be bound to regions close to replicating points. We performed control experiments with in vitro-formed complexes of T antigen and SV40 DNA. When these complexes were immunoprecipitated and HindIII digested we found, in agreement with previous studies, that only the origin containing the HindIII C fragment carried bound T antigen.

Tumor promoter 12-O-tetradecanoylphorbol 13-acetate stimulates simian virus 40 induction by DNA-damaging agents and tumor initiators

Simian virus 40 (SV40)-transformed Syrian hamster kidney cells produce infectious SV40 virus particles after treatments which damage DNA, such as UV irradiation or mitomycin C treatment. We have found that the induction of SV40 by DNA-damaging agents is greatly stimulated when a typical tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA), is present in the medium. Phorbol, which has a molecular structure similar to TPA but does not have any tumor-promoting activity, showed no such stimulatory effect on SV40 induction. This apparent synergistic effect of DNA-damaging agents and tumor promoter (TPA) was more pronounced when a tumor initiator, benzo [a]pyrene or 2-acetamido-fluorene, was combined with TPA. The effect of TPA on UV-triggered SV40 induction was greatly influenced by the timing of TPA addition to the culture medium, which was most efficient when addition of TPA was 5 to 20 h before UV irradiation. The effect of TPA, however, was not observed in SV40 rescue from hamster cells by cell fusion with permissive monkey (C7) cells.

Self-assembly of simian virus 40 large T antigen oligomers by divalent cations

In simian virus 40-transformed cells, simian virus 40 large T antigen can be detected in different forms separable by sucrose density gradient centrifugation. In our experiments, light forms sedimented around 5 to 7S, oligomers such as tetramers were detected around 16S, and higher aggregates sedimented in a broad distribution reaching above 23S. The oligomers sedimenting at and above 16S could be disassembled into the slowly sedimenting 5 to 7S forms by chelating agents [EDTA or ethylene bis(oxonitrilo)tetraacetate]. After the addition of divalent cations (CaCl2 or MgCl2) in excess of chelating agents, oligomeric forms reassembled and appeared in a sedimentation pattern resembling that observed before treatment with chelating agents. Time course studies permitted the identification of the 5 to 7S forms as precursors upon pulse-labeling (15 min); the 16S and higher oligomers were identified as the successors after a 14-h chase. Treatment of extracts of pulse-chase-labeled cells with chelating agents again disassembled the oligomers, whereas pulse-labeled precursors did not change their 5 to 7S sedimentation pattern. Adding an excess of divalent cations reassembled the pulse-chase-labeled T antigen to oligomers but did not influence the sedimentation behavior of pulse-labeled 5 to 7S precursors. It is therefore reasonable to assume that a posttranslational modulation induces divalent cation binding, leading finally to the oligomerization of T antigen. Thus, some of the multifunctional activities of T antigen can be dictated by divalent cation binding properties.