Regulation of the appearance of cytoplasmic RNAs from region 1 of the adenovirus 2 genome

The E1A 13S product of adenovirus 5 activates transcription of the cellular human HSP70 gene

Expression of the human gene encoding the major heat shock protein, HSP70, was induced during cell growth by serum stimulation and after infection with adenovirus 5. In this study we showed that HSP70 gene expression could be induced by adenovirus 5 infection, even in the absence of exogenous serum factors. Whereas serum stimulation induced the expression of the endogenous HSP70 gene, it had no effect on early adenovirus promoters. However, expression of both the cellular HSP70 gene and the adenovirus E3 promoter were activated during adenovirus infection. By using a collection of reconstructed mutant viruses, we identified the 13S product of the E1A region as the specific transcriptional trans-activator of the HSP70 gene.

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.

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.

An insertion mutation in the adenovirus type 12 early region 1A 13S mRNA unique region

An adenovirus type 12 mutant, in203S, was constructed to contain an insertion of two amino acids in the early region 1A (E1A) 13S mRNA-coding region and in the E1A 12S mRNA intron. in203S could not grow in HeLa and KB cells. Virus DNA replication was scarcely detected at a low multiplicity of infection, but was detected at a high multiplicity of infection. The transcription of early genes other than E1A was not detected at 13 h after infection, but became detectable after longer incubation. The transcription of the E1A gene was also reduced to about one-fifth of the wild-type level. The mutant induced fewer foci of smaller sizes than the wild type in rat 3Y1 and secondary rat kidney cells. The induction of cellular DNA synthesis was reduced in rat 3Y1 cells infected with in203S as compared with that in wild type-infected cells. These results show that the E1A 13S mRNA-derived polypeptide of adenovirus type 12 is required for activation of early genes, cell transformation, and induction of cellular DNA synthesis.

Molecular biology of adenovirus type 2 semipermissive infections. I. Viral growth and expression of viral replicative functions during restricted adenovirus infection

As an initial step toward understanding the mechanisms underlying host cell restriction of adenovirus 2 (Ad2) replication, we have studied various cell lines derived from hamster (CHO-K1), rat (CREF, NRK-49F, C-3, C-9), and mouse (3T3-Swiss) tissues to determine their degree of permissivity to Ad2 replication. For each cell line tested, the time course of Ad2 growth was determined; the yield of infectious virus, as measured by titration on HeLa cell monolayers, was reduced 3 to 5 logs. This result is independent of the multiplicity of infection at multiplicities between 4 and 100 plaque-forming units (PFU) per cell. The Western immunoblotting technique was used to quantitate the amounts of early proteins (E1A 45-54K proteins, E1B 21 and 58K proteins, E2A 72K DNA binding protein) and late structural proteins (hexon, fiber) produced during restricted infections. All cell lines expressed 72K DNA binding protein and variable levels of other early proteins. C-3, C-9, and NRK-49F cells expressed hexon as well as low, but detectable levels of fiber protein. Mouse 3T3-Swiss cells failed to synthesize any detectable levels of late structural proteins. DNA synthesis analysis indicated all rodent cell lines were capable of replicating viral DNA. A decreased rate of viral DNA synthesis was observed in CREF cells. Evidence is presented which suggests newly synthesized viral DNA is unstable in 3T3-Swiss cells.

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.

Vector expression of adenovirus type 5 E1a proteins: evidence for E1a autoregulation

