The molecular structure of the 9S mRNA from early region 1A of adenovirus serotype 2

Characterization of the 55-residue protein encoded by the 9S E1A mRNA of species C adenovirus

Early region 1A (E1A) of human adenovirus (HAdV) has been the focus of over 30 years of investigation and is required for the oncogenic capacity of HAdV in rodents. Alternative splicing of the E1A transcript generates mRNAs encoding multiple E1A proteins. The 55-residue (55R) E1A protein, which is encoded by the 9S mRNA, is particularly interesting due to the unique properties it displays relative to all other E1A isoforms. 55R E1A does not contain any of the conserved regions (CRs) present in the other E1A isoforms. The C-terminal region of the 55R E1A protein contains a unique sequence compared to all other E1A isoforms, which results from a frameshift generated by alternative splicing. The 55R E1A protein is thought to be produced preferentially at the late stages of infection. Here we report the first study to directly investigate the function of the species C HAdV 55R E1A protein during infection. Polyclonal rabbit antibodies (Abs) have been generated that are capable of immunoprecipitating HAdV-2 55R E1A. These Abs can also detect HAdV-2 55R E1A by immunoblotting and indirect immunofluorescence assay. These studies indicate that 55R E1A is expressed late and is localized to the cytoplasm and to the nucleus. 55R E1A was able to activate the expression of viral genes during infection and could also promote productive replication of species C HAdV. 55R E1A was also found to interact with the S8 component of the proteasome, and knockdown of S8 was detrimental to viral replication dependent on 55R E1A.

E1A induces phosphorylation of the retinoblastoma protein independently of direct physical association between the E1A and retinoblastoma products

We have studied the initial effects of adenovirus E1A expression on the retinoblastoma (RB) gene product in normal quiescent cells. Although binding of the E1A products to pRB could, in theory, make pRB phosphorylation unnecessary for cell cycle progression, we have found that the 12S wild-type E1A product is capable of inducing phosphorylation of pRB in normal quiescent cells. The induction of pRB phosphorylation correlates with E1A-mediated induction of p34cdc2 expression and kinase activity, consistent with the possibility that p34cdc2 is a pRB kinase. Expression of simian virus 40 T antigen induces similar effects. Induction of pRB phosphorylation is independent of the pRB binding activity of the E1A products; E1A domain 2 mutants do not bind detectable levels of pRB but remain competent to induce pRB phosphorylation and to activate cdc2 protein kinase expression and activity. Although the kinetics of induction are slower, domain 2 mutants induce wild-type levels of pRB phosphorylation and host cell DNA synthesis and yet fail to induce cell proliferation. These results imply that direct physical interaction between the RB and E1A products does not play a required role in the early stages of E1A-mediated cell cycle induction and that pRB phosphorylation is not, of itself, sufficient to allow quiescent cells to divide. These results suggest that the E1A products do not need to bind pRB in order to stimulate resting cells to enter the cell cycle. Indeed, a more important role of the RB binding activity of the E1A products may be to prevent dividing cells from returning to G0.

E1A induces phosphorylation of the retinoblastoma protein independently of direct physical association between the E1A and retinoblastoma products

We have studied the initial effects of adenovirus E1A expression on the retinoblastoma (RB) gene product in normal quiescent cells. Although binding of the E1A products to pRB could, in theory, make pRB phosphorylation unnecessary for cell cycle progression, we have found that the 12S wild-type E1A product is capable of inducing phosphorylation of pRB in normal quiescent cells. The induction of pRB phosphorylation correlates with E1A-mediated induction of p34cdc2 expression and kinase activity, consistent with the possibility that p34cdc2 is a pRB kinase. Expression of simian virus 40 T antigen induces similar effects. Induction of pRB phosphorylation is independent of the pRB binding activity of the E1A products; E1A domain 2 mutants do not bind detectable levels of pRB but remain competent to induce pRB phosphorylation and to activate cdc2 protein kinase expression and activity. Although the kinetics of induction are slower, domain 2 mutants induce wild-type levels of pRB phosphorylation and host cell DNA synthesis and yet fail to induce cell proliferation. These results imply that direct physical interaction between the RB and E1A products does not play a required role in the early stages of E1A-mediated cell cycle induction and that pRB phosphorylation is not, of itself, sufficient to allow quiescent cells to divide. These results suggest that the E1A products do not need to bind pRB in order to stimulate resting cells to enter the cell cycle. Indeed, a more important role of the RB binding activity of the E1A products may be to prevent dividing cells from returning to G0.

