Viral DNA in transformed cells. II. A study of the sequences of adenovirus 2 DNA IN NINE LINES OF TRANSFORMED RAT CELLS USING SPECIFIC FRAGMENTS OF THE VIRAL GENOME;

Adenovirus proteins from both E1B reading frames are required for transformation of rodent cells by viral infection and DNA transfection

To determine the requirements for the individual Ad2 E1B proteins during the transformation of rodent cells, viral mutants were constructed with genetic lesions disrupting the coding sequence of either the 175 amino acid residue (175R) or the 495 amino acid residue (495R) E1B proteins. Point mutations generating stop codons very early in the coding sequences were constructed to prevent the expression of amino-terminal protein fragments which might have biological activity. Mutant virus pm1722 contains a point mutation that terminates translation of the 175R protein after three amino acids. It was completely defective for transformation of CREF cells in virion- and DNA-mediated assays. In HeLa cells, pm1722 replicated as well as wild-type virus but produced an extreme cytopathic effect and fragmentation of host-cell DNA. Nonetheless, we provide evidence that the observed transformation defect is not due to the death of transformed cells. The mutant virus dl1520, a double mutant unable to synthesize the 495R protein, was also extremely defective for the transformation of CREF cells in virion- and viral DNA-mediated assays. This result is in contrast to studies with other Ad5 mutants with lesions in the equivalent protein. Possible explanations for this difference are discussed. Replication of dl1520 in HeLa cells was significantly reduced compared to wild-type. Studies with a third mutant virus, pm2022, which contains a stop codon after the second codon of the 495R protein, suggest that very low levels of 495R protein activity are sufficient for a productive infection and significant transforming activity.

Partition of E1A proteins between soluble and structural fractions of adenovirus-infected and -transformed cells

The partition of E1A proteins between soluble and structural framework fractions of human cells infected or transformed by subgroup C adenoviruses was investigated by using gentle cell fractionation conditions. A polyclonal antibody raised against a trpE-E1A fusion protein (K.R. Spindler, D.S.E. Rosser, and A. J. Berk, J. Virol. 132-141, 1984) synthesized in Escherichia coli was used to measure the steady-state levels of E1A proteins recovered in the various fractions by immunoblotting. The relative concentration of E1A proteins recovered in the soluble fraction of adenovirus type 2-infected cells was at least fivefold greater than the relative concentration in the corresponding fraction of transformed 293 cells. The observed distribution of E1A proteins was not altered by the sulfhydryl-blocking reagent N-ethylmaleimide. E1A proteins were recovered in nuclear matrix, chromatin, and cytoskeleton fractions after further fractionation of the structural framework fraction. However, the E1A protein species that could be identified by one-dimensional gel electrophoresis were not uniformly distributed among the subcellular fractions examined. The results obtained when fractionation was performed in the presence of the oxidation catalysts Cu2+ or (ortho-phenanthroline)2 Cu2+ indicate that E1A proteins can be efficiently cross-linked, via disulfide bonds, to the structural framework of both adenovirus-infected and adenovirus-transformed cells.

Region E1a of highly oncogenic adenovirus 12 in transformed cells protects against NK but not LAK cytolysis

The sensitivity of a library of adenovirus-transformed rat cell lines to lysis with highly enriched populations of rat NK cells and LAK cells activated in vitro by culture with recombinant human IL-2 was studied and correlated with the tumorigenic potential of these cell lines. The cell lines studied express the transforming E1 region of highly oncogenic Ad12 or nononcogenic Ad2. Two cell lines express recombinant E1A regions. In one the E1A genes were of Ad12 origin and the E1B region was derived from nononcogenic Ad5. In the other, the E1A region was from Ad5 and the E1B genes from Ad12. All cell lines tested which express the early region E1 of Ad12 are tumorigenic in syngeneic rats. The two cell lines which express only the E1A or the E1B genes of Ad12, and the Ad2-transformed cells did not induce tumors. Transformed cell lines which express the E1A region of nononcogenic Ad2 or Ad5 are efficiently killed by rat NK cells, but cells which express the Ad12 E1A genes are resistant to lysis by NK-enriched cell fractions even at high effector:target ratio; cells containing the Ad12 E1 region are also resistant to IFN-activated NK cells. Although such NK-resistant cells have a uniformly low level of class I MHC antigen, their resistance is not affected by MHC antigen level modulation by rat IFN. Ad12-transformed cells resistant to endogeneous NK cells, however, are efficiently lysed by LAK cells stimulated in vitro by recombinant IL-2. Sensitivity to LAK killing is unaffected by IFN treatment of target cells. These results show that expression of the E1A region of highly oncogenic Ad12 in the transformed cells, which confers resistance to endogeneous NK cells, fails to protect against lysis by LAK cells.

Adenovirus E1a proteins repress expression from polyomavirus early and late promoters

We have examined the effects of the E1a products of adenovirus types 5 and 12 on the expression of polyomavirus early and late promoters. In cotransfection experiments in HeLa cells, plasmids expressing the E1a region of adenovirus type 5 or 12 repressed both the early and late promoters of polyomavirus, and deletion analysis indicates that the polyomavirus enhancers were the target of the E1a repression. With mutants lacking enhancer sequences, the polyomavirus early promoter but not the late promoter was trans-activated by E1a. Chimeric mutant plasmids with deletions in the regulatory region that contained either the A enhancer or the B enhancer were repressed to the same extent, indicating that E1a can repress both elements. Polyomavirus variant plasmids with rearrangements in the regulatory region conferring activity in embryonal carcinoma stem cells were repressed by E1a as was the wild type, suggesting that the repressor function is quite general. We discuss a model in which the influence of E1a on the transcriptional activity of a gene is the sum of positive and negative effects on promoter and enhancer elements and discuss possible mechanisms of negative regulation of enhancer function.