We transiently expressed adenovirus type C E1a proteins in wild-type or mutant form from plasmid vectors which have different combinations of E1a and simian virus 40 enhancer elements and which contain the DNA replication origin of SV40 and can replicate in COS 7 cells. We measured the levels of E1a mRNA encoded by the vectors and the transition regulation properties of the protein products. Three vectors encoded equivalent levels of E1a mRNA in COS 7 cells: (i) a plasmid encoding the wt 289-amino acid E1a protein (this complemented the E1a deletion mutant dl312 for early region E2a expression under both replicative and nonreplicative conditions); (ii) a vector for the wt 243-amino acid E1a protein (this complemented dl312 weakly and only under conditions of high multiplicities of dl312); (iii) a mutant, pSVXL105, in which amino acid residues-38 through 44 of the 289-amino acid E1a protein (which includes two highly conserved residues) are replaced by 3 novel amino acids (this also complemented dl312 efficiently). A fourth vector, mutant pSVXL3 with which linker substitution shifts the reading frame to encode a truncated 70-amino acid fragment from the amino terminus of the 289-amino acid protein, was unable to complement dl312. Surprisingly, pSVXL3 overexpressed E1a mRNA approximately 30-fold in COS 7 cells in comparison with the other vectors. The pSVXL3 overexpression could be reversed by cotransfection with a wt E1a vector. We suggest that wt E1a proteins regulate the levels of their own mRNAs through the recently described transcription repression functions of the 289- and 243-amino acid E1a protein products and that pSVXL3 fails to autoregulate negatively.

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.

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.

Transcription termination within the E1A gene of adenovirus induced by insertion of the mouse beta-major globin terminator element

In induced erythroleukemia cells, transcription of the beta-globin gene terminates in a region 600-1500 nucleotides downstream of the poly(A) site. To determine whether this region of the mouse DNA functions to terminate transcription when moved to another genomic site, portions of the putative termination region have been inserted into the E1A transcription unit of the adenovirus (type 5) chromosome. Analysis of RNA labeled either in isolated nuclei or in whole cells early after infection with reconstructed viruses indicated that transcription is terminated if the inserted DNA is oriented in the same direction as in the beta-globin transcription unit and contains the globin poly(A) site plus an additional 1395 nucleotides downstream. In addition to halting transcription within the E1A unit, the insertion of the terminator region had a negative cis effect on the E1B transcription unit, which normally initiates 363 bp downstream of the site of the globin insert. The E1B transcription unit was the only early gene affected, and complementation of the terminator virus with a wild-type E1A gene did not restore transcription of the E1B gene.

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.

Splicing in adenovirus and other animal viruses

Splicing provides viruses with great genetic versatility. It is still too early to say whether this versatility is derived from ingeneous mechanisms evolved by necessity by the viruses, or whether the viruses indeed mimic cellular mechanisms. In any event, it is unlikely that cells will provide a single genomic cluster of genes that utilize splicing in such diverse ways as adenovirus, or the other viruses discussed here. And we may speculate that when the full role of splicing in adenovirus gene expression program is known, its import will continue to be a source of amazement!

Complementation of an adenovirus 5 immediate early mutant by human cytomegalovirus

The initiation of the lytic cycle of adenovirus 5 requires synthesis of a transcriptional activator encoded by the viral early genetic unit, E1a. Mutant viruses lacking E1a are defective. Human cytomegalovirus activated transcription of early adenovirus genes and complemented an E1a- adenovirus mutant in cells permissive for both viruses.

Accumulation of early and intermediate mRNA species during subgroup C adenovirus productive infections

The cytoplasmic, poly(A)-containing RNA species complementary to all regions of the adenovirus type 2 (Ad2) genome expressed before the onset of viral DNA synthesis have been examined by "Northern" blotting. HeLa cells infected with low multiplicities of Ad2 were harvested at 2-hr intervals until late in the infectious cycle to establish the temporal patterns of expression of each viral gene. Under these conditions, such late mRNA products as those of the L3 and L5 families were not detected until 14 hr after infection. Although individual mRNA species complementary to several genes showed different patterns of expression, the order of appearance in the cytoplasm of substantial concentrations of adenoviral mRNA species was E1A, E3, and E4 (4 to 6 hr), E2A and E2B (8 hr), 3.7- and 4.1-kb L1 mRNA species (10-12 hr), IX and IVa2 mRNAs (12 hr), and those encoded in the major late transcriptional unit, such as members of the L3 and L5 families (14 hr). The mRNA species encoding polypeptides IX and IVa2 were not produced when viral DNA synthesis was blocked, whereas the larger L1 mRNA species was made under these conditions. Two E2B mRNA species, some 5.0 and 7 kb, were observed at low concentrations at 8 hr after infection and their concentration increased until 24 hr after infection, as did that of the E2A mRNA species: the products of the E2 transcription unit appeared to be expressed coordinately and at a constant ratio throughout infection. Few of the early mRNA species decreased in concentration after the onset of the late phase of infection. Examination of the viral mRNA produced when protein synthesis was inhibited by 10 microM anisomycin added 3 hr after infection suggested that processing of certain viral, early transcripts was altered in the presence of the drug.