Adenovirus E1B 19-kilodalton protein overcomes the cytotoxicity of E1A proteins

Infection with adenovirus mutants carrying either point mutations or deletions in the coding region for the 19-kDa E1B gene product (19K protein) causes degradation of host cell and viral DNAs (deg phenotype) and enhanced cytopathic effect (cyt phenotype). Therefore, one function of the E1B 19K protein is to protect nuclear DNA integrity and preserve cytoplasmic architecture during productive adenovirus infection. When placed in the background of a virus incapable of expressing a functional E1A gene product, however, E1B 19K gene mutations do not result in the appearance of the cyt and deg phenotypes. This demonstrated that expression of the E1A proteins was responsible for inducing the appearance of the cyt and deg phenotypes. By constructing a panel of viruses possessing E1A mutations spanning each of the three E1A conserved regions in conjunction with E1B 19K gene mutations, we mapped the induction of the cyt and deg phenotypes to the amino-terminal region of E1A. Viruses that fail to express conserved region 3 (amino acids 140 to 185) and/or 2, (amino acids 121 to 185) or nonconserved sequences between conserved regions 2 and 1 of E1A (amino acids 86 to 120) were still capable of inducing cyt and deg. This indicated that activities associated with these regions, such as transactivation and binding to the product of the retinoblastoma susceptibility gene, were dispensable for induction of E1A-dependent cytotoxic effects. In contrast, deletion of sequences in the amino terminus of E1A (amino acids 22 to 107) resulted in extragenic suppression of the cyt and deg phenotypes. Therefore, a function affected by deletion of amino acids 22 to 86 of E1A is responsible for exerting cytotoxic effects in virally infected cells. Furthermore, transient high-level expression of the E1A region using a cytomegalovirus promoter plasmid expression vector was sufficient to induce the cyt and deg phenotypes, demonstrating that E1A expression alone is sufficient to exert these cytotoxic effects and that other viral gene products are not involved. Finally, placing E1A expression under the control of a strong promoter did not alter the requirement for E1B in the transformation of primary cells. One possibility is that the E1B 19K protein is required to overcome the cytotoxic effects of E1A protein expression and thereby enable primary cells to become transformed.

Enteric adenovirus type 40:E1B transcription map and identification of novel E1A-E1B cotranscripts in lytically infected cells

Adenovirus 40 (Ad40) is defective for growth in tissue culture but is complemented when the Ad2/5 or Ad12 E1B 55K protein is supplied in trans. Ad40 E1B mRNA has not been detected in E1-transformed cells, or at early times in lytically infected cells. In cells constitutively expressing the E1B region of Ad2, Ad40 E1B mRNAs are detected at late times in infection, after the onset of DNA replication. We have determined the Ad40 E1B transcription map from RNA produced at late times in infected KB16 cells, using S1 nuclease, primer extension, PCR-cDNA analysis, and Northern blotting. E1B transcripts corresponding to Ad2 14 S, 22 S, and 9 S mRNAs were identified but no 13 S mRNA equivalent was detected, a pattern similar to that seen in the Ad12 transcription map. The coding potential for E1B 19K, 55K, and 15K proteins and for ppIX is retained in the Ad40 transcripts. In addition we find novel E1A-E1B cotranscript counterparts of the 14 S and 22 S mRNAs. These contain the first 40 codons of the E1A first exon linked to a site 4-5 nt downstream of the E1B cap site, retaining all the coding potential of the E1B mRNAs. No new open reading frames are created by the junction, and the E1A ORF terminates with one codon added after the junction. Each E1A-E1B cotranscript is present in abundance comparable to that of its authentic E1B counterpart. The E1A-E1B junction is unusual in that it does not conform to splice consensus sequences and thus may not be generated by a conventional splicing mechanism.