Cleavage of vimentin in dense cell cultures. Inhibition upon transformation by type 5 adenovirus

The analysis on two-dimensional isoelectric focusing and SDS polyacrylamide gels (2D gels) of the Triton X-100 and high salt-insoluble fraction of fibroblast cell lines, certain epithelial cell lines and granulosa cells revealed various amounts of a vimetin cleavage product, with a more basic pI and with a MW (1,500-2,000) lower than that of intact vimentin. This cleavage product of vimentin which constituted as much as 30% of the total vimentin in an established rat embryo fibroblast cell line (CREF), was detected by a monoclonal antivimentin antibody in whole cell and Triton-insoluble extracts, and it has a phosphorylated variant which can be degraded to form the "staircase pattern" on 2D gels similarly to intact vimentin. This processing of vimentin occurred mainly in dense cell cultures and it could not be induced in sparse cell cultures by inhibiting DNA synthesis with ara C, or by arresting cell growth in medium containing 0.1% serum. Transformation of CREF cells with intact wild-type (H5wt) and host-range cold-sensitive mutants (H5hr1 or H5d1101) of type 5 adenovirus (Ad5), or transformation of CREF cells by Ca2+-mediated DNA transfection with the transforming E1a (0-4.5 map units) or E1a + E1b (0-11.5 map units) region of Ad5 inhibits the cleavage of vimentin in dense cultures only at temperatures which are permissive for expression of the transformed phenotype. The transformation of cells with bovine papilloma virus type 1, with T24 ras oncogene, or with RSV does not interfere with the cleavage of vimentin. The organization of the vimentin network in dense cultures, where the vimentin cleavage occurs, is very different from that of sparse untransformed and sparse or dense Ad5-transformed cells. The possibility that the acidic amino acid-rich C-terminus of vimentin is cleaved in dense cell cultures in conjunction with the reorganization of the vimentin network and the inhibition of this cleavage by transformation with Ad5, are discussed.

Cellular protein differences between nontumorigenic Ad5 and tumorigenic Ad12 transformed mouse cells

Murine fibroblasts transformed by adenovirus 12 (Ad12) show reduced class I major histocompatibility complex (MHC) antigen synthesis and form tumors in syngeneic mice whereas those transformed by adenovirus 5 (Ad5) show no alteration in class I antigen synthesis and do not form tumors (K. B. Eager, J. Williams, D. Breiding, S. Pan, B. Knowles, E. Appella, and R. P. Ricciardi (1985) Proc. Natl. Acad. Sci. USA 82, 5525-5529). Nearly 1500 metabolically labeled polypeptides from the Ad5 and Ad12 transformed cell lines as well as polypeptides from a nontransformed murine line of the same haplotype were compared by two-dimensional gel electrophoresis. In addition to the reduction of the class I H-2 transplantation antigens seen in the Ad12-transformed lines, we detect few but reproducible polypeptide differences between the tumorigenic and nontumorigenic cell lines.

Adenovirus type 12 early region 1A proteins repress class I HLA expression in transformed human cells

The adenovirus type 12 (Ad12) early region 1A (E1A) gene is thought to play a major role in repressing class I major histocompatibility complex expression in transformed rodent cells. However, since transformation by adenovirus requires both E1A and E1B genes, it has not been demonstrated whether the Ad12 E1A gene acts alone or synergistically with the E1B gene to accomplish this effect. Moreover, it is not known whether the repression of class I antigen synthesis by Ad12-transforming gene products occurs only in rodent cells. We show that the Ad12 E1A gene, in the absence of the E1B gene, is capable of greatly reducing the levels of class I HLA antigens and mRNAs in primary human cells transformed by the E1A gene of Ad12 and the large tumor antigen (T-antigen) gene of BK virus; control cells transformed by BK virus T-antigen gene alone or the highly related simian virus 40 T-antigen gene showed no apparent alteration in class I HLA expression. Human recombinant interferon gamma was able to restore synthesis of class I HLA antigens in transformed cells that produced Ad12 E1A proteins, indicating that these cells were not deficient for class I genes. These results strongly indicate that the Ad12 E1A proteins modulate class I gene expression by similar mechanisms in both transformed rodent and human cells.

Association of adenovirus early-region 1A proteins with cellular polypeptides

Extracts from adenovirus-transformed human 293 cells were immunoprecipitated with monoclonal antibodies specific for the early-region 1A (E1A) proteins. In addition to the E1A polypeptides, these antibodies precipitated a series of proteins with relative molecular weights of 28,000, 40,000, 50,000, 60,000, 80,000, 90,000, 110,000, 130,000, and 300,000. The two most abundant of these polypeptides are the 110,000-molecular-weight protein (110K protein) and 300K protein. Three experimental approaches have suggested that the 110K and 300K polypeptides are precipitated because they form stable complexes with the E1A proteins. The 110K and 300K polypeptides do not share epitopes with the E1A proteins, they copurify with a subset of the E1A proteins, and they bind to the E1A proteins following mixing in vitro. The 110K and 300K polypeptides are not adenoviral proteins, but are encoded by cellular DNA. Both the 12S and the 13S E1A proteins bind to the 110K and 300K species, and these complexes are found in adenovirus-transformed and -infected cells.

Role of the adenovirus early region 1B tumor antigens in transformation and lytic infection

We have investigated the contribution of each of the two adenovirus type 5 (Ad5) major early region 1b (E1b) proteins in cell transformation and in lytic infection. An Ad5 E1 plasmid, in which the reading frame for the 19-kDa E1b protein was abolished by a stop codon close to the initiation codon, transformed primary baby rat kidney (BRK) cells with an efficiency of about half of that of a wild type Ad5 E1 plasmid, whereas a plasmid with a mutation in the gene for the 58-kDa E1b protein transformed the same primary cells with only one-third of the wild type efficiency. Plasmids containing region E1a only or a plasmid carrying mutations in the genes for major E1b proteins all transformed primary cells with an efficiency of approximately 5% of wild type. To test the effect of the E1b mutations in virion-mediated cell transformation, the mutant E1b regions were introduced into intact viral genomes by overlap recombination and were subsequently used in a transformation assay on BRK cells. The 19 and 58-kDa mutant viruses were found to transform BRK cells with 11 and 25% of the efficiency of wild type virus, respectively. These results suggest that the 19-kDa E1b protein is essential for virus-mediated cell transformation, in agreement with results of others, but not for plasmid-mediated cell transformation. In lytic infection, the 19-kDa mutant virus was some 30-fold reduced in yield on HeLa cells, whereas the 58-kDa mutant virus was 3000-fold reduced in its ability to grow on HeLa cells at low multiplicity of infection, but showed a marked multiplicity-dependent leakiness. The 58-kDa mutant virus was not defective when its growth was assayed on human embryonic kidney (HEK) cells. This may indicate that cellular proteins are expressed in HEK cells that are functionally homologous to the 58-kDa E1b protein.