Construction and applications of a highly transmissible murine retrovirus shuttle vector

We develop a murine retrovirus shuttle vector system for the efficient introduction of selectable and nonselectable DNA sequences into mammalian cells and recovery of the inserted sequences as molecular clones. Three protocols allow rapid recovery of vector DNA sequences from mammalian cells. Two of the methods rely on SV40 T-antigen-mediated replication of the vector sequences and yield thousands of bacterial transformants per 5 X 10(6) mammalian cells. The majority of plasmids recovered by all three protocols exhibited the proper structure and were as active as the parental vector in the generation of transmissible retrovirus genomes upon transfection of mammalian cells. One of the rescue methods, which relies on "onion skin" replication and excision of an integrated provirus from the host chromosome, enables facile recovery of the chromosomal site of proviral integration. The system was also used to generate, and then efficiently recover, a cDNA version of a genomic insert from the adenovirus E1A region.

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.

Production of a monospecific antiserum against the early region 1A proteins of adenovirus 12 and adenovirus 5 by an adenovirus 12 early region 1A-beta-galactosidase fusion protein antigen expressed in bacteria

Antisera were prepared against the amino acid sequences encoded within the N-terminal half of the adenovirus 12 (Ad12) early region 1A (E1A) gene. This was accomplished by construction of a plasmid vector which encoded the N-terminal 131 amino acids of Ad12 E1A joined in frame to the coding sequence of beta-galactosidase. After induced synthesis in Escherichia coli, the Ad12 E1A-beta-galactosidase fusion protein (12-1A-FP) was extracted with urea and used to raise antibodies in rabbits. The 12-1A-FP antisera immunoprecipitated major phosphoproteins of 39,000 and 37,000 apparent molecular weights from Ad12-transformed and infected cells. The 12-1A-FP antisera also immunoprecipitated E1A phosphoproteins from Ad5-transformed and infected cells. Immunospecificity of the 12-1A-FP antisera was demonstrated by the ability of 12-1A-FP antigen to block immunoprecipitation of E1A proteins. Furthermore, E1A proteins immunoprecipitated from in vivo-labeled cells comigrated with those translated in vitro by RNA that had been hybridization selected to E1A DNA.

Splice junctions in adenovirus 2 early region 4 mRNAs: multiple splice sites produce 18 to 24 RNAs

We localized the splice junctions in adenovirus 2 early region 4 (E4) mRNAs. Processing of the E4 precursor RNA positioned the donor splice site of the 5' leader sequence adjacent to acceptor sites near the 5' ends of five of the six open reading regions in the E4 transcription unit. Of particular interest among the E4 mRNAs is an extensively spliced class which includes multiple species with sizes ranging from 1.1 to 0.75 kilobases (kb). Purified 1.1- to 0.75-kb mRNAs specified at least 10 polypeptides in vitro. We detected eight acceptor and two donor splice sites utilized in the deletion of the intron from the 3' portion of these mRNAs. E4 RNAs were isolated from the cytoplasm of infected cells at 5, 9, 12, and 18 h after infection. The E4 mRNAs were present throughout infection, but different members of the 1.1- to 0.7-kb class were predominant at each time assayed. Alternate splicing of the 3.0-kb E4 precursor RNA can generate as many as 25 mRNAs that encode at least 16 polypeptides.

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.