Adenovirus E1A under the control of heterologous promoters: wide variation in E1A expression levels has little effect on virus replication

Adenovirus early region 1A(E1A) encodes a heterogeneous family of proteins some of which function as transactivators and are required for efficient viral replication in HeLa cells. We have constructed adenovirus type 5 (Ad 5) mutants in which the E1A transcription unit is placed under the control of either the human beta-actin promoter or the human cytomegalovirus immediate early promoter. The level of E1A expression in cells infected with these mutants was several times higher than that in wild-type virus infections. When the same heterologous promoters were inserted upstream of, but in the opposite orientation to, the E1A transcription unit, the level of expression was greatly reduced with respect to wild-type levels of E1A. Despite this variation of at least 40-fold in the concentration of E1A proteins in infected cells, there was no significant difference between wild-type Ad 5 and any of the mutants in their ability to replicate in HeLa cells. These results suggest that very low levels of E1A proteins are sufficient for virus production in cultured cells and that wild-type Ad 5 produces an amount of E1A in excess of that required.

Isolation and characterization of novel adenovirus type 12 E1A mRNAs by cDNA PCR technique

The technique of cDNA polymerase chain reaction was used to study the heterogeneity of adenovirus type 12 early region 1A mRNAs in infected and transformed cells. In addition to 13 S and 12 S mRNAs, three transcripts of 10 S, 9.5 S, and 9 S could be isolated from infected HeLa cells. A fourth transcript of 11 S was found in transformed cells. The 9 S mRNA is generated by removing one intron (nucleotide (nt) 589 to nt 1143) from the RNA precursor. For the 11 S and 10 S mRNAs the primary transcript is spliced twice. The first splicing event removes a common intron from nt 589 to nt 715; the second splicing event removes introns deleted also during the processing of either the 13 S mRNA (leading to the 11 S mRNA) or the 12 S mRNA (leading to the 10 S mRNA). The 9.5 S mRNA is also generated by removing two introns (first splice junction: nt 588/852; the second splice junction is identical to that of the 12 S mRNA). In comparison to the 13 S and 12 S transcripts, the four smaller mRNAs alter the translational reading frames with the beginning of their second exons. In vitro translation of these mRNAs resulted in protein products with molecular weights of 10,100 (11 S and 10 S mRNAs), 10,500 (9.5 S mRNA), and 6200 (9 S mRNA) kDa.

Transactivation of the p53 oncogene by E1a gene products

Infection of quiescent rat kidney cells with human adenovirus is shown to transcriptionally stimulate (transactivate) the p53 oncogene. The increased transcription results in an accumulation of p53-specific mRNA in parallel with an increase in p53 protein levels, although there is a considerable delay between transcriptional activation and the detection of stable p53 mRNA and protein. The induction of p53 is detectable with two monoclonal antibodies recognizing different epitopes. The induction of p53 by adenovirus is delayed compared to induction by serum, and it occurs after the onset of adenovirus-induced cellular DNA replication. Thus, adenovirus-induced DNA replication bypasses a G0/G1 control point. Experiments with hydroxyurea show that p53 activation does not require continued cell cycling and thus is likely to be a direct consequence of viral gene expression. Finally, the induction of p53 is shown to be dependent on expression of the 289-residue product encoded by the viral E1a gene.

Analysis of phosphorylation sites in the exon 1 region of E1A proteins of human adenovirus type 5

Early region 1A (E1A) of human adenovirus type 5 (Ad5) produces five mRNAs that encode proteins of 55, 171, 217, 243, and 289 residues. We have shown previously that the major products of 289 and 243 residues are phosphorylated at a minimum of three sites of which one, Ser-89, is located in the amino terminal half of the protein. In the present report we show that these E1A proteins are also phosphorylated at a second site in this region located at Ser-96. The 171 and 217 residue E1A species were also tentatively identified and, as predicted, neither contained the Ser-89 or Ser-96 sites but both appeared to be phosphorylated at the same sites as 289R and 243R toward the carboxy terminus. Studies with mutants in which Ser-89 or Ser-96 were converted to alanine residues indicated that phosphorylation of Ser-89 but not Ser-96 induces the major shift in gel mobility of E1A products. However, neither site appears to be of major importance in the regulation of E1A-mediated transactivation or transformation.