Mutations in the E1a gene of adenovirus type 5 alter the tumorigenic properties of transformed cloned rat embryo fibroblast cells

The adenovirus type 5 mutants H5hr1 and H5dl101 contain modifications in the E1a gene affecting the 13S mRNA-encoded 289-amino acid polypeptide and exhibit a cold-sensitive transformation phenotype upon infection of cloned rat embryo fibroblast (CREF) cells. Transformed cell lines expressing solely E1a or E1a and E1b gene products derived from these viruses display enhanced anchorage-independent growth at 37 degrees C versus 32 degrees C and display a cytoskeletal architecture resembling untransformed fibroblastic CREF cells. In contrast, CREF cells transformed by H5wt or the E1a and E1b region of H5wt grow with similar efficiency in agar at 37 degrees C or 32 degrees C and exhibit an epithelioid morphology that is associated with an altered cytoskeleton. Regardless of the expression or presence of other viral early regions, including E1b, E2a, and E4 genes, specific CREF cell lines expressing an altered 289-amino acid protein and a wild-type 12S mRNA-encoded 243-amino acid protein were capable of inducing tumors in nude mice and in immunocompetent syngeneic Fischer rats. In sharp contrast, cells expressing a wild-type 289-amino acid protein were unable to induce tumors in either nude mice or syngeneic rats. The ability to induce tumors did not correlate with alterations in the pattern of viral DNA integration or differential expression of the E1a and E1b genes, nor was the tumor induction a consequence of unique properties of the immortal parental CREF cell line.

Adenovirus tumor-specific transplantation antigen is a function of the E1A early region

Viable recombinant adenoviruses that carry a portion of the type 12 E1A and E1B transcription units in a type 5 background were used to identify genes controlling expression of the adenovirus tumor-specific transplantation antigen (TSTA). The TSTA immunity is not crossreacting between the group A and group C adenovirus serotypes. Viruses carrying the E1A region (sub370-12E1A), or both E1A and E1B (sub370-12E1AB) regions of Ad12, induce a strong transplantation immunity against tumors induced by syngeneic cells transformed with adenovirus 12, but fail to induce any protection against syngeneic cells transformed with adenovirus 2. Immunization with the virus carrying only the E1B region (sub370-12E1B) of adenovirus 12 induces no immunity to adenovirus 12 transformed cell line, but confers a strong protection against cells transformed with adenovirus 2. These results provide strong evidence that the adenovirus tumor-specific transplantation antigen is a function of the E1A early region.

Tumorigenicity of adenovirus-transformed cells: region E1A of adenovirus 12 confers resistance to natural killer cells

Sensitivity of a library of rat cells transformed in vitro with viable recombinant adenoviruses to natural killer (NK) cells and allogeneic cytotoxic T cells has been studied and correlated with their oncogenic potential in syngeneic rats. All cell lines transformed with the sub370-12E1AB virus (containing E1A and E1B regions of Ad12) and with the sub370-12E1A virions (containing the E1A region of Ad12 and the E1B region of Ad5) showed a high degree of resistance to NK cells. The cell lines transformed with the sub370-12E1B virus (containing the E1A region of Ad5 and the E1B region of Ad12) were highly sensitive to NK cytotoxicity. While all cell lines transformed with virions containing the E1A genes of Ad5 expressed high levels of class I MHC antigen, only three of eight cell lines containing the E1A region of Ad12 showed detectable levels by flow cytometric analysis after staining with specific antibodies. All cell lines containing E1A genes of Ad5 were killed by in vitro generated allogeneic cytolytic T cells. Only three of eight cell lines containing the E1A region of Ad12 were killed by such CTLs; the level of cytotoxicity, however, did not reach that seen with the cells containing the E1A genes of Ad5. All cell lines containing the E1A and E1B genes of Ad12 were highly tumorigenic. Only two of four cell lines transformed with virus containing the E1A genes of Ad12 and E1B region of Ad5 were tumorigenic. The efficiency of tumor induction was low and the latent period was long confirming the importance of the E1B region. None of the cell lines transformed with virus containing the E1A region of Ad5 and the E1B genes of Ad12 were tumorigenic, reflecting their high degree of sensitivity to both natural and induced cellular immunity. Expression of the E1A region of Ad12 in transformed cells modulates not only the level of class I MHC antigens, but also confers resistance to NK cell cytotoxicity.

Detection of cellular proteins associated with human adenovirus type 5 early region 1A polypeptides

Antisera prepared against synthetic peptides corresponding to the amino and carboxy termini of human adenovirus type 5 (Ad5) early region 1A (E1A) proteins were used to identify polypeptides that are associated with these viral species in lytically infected KB cells. Proteins were sought which coprecipitated with E1A polypeptides using both sera and which were not recognized in extracts from mock-infected cells by either serum. Four such species were identified with apparent molecular weights of 68K, 65K, and a doublet at about 105K. A fifth species migrating with a molecular weight in excess of 250K was also identified consistently with E1A-C1 but not E1A-N1 serum. Addition of an excess of the appropriate synthetic peptide to the immunoprecipitation mixtures prevented the precipitation of all of these species. Mixing experiments demonstrated that all species were cellular proteins expressed in normal uninfected KB cells and in addition showed that an association with E1A proteins could take place in vitro. Studies carried out with the mutants pm975 and hr1 indicated that while the 105K doublet and the greater than 250K species were found with the products of both the 1.1- and 0.9-kb E1A mRNAs, 65K and 68K appeared to be primarily associated with those of the 1.1-kb mRNA. Finally, the 105K doublet and greater than 250K were shown to be phosphoproteins. These data indicated that Ad5 E1A proteins may function in a complex with cellular polypeptides which includes species of 105K, 68K, 65K, and possibly a large protein of greater than 250K.

Adenovirus 5 early region 1A host range mutants hr3, hr4, and hr5 contain point mutations which generate single amino acid substitutions

The early region 1A (E1A) gene of adenovirus 5 encodes two proteins, 289AA and 243AA, which are translated from mRNAs of 13S and 12S, respectively. These two E1A proteins are identical except for an internal stretch of 46 amino acids unique to the larger protein. The 289AA protein activates transcription from promoters of other early adenoviral genes. The adenovirus type 5 host range mutants hr3, hr4, and hr5 are unable to activate transcription of these early viral genes. We show here that hr3, hr4, and hr5 each contain a distinct missense mutation in the E1A gene. We first localized the mutations in a series of constructed wild-type-hr hybrid E1A genes by using a biological assay which can discriminate between functional and nonfunctional E1A proteins. We then identified the mutations by DNA sequencing. In hr3 lysine replaced methionine at position 176, and in hr4 phenylalanine replaced leucine at position 173; both substitutions occurred in the region unique to the 289AA protein. In hr5, due to the splicing patterns of the two mRNAs, asparagine replaced serine as the last amino acid in the unique region of the 289AA protein at position 185, while aspartic acid replaced glycine at position 139 in the 243AA protein, which is the last amino acid common to both proteins before the unique region. These results substantiate the role of the 289AA protein in transcriptional activation and underscore the importance of the unique region as the basis of the functional difference between the two E1A proteins. Implications as to how these mutations affect the structure and function of the E1A proteins in transcriptional activation and transformation are discussed.