Deletion and insertion mutations in early region 1a of type 5 adenovirus that produce cold-sensitive or defective phenotypes for transformation

On the basis of earlier findings showing that H5hr1 (hr1) is cold sensitive for transformation, a series of mutants were constructed so that they contained deletions or insertions in different sites of early region 1a (E1a) to ascertain: (i) whether the cold-sensitive phenotype of hr1 was the result of the identified single-base pair deletion of nucleotide 1,055 or due to a missense mutation at another site and (ii) what region and how much of the E1a 51-kilodalton protein is actually required to produce cell transformation. A mutant, H5dl101 (dl101), was constructed to contain a 5-base pair deletion of nucleotides 1,008 to 1,012, which produced a frameshift and a subsequent stop codon at nucleotide 1,241. This mutant, which should encode a truncated 33-kilodalton protein in place of the wild-type 51-kilodalton protein, had a cold-sensitive phenotype for transformation essentially identical to hr1. Consonant with this finding, a mutant (H5in106) engineered to contain a 16-base pair insertion initiated after nucleotide 1,009, with a stop codon beginning at the newly inserted nucleotide 1,013, also had a cold-sensitive phenotype like hr1 and dl101. It is striking, however, that a mutant (H5dl105) with a 69-base pair deletion beginning at nucleotide 1,003, and having a stop codon at nucleotide 1,544, was totally defective for transformation at any temperature. Transfection studies with plasmids containing the E1a or E1a and E1b genes of sub309, hr1, and dl101 further revealed that the cold-sensitive transformation phenotype observed could be exhibited in the absence of viral E1b gene expression.

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)

The role of adenovirus early region 1A in the regulation of early regions 2A and 1B expression

The role of the adenovirus early region 1A (E1A) in the regulation of expression of early regions 2A (E2A) and 1B (E1B) has been investigated by microinjection of cloned segments of viral DNA into cultured cells. Plasmids carrying adenoviral sequences that code for the DNA binding protein (DBP) associated with either the early or the late E2A promoter (at map coordinates 75 and 72, respectively), or both promoters, were injected into cell nuclei in the presence or absence of a plasmid DNA containing early region 1A. The injected cells were scored for the expression of the DNA binding protein by indirect immunofluorescence using specific antisera. These studies suggest that the product(s) encoded by early region 1A differentially control the expression of the DNA binding protein. DBP produced from the early promoter is stimulated by addition of the E1A region. By contrast E1A gene product(s) inhibit the synthesis of the DBP species transcribed from the late promoter. It has been shown previously by a variety of studies that E1A regulates E1B expression. However, experiments in which E1B DNA was microinjected together with E1A and E2A suggest that the DNA binding protein may have a stimulatory effect on the synthesis of E1B products.

The pattern of integration of viral DNA sequences in the adenovirus 5-transformed human cell line 293

The pattern of integrated adenovirus 5 DNA in the adenovirus 5-transformed human cell line 293 was analyzed by DNA blot hybridization. In contrast to a previous report, only sequences from the left end of the genome were detected. The viral DNA was contained in a unique DNA fragment generated by cleavage of 293 DNA with either EcoRI or BamHI, two enzymes which do not cut the integrated viral DNA. Further mapping studies indicated that the integrated sequence was probably colinear with viral DNA. The joining to host cell sequences occurred between viral nucleotide base pairs 1 and 270 (a site for BalI) on one end and 4123 (SmaI site) and 5372 (BalI site) at the other end. The viral DNA was not tandemly repeated. These results suggest that integration of viral sequences into the host genome probably occurred at a single site. If any subsequent duplications of viral DNA took place, then duplication of extensive cellular sequences also must have occurred.

Localization of the adenovirus E1Aa protein, a positive-acting transcriptional factor, in infected cells infected cells

The function of the adenovirus E1Aa protein (the product of the 13S E1A mRNA) during a productive viral infection is to activate transcription of the six early viral transcription units. To study the mechanism of action of this protein, a peptide which was 13 amino acids long and had a sequence unique to the protein product of the adenovirus 13S E1A mRNA (pE1Aa) was coupled to keyhole limpet hemocyanin and used to raise an antibody in rabbits. The resulting antiserum was specific to this protein and did not react with the protein product of the 12S E1A mRNA, which shares considerable sequence with the E1Aa protein. This antiserum was used to probe for the E1Aa protein in situ by indirect immunofluorescence and in extracts of infected HeLa cells. We found that the protein was associated with large cellular structures both in the nucleus and in the cytoplasm. The nuclear form of the protein was analyzed further and was found to purify with the nuclear matrix.