Role of the branch site/3'-splice site region in adenovirus-2 E1A pre-mRNA alternative splicing: evidence for 5'- and 3'-splice site co-operation

The adenovirus E1A gene encodes five overlapping mRNAs which are processed by alternative RNA splicing from a common pre-mRNA. To characterize cis-acting sequence elements which are of importance for the alternative 5'-splice site selection deletion and substitution mutants within the intron that is common to all E1A mRNAs were constructed. Deletion of the wild-type E1A branch site/polypyrimidine tract resulted in activation of a functionally redundant sequence located within an A/T rich sequence just upstream of the normal E1A lariat branch site. Removal of both regulatory sequences abolished in vivo splicing completely and did not lead to activation of cryptic 3'-splice sites at other locations in the E1A pre-mRNA. Furthermore we show that the sequence around the E1A branch site/3'-splice site region may have a more direct effect on the efficiency by which the alternative E1A 5'-splice sites are selected. Replacing the E1A branch site/3'-splice site region with the corresponding sequence from the second intron of the rabbit beta-globin gene or the first intron of the major late transcription unit resulted in drastic changes in E1A 5'-splice site selection. For example, with the E1A/beta-globin hybrid gene the 9S mRNA became the most abundant E1A mRNA to accumulate. This contrasts with the wild-type E1A gene in which almost undetectable levels of 9S mRNA were produced in transient expression assays. Our results strongly suggest that a cooperative interaction between 5'- and 3'-splice sites on a pre-mRNA determines the outcome of alternative splicing.

Expression of adenovirus E1B mutant phenotypes is dependent on the host cell and on synthesis of E1A proteins

Adenovirus mutants containing genetic alterations in the gene encoding the E1B 19,000-molecular-weight (19K) tumor antigen induce the degradation of host cell chromosomal DNA (deg phenotype) and enhanced cytopathic effect (cyt phenotype) after infection of HeLa and KB cells. The deg and cyt phenotypes are a consequence of viral early gene expression in the absence of the E1B 19K protein. The role of the E1A proteins in induction of the cyt and deg phenotypes was investigated by constructing E1A-E1B double mutant viruses. Viruses were constructed to express the individual E1A 13S, 12S, or 9S cDNA genes in the presence of a mutation in the gene encoding the E1B 19K tumor antigen. Expression of either the 13S or 12S E1A proteins in the absence of functional E1B 19K protein produced the deg and cyt phenotypes. In contrast, a virus which expressed exclusively the 9S E1A gene product in the absence of the E1B 19K gene product did not induce the deg and cyt phenotypes, even at high multiplicities of infection. Therefore, both the 13S and 12S E1A gene products could directly or indirectly cause the deg and cyt phenotypes during infection of HeLa cells with an E1B 19K gene mutant virus. Furthermore, the deg phenotype was found to be host cell type specific, occurring in HeLa and KB cells but not in growth-arrested human WI38 cells. These results indicate that expression of the E1A trans-activating and transforming proteins is necessary for the induction of the cyt and deg phenotypes and that host cell factors also play a role.

Structure and organization of the left-terminal DNA regions of fastidious adenovirus types 40 and 41

To determine whether differences in structure and organization of the transforming regions of Ad40 and Ad41 could explain the fastidious growth of these viruses, we have sequenced these regions and analyzed the structure of the corresponding mRNAs. These regions are 85% homologous to each other and exhibit 52% of homology to the analogous Ad5 region. Like the other adenoviruses, the Ad40 and Ad41 transforming regions contain two transcriptional units, E1a and E1b. Differences and similarities in the strategic sequences of these transcription units and Ad5 regulatory elements are discussed. Northern blotting and S1-nuclease analysis of RNA isolated from transformed cells reveal that in Ad40-transformed cells region E1a is transcribed into three partially overlapping mRNAs, while no transcription of region E1b can be detected. In Ad41-transformed cells only one E1a mRNA is found, comparable to the larger Ad40 E1a mRNA. In the Ad41 E1b region a single mRNA species is synthesized, which is comparable to the E1b messenger found in cells transformed by other adenovirus serotypes. Comparison of the large proteins encoded by the Ad40 and Ad41 E1a regions with the corresponding Ad5 E1a protein shows a relatively low homology in three conserved regions. These results are discussed in relation to the transforming capacity and the fastidious growth of Ad40 and Ad41 in conventional cell lines.