Monoclonal antibodies specific for adenovirus early region 1A proteins: extensive heterogeneity in early region 1A products

Hybridomas secreting monoclonal antibodies specific for the adenovirus early region 1A (E1A) proteins were prepared from BALB/c mice immunized with a bacterial trpE-E1A fusion protein. This protein is encoded by a hybrid gene that joins a portion of the Escherichia coli trpE gene and a cDNA copy of the E1A 13S mRNA (Spindler et al., J. Virol. 49:132-141, 1984). Eighty-three hybridomas that secrete antibodies which recognize the immunogen were isolated and single cell cloned. Twenty-nine of these antibodies are specific for the E1A portion of the fusion protein. Only 12 of the monoclonal antibodies can efficiently immunoprecipitate E1A polypeptides from detergent lysates of infected cells. E1A polypeptides were analyzed on one-dimensional, sodium dodecyl sulfate-polyacrylamide gels and two-dimensional, isoelectric focusing polyacrylamide gels. The E1A proteins that are specifically immunoprecipitated by the monoclonal antibodies are heterogeneous in size and charge and can be resolved into approximately 60 polypeptide species. This heterogeneity is due not only to synthesis from multiple E1A mRNAs, but also at least in part to post-translational modification. Several of the monoclonal antibodies divide the E1A polypeptides into immunological subclasses based on the ability of the antibodies to bind to the antigen. In particular, two of the monoclonal antibodies bind to the polypeptides synthesized from the 13S E1A mRNA, but not to other E1A proteins.

Expression of histocompatibility antigens H-2K, -D, and -L is reduced in adenovirus-12-transformed mouse cells and is restored by interferon gamma

Primary mouse cells transformed by adenovirus type 12 (Ad12) expressed negligible amounts of class I antigens H-2K, -D, and -L on the cell surface and were capable of forming tumors in syngeneic animals, whereas cells transformed by Ad5 continued to express class I antigens and were nontumorigenic. Cells from a tumor, generated by injection of Ad12-transformed mouse cells into a syngeneic mouse, also expressed low levels of H-2 antigens, indicating that this phenotype is maintained in vivo. In all Ad12-transformed cells, synthesis of the H-2 heavy chain was not detected whereas the beta 2-microglobulin light chain was synthesized. Furthermore, the level of cytoplasmic H-2 mRNA in the Ad12 lines was greatly reduced. Reduction of H-2 expression is instructed solely by the transforming region of the viral genome, since this repression occurred in cells transformed by a DNA fragment containing only Ad12 E1A and E1B genes. Addition of recombinant murine interferon gamma strongly stimulated expression of class I antigens in the Ad12 transformants as well as in cells from the Ad12 tumor. This result indicates that Ad12 does not preferentially transform cells that are deficient for class I genes and that Ad12 does not mutate the class I genes in cells it transforms. The correlation between tumorigenicity and loss of H-2 expression in Ad12-transformed cells is discussed.

Activation and inhibition of expression of the 72,000-Da early protein of adenovirus type 5 in mouse cells constitutively expressing an immediate early protein of herpes simplex virus type 1

It has been previously reported that immediate early proteins of pseudorabies and cytomegalo viruses can substitute for the products of the human adenovirus type 5 (Ad5) E1A gene in the activation of early Ad5 transcription. In the present report the effect of one of the herpes simplex virus type 1 (HSV-1) immediate early genes, ICP4, on Ad5 early gene expression has been examined using mouse cell lines that constitutively express ICP4. These lines as well as nonproducers were infected with wild-type (wt) Ad5 or with various Ad5 E1A mutants and the levels of expression of the Ad5 E2A 72K DNA binding protein were measured by immunoprecipitation with a monoclonal antibody specific for 72K. With dl 312, which lacks E1A, some 72K expression was seen in nonproducer lines but levels were considerably higher in the producer lines. A similar result was also obtained using dl 312-infected nonproducer cells that were superinfected with HSV-1 virions. These data suggest that HSV-1 ICP4 can substitute for E1A in the activation of expression of early Ad5 proteins. With wt Ad5, 72K was also expressed at high levels in nonproducer mouse cells, however, in the ICP4 producer cell lines, a marked inhibition of 72K expression was observed and this inhibition correlated with the amount of ICP4 present. Using the E1A mutants pm 975 and hr 1, this inhibition was found to be specific for the products of the 1.1-kb E1A mRNA. These data suggest that ICP4 and E1A proteins either directly inhibit each other, or more likely, operate independently and competitively on factors required for viral gene activation.

Immortalization of rat embryo fibroblasts by an adenovirus 2 mutant expressing a single functional E1a protein

Expression of the adenovirus E1a and E1b genes is required for transformation of nonpermissive rodent cells. Differential splicing of the E1a precursor RNA molecules results in the production of two early mRNAs, 13S and 12S, which encode a 289-amino-acid-residue (289R) and 243R protein, respectively. Previously we constructed a mutant virus, dl231, which can only produce normal 289R protein from the E1a gene. In this report we demonstrate that dl231 induced focal transformation of primary rat embryo fibroblasts at 20% of the level of wild-type virus. dl231 transformants were immortalized and produced normal levels of E1a 13S and E1b mRNAs but only minute levels of defective E1a 12S mRNA. These transformants only minimally expressed the transformation phenotype and were similar to untransformed cells. Unlike wild-type transformants, they had a more fibroblastic morphology, were contact inhibited, grew to only low saturation density, and were limited in their ability to grow in an anchorage-independent manner in soft agar. We conclude that the 289R E1a protein can mediate immortalization of primary cells and that the 243R E1a protein is required to elicit the full transformation phenotype.

Organization of early region 1B of human adenovirus type 2: identification of four differentially spliced mRNAs

The mRNAs from early region 1B of adenovirus type 2 have been studied by Northern blot, S1 nuclease, and cDNA analysis. Two novel mRNAs, designated 14S and 14.5S, have been observed in addition to the previously identified 9S, 13S, and 22S mRNAs. They are 1.26 and 1.31 kilobases long and differ from the 13S and 22S mRNAs in being composed of three exons instead of two. Their two terminal exons are the same as those present in the 13S mRNA, whereas the middle exon is unique to each of the two novel mRNA species. The structures of the 14S and 14.5S mRNAs allow the prediction of their coding capacities: both mRNA species, like the 22S and 13S mRNAs, contain an uninterrupted translational reading frame encoding a 21,000-molecular-weight (21K) polypeptide. The 14S mRNA can, in addition, encode a 16.5K polypeptide which shares N-terminal and C-terminal sequences with the 55K polypeptide, known to be encoded by the 22S mRNA. The 14.5S mRNA species encodes a hypothetical 9.2K polypeptide which has the same N terminus as the 55K polypeptide but a unique C terminus. The two mRNAs differ in their kinetics of appearance; the 14.5S mRNA is preferentially expressed late after infection in contrast to the 14S mRNA, which is present in approximately equal amounts early and late after infection. Taken together with previously published information the results suggest that early region 1B of adenovirus type 2 encodes five proteins in addition to virion polypeptide IX. These have predicted molecular weights of 55,000, 21,000, 16,500, 9,200, and 8,100.