Functional analysis of the nucleotide sequence surrounding the cap site for adenovirus type 5 region E1A messenger RNAs

We have constructed a set of small, dispersed deletion mutations in the sequences surrounding the cap site of the adenovirus type 5 region E1A transcription unit. The effects of the mutations on E1A transcription were studied in vitro using a HeLa cell-free extract, and in vivo by reconstructing the mutations back into intact viral chromosomes and analyzing E1A messenger RNAs synthesized after infection of HeLa cells. The sequence between -35 and +20 (relative to the cap site at +1) was important for efficient E1A transcription and cap site selection in vitro. This region includes the "TATA" homology, which appeared essential for transcription. Sequences upstream of -35 were dispensable for transcription in vitro. Different results were found upon analysis of the same set of deletions in vivo. None of the mutations affected the steady-state levels of cytoplasmic, E1A-specific mRNAs found in infected HeLa cells by more than twofold. Deletions of the TATA homology, however, generated E1A mRNAs with heterogeneous 5' ends, and deletions downstream of the homology displaced the 5' end of mRNAs by about the size of the deletion.

Identification of adenovirus genes that require template replication for expression

The relationship between adenovirus type 2 DNA replication and expression of intermediate stage viral genes was investigated. The 1.03-kilobase mRNA from early region 1b (E1b) and the mRNAs coding for proteins IX and IVa2 were first detected between 6 and 8 h postinfection. Inhibition of viral DNA replication with hydroxyurea prevented expression of the IX and IVa2 mRNAs, but not of the E1b mRNA. Pulse-labeling experiments demonstrated that the block of IX and IVa2 expression in hydroxyurea-treated cells was at the level of transcription. By a series of superinfection experiments, it was determined that the viral and cellular factors present during the late stage of adenovirus infection are insufficient to activate IX gene expression. The viral DNA template must first replicate before IX transcription can begin.

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.

Localization of the adenovirus E1Aa protein, a positive-acting transcriptional factor, in infected cells infected cells

The function of the adenovirus E1Aa protein (the product of the 13S E1A mRNA) during a productive viral infection is to activate transcription of the six early viral transcription units. To study the mechanism of action of this protein, a peptide which was 13 amino acids long and had a sequence unique to the protein product of the adenovirus 13S E1A mRNA (pE1Aa) was coupled to keyhole limpet hemocyanin and used to raise an antibody in rabbits. The resulting antiserum was specific to this protein and did not react with the protein product of the 12S E1A mRNA, which shares considerable sequence with the E1Aa protein. This antiserum was used to probe for the E1Aa protein in situ by indirect immunofluorescence and in extracts of infected HeLa cells. We found that the protein was associated with large cellular structures both in the nucleus and in the cytoplasm. The nuclear form of the protein was analyzed further and was found to purify with the nuclear matrix.

Splicing of adenovirus 2 early region 1A mRNAs is non-sequential

The r-strand of early region 1A (E1A) of adenovirus serotype 2 is transcribed into three completely overlapping messenger RNA species, a 9S, a 12S and a 13S mRNA. These three mRNAs are processed from a common colinear RNA precursor and differ only with regard to the size of the intron removed during mRNA maturation. We have studied the processing pathways for the E1A mRNAs by using an assay for transient expression of recombinant plasmids containing the E1A region. All three region E1A mRNAs are synthesized and transported to the cytoplasm in sufficient quantities to permit a detailed study of their structure by S1 endonuclease analysis and primer extension. Additionally, we show that the 72 base-pair repeat from simian virus 40 (SV40), when located upstream of the E1A promoter stimulates expression of the E1A mRNAs five- to tenfold. In order to determine whether splicing of the E1A mRNAs is sequential, i.e. whether the 13S and 12S mRNAs can serve as intermediates in splicing, we constructed two plasmids that lack the intervening sequences that are removed during the maturation of the 12S and 13S mRNAs, respectively. From an analysis of the RNAs produced after transfection with these deletion mutants, the following major conclusions can be made. (1) Splicing of the E1A mRNAs is non-sequential, i.e. the 13S, 12S and 9S RNAs are generated by separate splicing events using the nuclear colinear transcript as the only precursor RNA. (2) RNA splicing is not a prerequisite for an efficient transport of the E1A mRNAs to the cytoplasm. (3) The 13S RNA can be further processed to 12S and 9S RNA species. These splicing events are, however, illegitimate and give rise to 12S and 9S RNAs that both lack one nucleotide at the splice junction. (4) A coupling between splicing and nuclear transport is most likely required in vivo to prevent illegitimate splicing of the 13S mRNA. (5) The 12S RNA does not serve as a precursor for further processing to the 9S RNA. (6) Splicing of the E1A mRNAs followed strictly the G-T-A-G rule.