Adenovirus E1A coding sequences that enable ras and pmt oncogenes to transform cultured primary cells

Plasmids expressing partial adenovirus early region 1A (E1A) coding sequences were tested for activities which facilitate in vitro establishment (immortalization) of primary baby rat kidney cells and which enable the T24 Harvey ras-related oncogene and the polyomavirus middle T antigen (pmt) gene to transform primary baby rat kidney cells. E1A cDNAs expressing the 289- and 243-amino acid proteins expressed both E1A transforming functions. Mutant hrA, which encodes a 140-amino acid protein derived from the amino-terminal domain shared by the 289- and 243-amino acid proteins, enabled ras (but not pmt) to transform and facilitated in vitro establishment to a low, but detectable, extent. These studies suggest that E1A functions which collaborate with ras oncogenes and those which facilitate establishment are linked. Furthermore, E1A transforming functions are not associated with activities of the 289-amino acid E1A proteins required for efficient transcriptional activation of viral early region promoters.

Adenovirus E1A coding sequences that enable ras and pmt oncogenes to transform cultured primary cells

Plasmids expressing partial adenovirus early region 1A (E1A) coding sequences were tested for activities which facilitate in vitro establishment (immortalization) of primary baby rat kidney cells and which enable the T24 Harvey ras-related oncogene and the polyomavirus middle T antigen (pmt) gene to transform primary baby rat kidney cells. E1A cDNAs expressing the 289- and 243-amino acid proteins expressed both E1A transforming functions. Mutant hrA, which encodes a 140-amino acid protein derived from the amino-terminal domain shared by the 289- and 243-amino acid proteins, enabled ras (but not pmt) to transform and facilitated in vitro establishment to a low, but detectable, extent. These studies suggest that E1A functions which collaborate with ras oncogenes and those which facilitate establishment are linked. Furthermore, E1A transforming functions are not associated with activities of the 289-amino acid E1A proteins required for efficient transcriptional activation of viral early region promoters.

Lytic and transforming functions of individual products of the adenovirus E1A gene

To distinguish the individual roles of the 13S, 12S, and 9S adenovirus E1A gene products, we isolated the corresponding cDNA clones and recombined them into both plasmids and viruses. Only the expected E1A mRNA products were made from the corresponding 12S and 13S viruses. The 9S mRNA was detected when the 9S virus was coinfected with the 13S virus but not when either virus was infected alone. The 13S virus formed plaques equally well in 293 cells, HeLa cells, and A549 cells, a human lung oat cell carcinoma line. Plaque titers of the 12S virus were much reduced in HeLa and A549 cells compared with 293 cells, although the 12S virus is multiplicity-dependent leaky in both HeLa and A549 cells. A549 cells were significantly more permissive than HeLa cells for growth of the 12S virus. In A549 cells even at low multiplicities of infection the final yield of 12S virus eventually approached the maximum yield from 293 cells. Expression from the adenovirus early region 2 and early region 3 promoters in HeLa cells was activated in the presence of a 13S cDNA E1A region but not in the presence of a 12S E1A cDNA region. Although defective for lytic growth in HeLa cells, the 12S virus immortalized BRK cells at very high efficiency, whereas infection of these cells with 13S virus, as with wild-type E1A virus, resulted mainly in cell death. The 13S product does have an immortalization function, however, revealed in the absence of adenovirus lytic functions when a plasmid containing the E1A 13S cDNA region was transfected into BRK cells. The 9S virus failed to immortalize infected BRK cells or to interfere with focus formation when coinfected with the 12S virus.

