Synthesis and genomic site for an adenovirus type 2 early glycoprotein

Functions and mechanisms of action of the adenovirus E3 proteins

In the evolutionary battle between viruses and their hosts, viruses have armed themselves with weapons to defeat the host's attacks on infected cells. Various proteins encoded in the adenovirus (Ad) E3 transcription unit protect cells from killing mediated by cytotoxic T cells and death-inducing cytokines such as tumor necrosis factor (TNF), Fas ligand, and TNF-related apoptosis-inducing ligand (TRAIL). The viral protein E3-gp19 K blocks MHC class-I-restricted antigen presentation, which diminishes killing by cytotoxic T cells. The receptor internalization and degradation (RID) complex (formerly E3-10.4 K/14.5 K) stimulates the clearance from the cell surface and subsequent degradation of the receptors for Fas ligand and TRAIL, thereby preventing the action of these important immune mediators. RID also downmodulates the epidermal growth factor receptor (EGFR), although what role, if any, this function has in immune regulation is uncertain. In addition, RID antagonizes TNF-mediated apoptosis and inflammation through a mechanism that does not primarily involve receptor downregulation. E3-6.7 K functions together with RID in downregulating some TRAIL receptors and may block apoptosis independently of other E3 proteins. Furthermore, E3-14.7 K functions as a general inhibitor of TNF-mediated apoptosis and blocks TRAIL-induced apoptosis. Finally, after expending great effort to maintain cell viability during the early part of the virus replication cycle, Ads lyse the cell to allow efficient virus release and dissemination. To perform this task subgroup C Ads synthesize a protein late in infection named ADP (formerly E3-11.6 K) that is required for efficient virus release. This review focuses on recent experiments aimed at discovering the mechanism of action of these critically important viral proteins.

Control of adenovirus early gene expression: posttranscriptional control mediated by both viral and cellular gene products

An adenovirus type 5 host range mutant (hr-1) located in region E1A and phenotypically defective in expressing viral messenger ribonucleic acid (RNA) from other early regions (Berk et al., Cell 17:935-944, 1979) was analyzed for accumulation of viral RNA in the presence of protein synthesis inhibitors. Nuclear RNA was transcribed from all early regions at the same rate, regardless of whether the drug was present or absent. As expected, low or undetectable levels of RNA were found in the cytoplasm of hr-1-infected cells compared with the wild-type adenovirus type 5 in the absence of drug. When anisomycin was added 30 min before hr-1 infection, cytoplasmic RNA was abundant from early regions E3 and E4 when assayed by filter hybridization. In accordance, early regions E3 and E4 viral messenger RNA species were detected by the S1 endonuclease mapping technique only in hr-1-infected cells that were treated with the drug. Similar results were obtained by in vitro translation studies. Together, these results suggest that this adenovirus type 5 mutant lacks a viral gene product necessary for accumulation of viral messenger RNA, but not for transcription. It is proposed that a cellular gene product serves as a negative regulator of viral messenger RNA accumulation at the posttranscriptional level.

Lack of effect of mouse adenovirus type 1 infection on cell surface expression of major histocompatibility complex class I antigens

It has been proposed that adenoviruses establish and maintain persistent infections by reducing the class I major histocompatibility complex-associated presentation of viral antigens to cytotoxic T lymphocytes, leading to ineffective cell-mediated immunity and impaired clearance of infected cells (W.S.M. Wold and L. R. Gooding, Virology 184:1-8, 1991). Early region 3 of human adenovirus types 2 and 5 encodes a 19-kDa glycoprotein that associates with the class I major histocompatibility complex (MHC) antigens in the endoplasmic reticulum and prevents their maturation and transport to the cell surface. Early region 1A of human adenovirus type 12 encodes a protein that inhibits class I MHC mRNA production at the transcriptional or posttranscriptional processing level. Unlike human adenovirus infections, however, mouse adenovirus type 1 (MAV-1) infection of a variety of cell types did not affect the surface expression of 10 different mouse class I MHC allotypes. MAV-1-infected cells also regenerated cell surface class I MHC antigens following proteolytic removal as efficiently as mock-infected cells. The ability of cells to present antigen to class I MHC (Kb)-ovalbumin-specific T-cell hybridoma cells was likewise unaltered by MAV-1 infection. Thus, the ability of MAV-1 to persist cannot be explained by the model of reduced class I MHC-associated antigen presentation proposed for human adenoviruses.