Adenovirus E1a proteins repress transcription from the SV40 early promoter

Using a transient expression assay in HeLa cells, we show that products from the adenovirus-5 E1a transcription unit repress transcription from the SV40 early promoter. The repression is unrelated to T antigen autoregulation, occurs maximally with low concentrations of E1a expression plasmid, is exerted at the transcriptional level, and requires functional E1a protein. The 289 and 243 amino acid E1a proteins are equally effective at repressing transcription. Since only the 289 amino acid protein is efficient at activating transcription, we conclude that activation and repression are separate E1a functions. We discuss possible mechanisms for E1a repression and the relationship of repression to the function of E1a in cell immortalization and transformation.

The use of monoclonal antibodies to study the proteins specified by the transforming region of human adenoviruses

Monoclonal antibodies against two of the proteins specified by one of the transforming genes (early region 1B) of human adenovirus type 2 have been produced and characterized. Two clones (RA1 and PA6), generated by fusion of mouse myeloma NSO cells with splenocytes from rats immunized with whole-cell lysates of an adenovirus-transformed rat cell line (F19), secreted antibodies against a 58 kDa protein. Another clone (DC1) produced antibodies against the same protein, and resulted from fusion of immune rat splenocytes with the rat myeloma Y3.Ag.1.2.3. Immunoprecipitation studies showed that all three antibodies recognized [35S]-methionine-labelled 58 kDa protein, and phosphorylated derivatives of the 58 kDa protein labelled with [32P]orthophosphate present in infected human cells. One clone (EC3) produced antibody against a 19 kDa protein also encoded by early region 1B, but not sharing sequence homology with 58 kDa. The identity of the 19 kDa protein recognized by the EC3 antibody was established by immunoprecipitation from lysates of labelled-infected cells and from products of cell-free translation directed by mRNA isolated from adenovirus 2-infected cells. Indirect immunofluorescent-antibody staining of infected human cells using the RA1 and EC3 antibodies revealed a nuclear location of the 58 kDa protein and a mainly cytoplasmic location of the 19 kDa protein.

Evidence that a second tumor antigen coded by adenovirus early gene region E1a is required for efficient cell transformation

The expression of the adenovirus (Ad) early coding region 1a (E1a) is required for virus-induced cell transformation and for the activation of other viral early genes and some cellular genes. Two overlapping early mRNAs of 13S and 12S that are transcribed from this region code for a 289-amino acid protein and a 243-amino acid protein, respectively. Earlier studies have shown that the 289-amino acid protein is essential for cell transformation. We have constructed an Ad type 2 (Ad2) deletion mutant (dl231) in which the intervening sequence for the 13S mRNA is precisely removed. Mutant dl231 is completely viable in human KB cells and produces normal amounts of 13S mRNA but much reduced amounts of a defective 12S mRNA. Mutant dl231 induces focal transformation of established rat embryo fibroblasts at a frequency one-fifth to one-half that of wild-type virus. However, the transformed cells are defective in their ability to form anchorage-independent colonies on semisolid medium. Therefore, our results demonstrate that the 243-amino acid protein is required for full transformation of rat embryo cells.

UV irradiation stimulates levels of p53 cellular tumor antigen in nontransformed mouse cells

Elevated levels of the p53 cellular tumor antigen have been previously observed in proliferating and transformed mammalian cells. We found that nontransformed mouse cells treated with either UV light or a UV-mimetic chemical carcinogen exhibited a rapid increase in the amount of p53. This stimulation can be explained, at least in part, on the basis of a post-translational stabilization of p53 which is independent of replicative DNA synthesis, consistent with p53 not being an adventitious product of proliferating cells. The results presented here are interpreted in light of the general hypothesis that p53 is involved in the preparation of mammalian cells for DNA synthesis.

Mutations in the gene encoding the adenovirus early region 1B 19,000-molecular-weight tumor antigen cause the degradation of chromosomal DNA

The adenovirus mutant Ad2ts111 has been previously shown to contain a mutation in the early region 2A gene encoding the single-stranded-DNA-binding protein that results in thermolabile replication of virus DNA and a mutation in early region 1 that causes degradation of intracellular DNA. A recombinant virus, Ad2cyt106, has been constructed which contains the Ad2ts111 early region 1 mutation and the wild-type early region 2A gene from adenovirus 5. This virus, like its parent Ad2ts111, has two temperature-independent phenotypes; first, it has the ability to cause an enhanced and unusual cytopathic effect on the host cell (cytocidal [cyt] phenotype) and second, it induces degradation of cell DNA (DNA degradation [deg] phenotype). The mutation responsible for these phenotypes is a single point mutation in the gene encoding the adenovirus early region 1B (E1B) 19,000-molecular-weight (19K) tumor antigen. This mutation causes a change from a serine to an asparagine in the 20th amino acid from the amino terminus of the protein. Three other mutants that affect the E1B 19K protein function have been examined. The mutants Ad2lp5 and Ad5dl337 have both the cytocidal and DNA degradation phenotypes (cyt deg), whereas Ad2lp3 has only the cytocidal phenotype and does not induce degradation of cell DNA (cyt deg+). Thus, the DNA degradation is not caused by the altered cell morphology. Furthermore, the mutant Ad5dl337 does not make any detectable E1B 19K protein product, suggesting that the absence of E1B 19K protein function is responsible for the mutant phenotypes. A fully functional E1B 19K protein is not absolutely required for lytic growth of adenovirus 2 in HeLa cells, and its involvement in transformation of nonpermissive cells to morphological variants is discussed.

UV irradiation stimulates levels of p53 cellular tumor antigen in nontransformed mouse cells

Elevated levels of the p53 cellular tumor antigen have been previously observed in proliferating and transformed mammalian cells. We found that nontransformed mouse cells treated with either UV light or a UV-mimetic chemical carcinogen exhibited a rapid increase in the amount of p53. This stimulation can be explained, at least in part, on the basis of a post-translational stabilization of p53 which is independent of replicative DNA synthesis, consistent with p53 not being an adventitious product of proliferating cells. The results presented here are interpreted in light of the general hypothesis that p53 is involved in the preparation of mammalian cells for DNA synthesis.