Individual adenovirus type 5 early region 1A gene products elicit distinct alterations of cellular morphology and gene expression

Adenovirus early region 1A (E1A), which gives rise to three overlapping transcripts, was inserted into a murine leukemia virus-derived vector, and recombinant viruses were used to prepare permanent cell lines of NIH 3T3 cells containing DNA copies of the individual 13S, 12S, and 9S mRNAs. Integrated proviral copies of the recombinant genomes were rescued as bacterial plasmids from each of the cell lines, and the DNA sequence of E1A was demonstrated to be a precise copy of the individual transcripts. The DNA copies were shown to be expressed as part of the full-length retroviral transcript by S1 nuclease analysis, and the synthesis of their encoded polypeptides was demonstrated by immunoprecipitation. Those cell lines expressing the polypeptide encoded by the 13S transcript were shown to contain that function required for regulating the accumulation of mRNAs from adenovirus early genes by their ability to complement the adenovirus type 5 E1A deletion mutant dl312. Cell lines expressing polypeptides encoded by the 13S, 12S, and 9S transcripts showed characteristic alterations in morphology. Two-dimensional gel electrophoresis of total cellular protein derived from the three cell lines demonstrated that each E1A gene product elicits specific alterations in the patterns of proteins expressed. Studies of the expression of two specific genes, those encoding fibronectin and collagen type 1, indicated that the observed alteration in levels of the two proteins results from a reduction in RNA levels induced by E1A functions.

Analysis of multiple forms of human adenovirus type 5 E1A polypeptides using an anti-peptide antiserum specific for the amino terminus

Studies were carried out to further characterize the proteins coded for by the early 1A region (E1A) of human adenovirus type 5 (Ad5) in lytically infected cells using an antiserum prepared against a synthetic octapeptide corresponding to the amino termini of E1A products as well as another anti-peptide serum specific for the carboxy termini. Both sera precipitated the same collection of four major and two minor acidic phosphoproteins and it was confirmed that each of the 1.1- and 0.9-kb E1A mRNAs code for two major and one minor species. These data also indicated that none of the E1A species was produced by proteolytic cleavage. The deletion mutant dl313 which lacks DNA coding for the last 70 C-terminal amino acids of E1A products also produced multiple species which suggested that post-translational modifications involved in their generation do not take place in this region of the proteins. The N-terminal serum was effective in detecting neither the truncated 1.1-kb mRNA product predicted for the mutant hr1 nor the product of the small 0.6-kb E1A mRNA, suggesting that these species are either very short-lived in infected cells or exist in a conformation in which the amino terminus is inaccessable to the antibody.

Splicing of the adenovirus-2 E1A 13S mRNA requires a minimal intron length and specific intron signals

The adenovirus E1A region encodes three overlapping mRNAs, designated 9S, 12S and 13S. They differ from each other with regard to the length of the intron which is removed by RNA splicing. We have constructed E1A genes with deletions and insertions in the intervening sequence that is common to all three E1A mRNAs, in a search for signals which influence splicing of the 13S mRNA. Mutant plasmids were transfected into HeLa cells and the transiently expressed E1A mRNAs characterized by the S1 protection assay. The results show that five upstream and 20 downstream nucleotides are sufficient to allow for a correct utilization of the 5'-splice junction for the E1A 13S mRNA. Moreover, we show that a minimal intron length of 78 nucleotides is required for efficient 13S mRNA splicing. The ability of mutants with large intron deletions to maturate a 13S mRNA could partially be restored by expanding the intron length with phage lambda sequences. However, in no case was the normal splicing efficiency obtained with these mutants. In contrast, one mutant in which sequences from the authentic 13S mRNA intron were used to expand the intron expressed almost normal levels of 13S mRNA, thus suggesting that signals which specifically promote 13S mRNA splicing exist.

The release of growth arrest by microinjection of adenovirus E1A DNA

The induction of DNA synthesis in growth-arrested mouse fibroblasts (NIH 3T3) was studied by microinjection of different constructs of adenovirus DNA using SV40 DNA and plasmid DNA as positive and negative controls. The E1A region of adenovirus types 2 and 12 appears to be sufficient to induce cellular DNA synthesis after growth arrest in approximately 30% of the cells and both 13S and 12S cDNA constructs mediate this effect. The presence of the E1A protein products as assayed by immunofluorescence does not strictly correlate with the induction of DNA synthesis in microinjected cells in contrast to the SV40 large T-antigen. Microinjection of truncated fragments of the Ad12 E1A region suggests, however, that the protein products of 12S and 13S may be involved in the induction process. A sequence comparison of the SV40 T-antigen and the adenovirus E1A products identified a region of significant homology providing a basis for a hypothesis concerning the evolution of T-antigen genes in DNA viruses.