Far upstream initiation sites for adenovirus early region 1A transcription are utilized after the onset of viral DNA replication

Adenovirus early region 1A (E1A) is the first transcription unit expressed after infection. It encodes a protein which controls the expression of all other early viral genes. The E1A mRNAs have one major capped 5' terminus which maps 31 nucleotides downstream from a T-A-T-A sequence (C. Baker and E. Ziff. J. Mol. Biol. 149:189-221, 1981). In addition, a minor set of E1A mRNAs are observed during the early phase of infection which have 5' termini mapping at approximately -160, -185, and -230 relative to the major cap site (Osborne et al., Cell 29:139-148, 1982). Here we report the occurrence of another set of minor E1A mRNAs which were observed exclusively after the initiation of viral DNA replication. These late specific E1A mRNAs had cap sites which mapped at approximately -300, -325, -360, and -375 relative to the major cap site. The appearance of these minor late E1A mRNAs was blocked by the DNA synthesis inhibitor cytosine arabinoside. These same late specific E1A mRNAs were synthesized from E1A-containing plasmids which replicate in monkey cells. This demonstrated that neither late specific adenovirus proteins nor adenovirus-specific chromatin structure was required for the production of the late specific E1A mRNAs. Adenovirus mutants in which the E1A T-A-T-A box region had been deleted also synthesized the corresponding deleted forms of the late specific mRNAs after initiation of DNA replication. These results indicate that the process of DNA replication alters the specificity of E1A transcription initiation in a promoter region which is at least 375 nucleotides in length.

Transcription of adenovirus 5 early region 1b is elevated in permissive cells infected by a mutant with an upstream deletion

Early region 1b (E1b) of adenovirus 5 consists of a single transcription unit that lies from 1,702 to 4,070 nucleotides from the conventional left end of the genome. The effect of mutations that map upstream of E1b on the production of E1b mRNA was examined in vivo with mutants defective in gene functions from the neighboring early region 1a (E1a) transcription unit (499 to 1,632 nucleotides from the left end). These host range mutants replicate in the adenovirus 5-transformed human cell line 293. E1b mRNA accumulation was assayed by DNA-RNA hybridization late after productive infection when the E1b transcripts are abundant in the cytoplasm. Cells infected by wild-type virus, mutant dl311, or mutant hr1. The elevated levels of E1b mRNA were also detected in steady-state nuclear RNA, pulse-labeled polyadenylated nuclear RNA, and pulse-labeled total nuclear RNA. These data indicate that E1b transcription was elevated in human 293 cells infected with dl312. There was no evidence of increases in genomic DNA in dl312-infected cells, suggesting that the rate of transcription may be elevated. When mixed infections with a 10-fold excess of either dl312 or wild-type virus were performed, the phenotype was that of the more abundant genome. This result suggests that the respective phenotypes were cis dominant. The increased rate of transcription can be attributed to cis-active regulatory effects of the deletion of nucleotides 448 to 1,349 in mutant dl312 DNA.