Complete transformation by adenovirus 2 requires both E1A proteins

Rodent cells transformed by adenovirus 2 (Ad2) express two highly related viral proteins of 289 and 243 amino acids encoded in early region 1A (E1A). Transformation studies were performed with adenovirus mutants that express only one or the other E1A protein. We found that the 289 amino acid protein, which has transcription inducing activity, and the 243 amino acid protein, which has little if any of this activity, were both required to produce the fully transformed phenotype. Expression of either E1A protein induced a partially transformed phenotype. The 243 amino acid protein was particularly important for anchorage independent growth. As found in previous studies with several other E1A mutants, the process of transformation by the mutant that expresses the 243 amino acid protein only was cold-sensitive. While the 289 amino acid protein is the only E1A protein required for efficient viral replication under standard cell culture conditions, the 243 amino acid protein in addition to the 289 amino acid protein was found to be required for efficient viral replication in growth-arrested human cells.

Adenovirus type 5 early region 1b gene product is required for efficient shutoff of host protein synthesis

To determine the role adenovirus 5 early region 1b-encoded 21- and 55-kilodalton proteins play in adenovirus productive infection, mutants have been isolated which were engineered to contain small deletions or insertions at 5.8, 7.9, or 9.6 map units. By using an overlap recombination procedure involving H5dl314 (delta 3.7 to 4.6 map units) DNA cleaved at 2.6 map units with ClaI and the adenovirus 5 XhoI-C (0 to 15.5 map units) fragment containing the desired mutation, viral mutants were isolated by their ability to produce plaques on KB cell line 18, which constitutively expresses only viral early region 1b functions (Babiss et al., J. Virol. 46:454-465, 1983). DNA sequence analysis of the viral mutants isolated (H5dl118, H5dl110, H5in127, and H5dl163) indicates that all of the viruses contain mutations which affect the 55-kilodalton protein, whereas dl118 should also produce a truncated form of the 21-kilodalton protein. When analyzed for their replication characteristics in HeLa cells, all of the mutant viruses exhibited extended eclipse periods and effected yields that were reduced to 10% or less of that produced by H5sub309 (parent virus of the mutants which is phenotypically identical to wild-type adenovirus 5). When compared with characteristics of sub309, the early and late transcription and DNA replication of the mutants were similar, but synthesis of late polypeptides and late cytoplasmic mRNAs was greatly reduced. Quantitation of mutant virus-specific late mRNAs associated with polysomes revealed a threefold reduction when compared with that of sub309. Analysis of infected cell extracts further revealed that these mutants were incapable of efficiently shutting off host cell protein synthesis, suggesting that the 55-kilodalton protein plays a role in this process. These data suggest that early region 1b products may function by interacting with additional viral or host cell macromolecules to modulate host cell shutoff or that some late viral mRNA or polypeptide may potentiate this reaction.

Isolation and characterization of four adenovirus type 12-transformed human embryo kidney cell lines

Four transformed cell lines were established from cultures of human embryo kidney (HEK) cells microinjected or transfected with cloned adenovirus 12 (Ad12) EcoRI-C DNA (0 through 16.5 map units of the left-hand end of the viral genome). Each cell line showed a different growth pattern. Southern blotting demonstrated that all of the cell lines contained Ad12-specific DNA sequences, but in the microinjected isolates these were at a much lower copy number than in the transfected isolate. Two cell lines (Ad12 HEK 1 and 3) appeared to contain tandemly repeated Ad12 EcoRI-C DNA fragments. Immunoprecipitation and Western blotting confirmed that Ad12 early region 1 (E1) proteins were being expressed by all four of the transformed cell lines, but indicated that E1A polypeptide expression was considerably less than E1B polypeptide expression. All of the Ad12-transformed HEK cell lines were tumorigenic when inoculated intracranially into athymic nude mice.

Isolation and characterization of four adenovirus type 12-transformed human embryo kidney cell lines

Four transformed cell lines were established from cultures of human embryo kidney (HEK) cells microinjected or transfected with cloned adenovirus 12 (Ad12) EcoRI-C DNA (0 through 16.5 map units of the left-hand end of the viral genome). Each cell line showed a different growth pattern. Southern blotting demonstrated that all of the cell lines contained Ad12-specific DNA sequences, but in the microinjected isolates these were at a much lower copy number than in the transfected isolate. Two cell lines (Ad12 HEK 1 and 3) appeared to contain tandemly repeated Ad12 EcoRI-C DNA fragments. Immunoprecipitation and Western blotting confirmed that Ad12 early region 1 (E1) proteins were being expressed by all four of the transformed cell lines, but indicated that E1A polypeptide expression was considerably less than E1B polypeptide expression. All of the Ad12-transformed HEK cell lines were tumorigenic when inoculated intracranially into athymic nude mice.

Transformation by human adenoviruses

When, approximately 10 years ago, it was shown that the functions essential for cell transformation were localized in a small region of the adenovirus genome, a DNA segment which at that time was thought to be capable of encoding two or three average-sized proteins at most, it seemed reasonable to hope that an understanding of the mechanisms by which adenoviruses transform cells might be quickly achieved. While such optimism might be forgiven, it was quite clearly naive in the extreme. As a consequence of mRNA splicing and the use of overlapping reading frames the number of proteins encoded within E1 is 2-3-times greater than would have been predicted a decade ago, and post-translational modifications may add another dimension of complexity. In fact it has taken nearly all of the past decade just to identify the proteins encoded in E1 and to characterize them in the most rudimentary way. However, we have now entered a period in which new information is accumulating at an extremely rapid rate as a result of several major technical and fundamental advances. Chief among these are the use of recombinant DNA techniques, particularly site-directed mutagenesis, which combined with methods for introducing mutations made in cloned sequences back into infectious virus, clearly represents a powerful approach to studying the functions of transforming proteins. In addition, the ability to express transforming proteins in bacteria and to produce large amounts of highly purified proteins which previously were only just detectable in infected and transformed cells is a major breakthrough. Advances in immunological techniques, particularly the development of monoclonal antibodies and antisera against synthetic peptides, have enormously simplified the task of detecting and characterizing E1 proteins. Finally, recent results suggesting that adenovirus transforming proteins may be functionally and structurally similar to other oncogenes brings a new perspective to the study of oncogenic transformation. Have all the proteins involved in transformation by adenoviruses been identified? It seems probable that all those virally coded proteins which play a major role are now known but of course minor players in the cast could still be waiting in the wings. We have pointed out that viral functions encoded outside region E1 may have some importance at least in initiation of transformation by virions and have speculated on the possibility that one or more of these may be involved in the integration of viral DNA into the host cell chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenovirus type 12 specific cell surface antigen in transformed cells is a product of the E1b early region