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.

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.

E1a regions of the human adenoviruses and of the highly oncogenic simian adenovirus 7 are closely related

Simian adenovirus 7 (SA7) is a highly oncogenic virus, capable of causing tumors in hamsters upon the direct injection of viral DNA. We determined the transcriptional organization of the transforming region and compared it with that of the human adenoviruses. This analysis demonstrated that there are two independently promoted transcription units similar to the E1a and E1b regions of the human adenoviruses. The nucleotide sequence of the SA7 E1a region demonstrated considerable homology with the human adenoviruses, both in the sequences that regulate E1a expression and in the encoded polypeptides. The amino acid homology was reflected in the ability of SA7 to complement the growth of human adenoviruses mutant in the E1a region. Furthermore, we found two regions of amino acid homology unique to SA7 and the highly oncogenic human adenovirus 12.

Transcription control region within the protein-coding portion of adenovirus E1A genes

A single-base deletion within the protein-coding region of the adenovirus type 5 early region 1A (E1A) genes, 399 bases downstream from the transcription start site, depresses transcription to 2% of the wild-type rate. Complementation studies demonstrated that this was due to two effects of the mutation: first, inactivation of an E1A protein, causing a reduction by a factor of 5; second, a defect which acts in cis to depress E1A mRNA and nuclear RNA concentrations by a factor of 10. A larger deletion within the protein-coding region of E1A which overlaps the single-base deletion produces the same phenotype. In contrast, a linker insertion which results in a similar truncated E1A protein does not produce the cis-acting defect in E1A transcription. These results demonstrate that a critical cis-acting transcription control region occurs within the protein coding sequence in adenovirus type 5 E1A. The single-base deletion occurs in a sequence which shows extensive homology with a sequence from the enhancer regions of simian virus 40 and polyomavirus. This region is not required for E1A transcription during the late phase of infection.

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.

Adenovirus cyt+ locus, which controls cell transformation and tumorigenicity, is an allele of lp+ locus, which codes for a 19-kilodalton tumor antigen

The early region E1b of adenovirus type 2 (Ad2) codes for two major tumor antigens of 53 and 19 kilodaltons (kd). The adenovirus lp+ locus maps within the 19-kd tumor antigen-coding region (G. Chinnadurai, Cell 33:759-766, 1983). We have now constructed a large-plaque deletion mutant (dl250) of Ad2 that has a specific lesion in the 19-kd tumor antigen-coding region. In contrast to most other Ad2 lp mutants (G. Chinnadurai, Cell 33:759-766, 1983), mutant dl250 is cytocidal (cyt) on infected KB cells, causing extensive cellular destruction. Cells infected with Ad2 wt or most of these other Ad2 lp mutants are rounded and aggregated without cell lysis (cyt+). The cyt phenotype of dl250 resembles the cyt mutants of highly oncogenic Ad12, isolated by Takemori et al. (Virology 36:575-586, 1968). By intertypic complementation analysis, we showed that the Ad12 cyt mutants indeed map within the 19-kd tumor antigen-coding region. The transforming potential of dl250 was assayed on an established rat embryo fibroblast cell line, CREF, and on primary rat embryo fibroblasts and baby rat kidney cells. On all these cells, dl250 induced transformation at greatly reduced frequency compared with wt. The cells transformed by this mutant are defective in anchorage-independent growth on soft agar. Our results suggest that the 19-kd tumor antigen (in conjunction with E1a tumor antigens) may play an important role in the maintenance of cell transformation. Since we have mapped the low-oncogenic or nononcogenic Ad12 cyt mutants within the 19-kd tumor antigen-coding region, our results further indicate that the 19-kd tumor antigen also directly or indirectly plays an important role in tumorigenesis of Ad12. Our results show that the cyt+ locus is an allele of the lp+ locus and that the cyt phenotype may be the result of mutations in specific domains of the 19-kd tumor antigen.