Newly formed mRNA lacking polyadenylic acid enters the cytoplasm and the polyribosomes but has a shorter half-life in the absence of polyadenylic acid

Labeled adenovirus type 2 nuclear RNA molecules from cells treated with 3'-deoxyadenosine (3'dA) were earlier reported to lack polyadenylic acid [poly(A)], but to be correctly spliced in the nucleus (M. Zeevi et al., Cell 26:39-46, 1981). We have now found that the shortened mRNA molecules, lacking poly(A), can also be found in the cytoplasm of 3'dA-treated cells in association with the polyribosomes. In addition, the accumulation of labeled, nuclear adenovirus-specific RNA complementary to early regions 1a, 1b, and 2 of the adenovirus genome was approximately equal in 3'dA-treated and control cells. At the initial appearance of newly labeled adenovirus type 2 RNA (10 min) in the cytoplasm, there was one-half as much labeled RNA in 3'dA-treated cells as in the control. However, control cells accumulated additional mRNA in the cytoplasm very rapidly in the first 40 min of labeling, whereas the 3'dA-treated cells did not. Therefore, it appears that the correctly spliced, poly(A)- mRNA molecules that are labeled in the presence of 3'dA can be transported from the nucleus with nearly the same frequency and the same exit time as in control cells and can be translated in the cytoplasm but have a much shorter half-life than the poly(A)+ mRNA molecules from control infected cells. From these results it is suggested that the role of poly(A) may be entirely to increase the longevity of cytoplasmic mRNA.

Selective inhibition of adenovirus type 2 early region II and III transcription by an anisomycin block of protein synthesis

The transcription of adenovirus type 2 genes proceeds through a broad three-phase program. From 1 to 4 h postinfection six early transcription units (EIa, EIb, EII, EIII, EIV, and the promoter-proximal segment of the late transcription unit) are activated. From 4 to 6 h postinfection transcription of the early genes is depressed. After the onset of viral DNA replication at approximately 6 h postinfection, the transcript from the late promoter is antiterminated, and this transcript dominates viral RNA synthesis. The early activation period also proceeds through a series of stages; early regions EIa and EIV are activated first, followed by early region EII. We show that in the presence of anisomycin, a stringent inhibitor of protein synthesis, nuclear RNA and cytoplasmic polyadenylated RNA from regions EIa and EIV accumulate at normal rates, whereas RNAs from regions EII and EIII do not accumulate. We also show that failure to accumulate RNAs from regions EII and EIII is due to reduction of the rate of transcription by greater than 90%. We conclude that the regulation of the promoters for early regions EII and EIII is distinct from the mechanism that operates on the other early transcription units. The promoters for early regions EII and EIII diverge and lie approximately 500 nucleotides apart. We examined the structure of viral chromatin in this region early during infection by mild DNase I digestion in isolated nuclei and indirect end labeling with a DNA probe near these promoters. In control, drug-free cells where EII and EIII are transcribed and in anisomycin-treated cells where EII and EIII are not transcribed we detect the same regular DNase I pattern. We suggest that regulation of EII and EIII is not mediated through a change in gross chromatin structure, but rather by a viral effector, possibly a product of region EIa.

Variety in the level of gene control in eukaryotic cells

In light of present progress in the understanding of how messenger RNA is constructed in eukaryotic cells, the levels of gene control are discussed. Transcriptional control, assessed by the rate of synthesis of specific nuclear RNA, is strongly indicated to be the most frequent level of control. Definition of eukaryotic transcription units as simple (encoding one protein) and complex (encoding two or more proteins) affords a framework in which to discuss control at the level of RNA processing for which several examples also are known. Finally, differential stability of cytoplasmic mRNAs and differential translation, both well established, are briefly described.

Synthesis of human adenovirus early RNA species is similar in productive and abortive infections of monkey and human cells

Northern (RNA) blot analysis has been used to show that synthesis of early mRNA species is similar in monkey cells productively or abortively infected with human adenovirus. mRNA species from all five major early regions (1A, 1B, 2, 3, 4) are identical in size and comparable in abundance whether isolated from monkey cells infected with adenovirus type 2 or with the host range mutant Ad2hr400 or coinfected with adenovirus type 2 plus simian virus 40. The mRNA species isolated from monkey cells are identical in size to those isolated from human cells. Production of virus-associated RNA is also identical in productive and abortive infections of monkey cells. Synthesis of virus-associated RNA is, however, significantly greater in HeLa cells than in CV1 cells at late times after infection regardless of which virus is used in the infection.