Six syngeneic rat cell lines transformed with isolated or cloned left end fragments of adenovirus type 12 (Ad12) DNA were used in a study of the Ad12-specific cell surface antigen. Cells transformed with EcoRI-C, SalI-C, and HindIII-G fragments of Ad12 DNA fragment-transformed express the E1a and a part of or a complete E1b early regions. Two AccI-H DNA fragment-transformed cell lines contain and express only the E1a region. All these cells contain nuclear Ad12 antigen. Cytotoxic antibodies raised against syngeneic EcoRI-C DNA fragment-transformed cells kill EcoRI-C, SalI-C and HindIII-G fragment-transformed cells, but fail to kill cells transformed with AccI-H DNA fragment. Immunofluorescence analysis shows that such antibodies, which stain the surface of cells expressing the E1b region, do not stain the surface of AccI-H DNA fragment-transformed cells. Cells transformed with AccI-H fragment are also not killed by secondary cytolytic T cells, raised and effective against cells transformed with EcoRI-C, SalI-C, and HindIII-G DNA fragments. Cells transformed with AccI-H fragment do not elicit cytolytic T cells against any of the studied cell lines. The only Ad12-specific product shared by all cell lines killed in cytolytic assays which is absent from AccI-H fragment-transformed cells is the E1b 18K protein. Since it has also been shown in other studies that this protein is associated with the cellular membrane, the simplest interpretation of these data is that the Ad12-specific cell surface antigen in transformed rat cells is a product of the left end of the E1b region.

Morphological transformation of human adenoviruses is determined to a large extent by gene products of region E1a

The role of region E1a and E1b of human adenovirus (Ad) types 5 and 12 in determining the morphology of transformed colonies has been studied. Primary baby rat kidney cells were transfected with a mixture of plasmids containing Ad5 region E1a and Ad12 region E1b, or vice versa, and the morphology of the resulting transformed colonies was studied. It was found that the morphology of the colonies was correlated with the identity of the E1a region present in the cells; i.e., colonies transformed by Ad5 E1a plus Ad12 E1b resembled Ad5-transformed colonies, whereas those transformed by Ad12 E1a plus Ad5E1b resembled Ad12-transformed colonies. This suggested that the morphology of Ad-transformed cells is determined mainly by region E1a. To exclude the possibility that this phenomenon is due to an E1a-mediated and serotype-specific regulation of E1b expression and that the transformed phenotype is largely determined by region E1b, the experiments were repeated with Ad5 E1b plasmids in which the transcription regulation sequences had been replaced by the SV40 early promoter segment. In these plasmids E1b expression has become independent of region E1a. Foci of cells transformed by these E1b-SV40 promoter plasmids in the presence of either E1a of Ad5 or E1a of Ad12 again showed the same phenomenon, i.e., an Ad5-specific morphology with Ad5 E1a and Ad12-specific morphology with Ad12 E1a. Preliminary evidence showing that region E1b may regulate the concentration of E1a transcripts is discussed.

E1A control of gene expression is mediated by sequences 5' to the transcriptional starts of the early viral genes

A product of the adenovirus E1A gene is a positive regulator of early viral gene expression. In this report we show that E1A regulates at the transcriptional level and that sequences located 5' to the early viral regions contain sites which confer regulation by the E1A gene product. We constructed chimeric genes in which the sequences at the 5' end of the E2A, E3, and E4 regions were fused to the structural sequences of either the herpes simplex virus thymidine kinase gene, the bacterial gene encoding the enzyme neomycin phosphotransferase, or the chloramphenicol acetyltransferase gene. In all cases, expression of the chimeric genes was induced by a product of the E1A region. It was also found that the insertion of a fragment from the left-hand end of the adenovirus type 5 genome into a plasmid harboring the thymidine kinase gene resulted in elevated frequencies of transformation of TK- cells to TK+. The elevated transformation frequencies were only detected when the insert and tk gene were covalently joined. This effect occurred even when the insert was several kilobase upstream from, and regardless of its orientation to, the transcriptional initiation site of the tk gene. We propose that this region of the adenovirus type 5 genome harbors a cis-acting enhancer of transcription.

Intracellular localization of adenovirus type 5 tumor antigens in productively infected cells

The intracellular localization of tumor antigens of human adenovirus type 5 (Ad5) during lytic infection of KB cells has been studied. The cells were pulse labeled with [35S]methionine early after infection and early proteins of 58,000 D (58K), 44K, 19K, 18.5K, and 14K detectable by immunoprecipitation with hamster antitumor serum were assayed for association with cytoplasm, nucleoplasm, chromatin, cytosol, cytoskeleton, and membranes. The 44,000 D (44K) tumor antigen encoded in early region 1A (E1A: 0-4.4%) was recovered in approximately equal amounts from cytoplasmic and nucleoplasmic fractions of pulse-labeled cells and within the cytoplasmic compartment was found in the cytosol as well as associated with the cytoskeleton. The E1B-58K (E1B: 4.5-11.2%) antigen was also found to be associated with the cytoplasmic and nucleoplasmic fractions in approximately equal amounts but unlike the E1A-44K showed no affinity for cytoskeletons. Pulse-chase and immunofluorescence experiments suggested the 58K antigen accumulated in the nucleus late in infection. The E1B-19K antigen was found almost exclusively associated with the membrane fraction of infected KB cells and was resolved in polyacrylamide gels into two related species of 18.5K and 19K. Immunofluorescence studies on the E1B 18.5-19K doublet suggested that within a population of infected HeLa cells a small minority seemed to be expressing copious amounts of stainable antigen. Cell fractionation and immunofluorescence studies showed that the E4-14K antigen was a nuclear protein and the only antigen in this study which showed a significant association with a nuclear subfraction composed almost entirely of histones. The implications of these findings for the roles of the Ad5 tumor antigens in lytic infection and transformation are discussed.

Expression of region E1b of human adenoviruses in the absence of region E1a is not sufficient for complete transformation

Previous work has suggested that morphological transformation of cultured cells by human adenoviruses of subgroups A, B, and C is predominantly a function of early region 1b (E1b), and that region E1a has a role in immortalization. To test the hypothesis that region E1b is essentially responsible for the induction of the transformed phenotype the transforming activity of region E1b in the absence of region E1a was reinvestigated. In agreement with previous results, region E1b had no detectable transforming activity in primary baby rat kidney (BRK) cells nor in established rat cell lines. Since recent experimental evidence indicates that expression of E1b is blocked by a cellular factor which is inactivated by region E1a products, the regulatory signals in front of the coding sequence of region E1b were removed and replaced by the early promoter of SV40. These E1b-SV40pr plasmids had no detectable transforming activity in primary BRK cells, but they transformed normally in the presence of region E1a plasmids, demonstrating that both subregions are required for complete transformation of primary BRK cells. Transfection of the established rat cell line 3Y1 with the E1b-SV40pr plasmids did not result in complete morphological transformation either. Cotransfection of 3Y1 cells with E1b-SV40pr plasmids and pAG60 (a plasmid which harbors the kanamycin-resistance gene of Tn5) resulted in the appearance of foci of cells resistant to the antibiotic G-418. These colonies expressed the region E1b polypeptides to levels comparable to those found in cells transformed with intact region E1. Despite the presence of the E1b proteins the cells appeared essentially untransformed, in contrast to foci obtained after cotransfection of 3Y1 cells with mixtures of p5XhoI C (comprising region E1 DNA) and pAG60. These results indicate that complete transformation is a function of both regions E1a and E1b and that region E1a must have an important role in morphological transformation.