Transformation properties of type 5 adenovirus mutants that differentially express the E1A gene products

Type 5 adenovirus mutants that differentially express E1A 13S, 12S, or 9S mRNAs were constructed to study the role of their gene products in transformation. H5dl520 expresses the 243-amino-acid (AA) protein encoded in the 12S mRNA but not the 13S mRNA-encoded 289-AA protein. This mutant transformed both cloned rat embryo fibroblast (CREF) cells and baby rat kidney (BRK) cells at a frequency 40-100 times greater than did wild-type viruses. In addition, all of the foci produced were fibroblastic and grew very slowly at 32 degrees C. In contrast, H5dl21, which was mutated so that only the 54-AA protein encoded by the 9S mRNA was synthesized, did not transform either cell type. DNA transfection studies with plasmids from which these mutants were constructed demonstrated that the differences in transformation frequencies were not as marked. The plasmid pD1/D2, which directs the synthesis of the 54-AA protein only, was found to transform baby rat kidney cells at low frequency, provided the gene was linked to a fragment from the simian virus 40 genome containing the transcriptional enhancer element.

Transcription control region within the protein-coding portion of adenovirus E1A genes

A single-base deletion within the protein-coding region of the adenovirus type 5 early region 1A (E1A) genes, 399 bases downstream from the transcription start site, depresses transcription to 2% of the wild-type rate. Complementation studies demonstrated that this was due to two effects of the mutation: first, inactivation of an E1A protein, causing a reduction by a factor of 5; second, a defect which acts in cis to depress E1A mRNA and nuclear RNA concentrations by a factor of 10. A larger deletion within the protein-coding region of E1A which overlaps the single-base deletion produces the same phenotype. In contrast, a linker insertion which results in a similar truncated E1A protein does not produce the cis-acting defect in E1A transcription. These results demonstrate that a critical cis-acting transcription control region occurs within the protein coding sequence in adenovirus type 5 E1A. The single-base deletion occurs in a sequence which shows extensive homology with a sequence from the enhancer regions of simian virus 40 and polyomavirus. This region is not required for E1A transcription during the late phase of infection.

Characterization of the major mRNAs from adenovirus 2 early region 4 by cDNA cloning and sequencing

The sequences at the splice junctions of many early region 4 (E4) mRNAs from adenovirus 2 (Ad2) were determined by analysis of cDNA clones. The cDNAs were synthesized from poly(A)+ mRNA isolated from HeLa cells early during Ad2 infection. A standard library was constructed, in pBR322, from double stranded cDNAs initiated by oligo-dT priming. Approximately 1% of total recombinants contained E4 sequences, however among eighty clones analyzed in detail, only four contained the 5' leader sequence. A second library was prepared using a new method that led to a greatly increased representation of desired clones. This method employed oligo-dT to prime the synthesis of the first strand and an oligonucleotide ligated to pBR322, whose sequence was present in the 5' leader, to prime the synthesis of the second strand. With this method the percentage of recombinants containing E4 sequences ranged between 15 and 50% of the total colonies. Virtually all of these E4 cDNA clones contained the 5' leader sequence and several hundred were analyzed by comparing the results from single channel dideoxy sequencing reactions. Nine unique sequence patterns were identified and representative clones were completely sequenced.

Adenovirus 2 early region 1A stimulates expression of both viral and cellular genes

The ability of products from the adenovirus early region 1A to stimulate viral and cellular gene expression has been studied, using a transient expression assay in HeLa cells. We show that the E1A 13S mRNA encodes a diffusible product which is capable of stimulating transcription of adenovirus genes as well as the rabbit beta-globin gene. The E1A 12S mRNA has no detectable stimulatory effect on either cellular or viral genes. Although being able to stimulate both types of genes, we find that the E1A regulatory protein enhances viral gene expression approximately 10 times more than beta-globin gene expression. We also find that when connected to the cis-acting SV40 enhancer element, the beta-globin gene cannot be further stimulated by the trans-acting E1A product. Finally, we find that transfection of either adenovirus or the beta-globin gene into 293 cells, which constitutively expresses the E1A gene products, leads to an enhanced expression which is 10- to 20-fold higher than obtained by co-transfection of HeLa cells. The 293 cells thus provide a simple assay to demonstrate E1A-mediated transcriptional regulation.

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)