E1A control of gene expression is mediated by sequences 5' to the transcriptional starts of the early viral genes

A product of the adenovirus E1A gene is a positive regulator of early viral gene expression. In this report we show that E1A regulates at the transcriptional level and that sequences located 5' to the early viral regions contain sites which confer regulation by the E1A gene product. We constructed chimeric genes in which the sequences at the 5' end of the E2A, E3, and E4 regions were fused to the structural sequences of either the herpes simplex virus thymidine kinase gene, the bacterial gene encoding the enzyme neomycin phosphotransferase, or the chloramphenicol acetyltransferase gene. In all cases, expression of the chimeric genes was induced by a product of the E1A region. It was also found that the insertion of a fragment from the left-hand end of the adenovirus type 5 genome into a plasmid harboring the thymidine kinase gene resulted in elevated frequencies of transformation of TK- cells to TK+. The elevated transformation frequencies were only detected when the insert and tk gene were covalently joined. This effect occurred even when the insert was several kilobase upstream from, and regardless of its orientation to, the transcriptional initiation site of the tk gene. We propose that this region of the adenovirus type 5 genome harbors a cis-acting enhancer of transcription.

Transformation of rodent cells by DNA extracted from transformation-defective adenovirus mutants

Complementation group II host range mutants of adenovirus type 5 which map in early region 1B (E1B, 4.5 to 11.0 map units) have been shown to be defective for the synthesis of the E1B 58,000-dalton (58K) antigen in infections of HeLa or KB cells (Lassam et al., Cell 18:781-791, 1979) and unable to transform cultured rodent cells (Graham et al., Virology 86:10-21, 1978). In this report we show that DNA extracted from group II mutants hr6 and hr50 can transform rat cells with the same efficiency as wild-type DNA. Furthermore, group II mutant-transformed hamster cells were shown to contain no detectable E1B 58K tumor antigen but were capable of inducing tumors in newborn hamsters. Hamster cell lines 1019-3 and 1019-C3, transformed by hr50 DNA, produced no detectable quantities of either the E1B 58K or 19K antigen but nonetheless exhibited a fully transformed oncogenic phenotype. Our results show that the E1B 58K antigen is not absolutely required for oncogenic transformation and suggest that even cells lacking the 19K protein can be oncogenic.

Identification of human adenovirus early region 1 products by using antisera against synthetic peptides corresponding to the predicted carboxy termini

Synthetic peptides were prepared which corresponded to the carboxy termini of the human adenovirus type 5 early region 1B (E1B) 58,000-molecular-weight (58K) protein (Tyr-Ser-Asp-Glu-Asp-Thr-Asp) and of the E1A gene products (Tyr-Gly-Lys-Arg-Pro-Arg-Pro). Antisera raised against these peptides precipitated polypeptides from adenovirus type 5-infected KB cells; serum raised against the 58K carboxy terminus was active against the E1B 58K phosphoprotein, whereas serum raised against the E1A peptide immunoprecipitated four major and at least two minor polypeptides. These latter proteins migrated with apparent molecular weights of 52K, 50K, 48.5K, 45K, 37.5K, and 35K, and all were phosphoproteins. By using tryptic phosphopeptide analysis, the four major species (52K, 50K, 48.5K, and 45K) were found to be related, as would be expected if all were products of the E1A region. The ability of the antipeptide sera to precipitate these viral proteins thus confirmed that the previously proposed sequence of E1 DNA and mRNA and the reading frame of the mRNA are correct. Immunofluorescent-antibody staining with the antipeptide sera indicated that the 58K E1B protein was localized both in the nucleus and in the cytoplasm, especially in the perinuclear region. The E1A-specific serum also stained both discrete patches in the nucleus and diffuse areas of the cytoplasm. These data suggest that both the 58K protein and the E1A proteins may function in or around the nucleus. These highly specific antipeptide sera should allow for a more complete identification and characterization of these important viral proteins.

Induction of cytotoxic T cell precursors in vivo. Role of T helper cells

Strain AS rats respond with two populations of cytotoxic T lymphocytes to stimulation in vitro by the major histocompatibility complex (MHC)-incompatible strain HL rat tumor (HL-A2T2). One is specific for MHC alloantigens present on both HL-A2T2 and normal HL targets, the other is tumor specific. The activation of these killer cells requires helper T lymphocytes. The tumor-specific helper cells depend on syngeneic radioresistant accessory cells to present the tumor antigens in an immunogenic form. The appropriate helper-accessory cell interaction results in the production of soluble factors which then induce the maturation of precursor cells into effective killer cells. Studies with a procedure for inducing negative selection of T cells in vivo showed that short-term exposure to HL-A2T2 tumor induced selection only for TH but not cytotoxic T lymphocyte precursors (CTLp). Simultaneous injection of supernatants from concanavalin A-activated spleen cell cultures, however, did produce selection of CTLp. These and other findings suggest that under normal circumstances in vivo, both signals (recognition of antigen and acceptance of maturation factors) are provided in the vicinity of an antigen presenting macrophage-like accessory cell.

Covalently closed circles of adenovirus 5 DNA

The genome of adenoviruses is a double-stranded linear DNA molecule with inverted terminal repeats about 100 base pairs (bp) in length and a terminal protein covalently linked to the 5' nucleotide of each strand. Both of these features permit the formation of DNA circles, the inverted repeats allowing the circularization of single-stranded DNA and the terminal protein the joining of one or more molecules to yield double-stranded circles or concatemers. However, although the existence of covalently closed circles has been postulated, double-stranded viral DNA purified from virions or infected cells by conventional methods (that is, using proteases and phenol or chloroform) has always been obtained in a linear form. Here, we present evidence for the existence in adenovirus 5 (Ad5) infected cells of novel structures resulting from covalent head-to-tail joining of viral DNA molecules and show that these structures are due at least in part to the formation of covalently closed circles.