Sequence analysis of putative E3, pVIII, and fiber genomic regions of a porcine adenovirus

The complete nucleotide sequence of the putative early transcriptional region 3 (E3), plus the hexon-associated polypeptide VIII (pVIII) gene, and the N-terminus of the fiber protein gene of a porcine adenovirus isolate, NADC-1, was determined. The E3 region of NADC-1 was found to be 1879 bp and located between 80 and 85.8 map units. Eight open reading frames (ORFs) and three polyadenylation signals were identified in the r strand. The amino acid sequences predicted to be encoded by ORFs 1 and 8 were compared to the amino acid sequences of human adenovirus type 2 (Ad2) pVIII and fiber protein and found to be 60% and 55% similar, respectively. The amino acid sequence predicted to be encoded by ORF 4 was compared to the human Ad5 14.7 kDa protein and the C-terminus of the amino acid sequence predicted to be encoded by ORF 11 of the bovine Ad3 E3 region and found to be 36% and 41% similar, respectively. A potential signal sequence was identified in ORF 5.

The E3-11.6K protein of adenovirus is an Asn-glycosylated integral membrane protein that localizes to the nuclear membrane

The 11,600 MW (101 amino acids; 11.6K) protein of adenovirus 2 (Ad2) is a protein of unknown function which is synthesized in low amounts during early stages of infection but in very high amounts at late stages. The 11.6K protein migrates as three major groupings of diffuse bands of ca. 14K, 21K, and 31K on SDS-PAGE, indicating that 11.6K undergoes post-translational modification. We show here that 11.6K is Asn-glycosylated with complex (endo H-resistant) oligosaccharides and that 11.6K is an integral membrane protein. Immunofluorescence indicated that 11.6K initially is associated with the endoplasmic reticulum and Golgi apparatus and that it ultimately localizes to the nuclear membrane. The 11.6K protein is predicted to have a single signal-anchor sequence at residues 41-62 and only one potential Asn-linked glycosylation site at residue 14; thus, 11.6K must be oriented in the membranes with its NH2-terminus in the lumen and its COOH-terminus in the cytoplasm. The signal-anchor and glycosylation features of 11.6K are preserved in Ad2 and Ad5 (group C), and in Ad3 and Ad7 (group B), but the sequence of 11.6K is more diverged among these serotypes than is the sequence of most other adenovirus proteins.

The E3-14.5K integral membrane protein of adenovirus that is required for down-regulation of the EGF receptor and for prevention of TNF cytolysis is O-glycosylated but not N-glycosylated

The adenovirus E3-14.5K protein is a cytoplasmic integral membrane protein that functions in concert with the E3-10.4K protein to down-regulate the epidermal growth factor receptor and to prevent tumor necrosis factor cytolysis in adenovirus-infected cells. The 14.5K protein migrates as multiple bands in SDS-PAGE, indicating that it undergoes post-translational modification. The 14.5K protein is known to be phosphorylated on serine. We show here that 14.5K can be metabolically labeled with [3H]glucosamine, that the label is labile to alkali, and that the SDS-PAGE band pattern is simplified in a cell line that is defective in O-glycosylation. Thus, 14.5K is O-glycosylated, probably at a single site in the NH2-terminal lumenal domain. The protein was not metabolically labeled with [3H]mannose, and its SDS-PAGE band pattern was not affected by tunicamycin treatment in vivo or endo F treatment in vitro; thus, 14.5K is not N-glycosylated. There was no evidence that the 10.4K protein is glycosylated, and the 10.4K protein was not required for glycosylation of 14.5K. Virtually all 14.5K molecules appear to contain the core disaccharide Gal beta 1-3GalNAc alpha 1-Ser/Thr which is commonly found on mucin-type O-glycoproteins, and neuraminidase digestion experiments indicated that this disaccharide contains terminal sialic acid.

The adenovirus E3 14.5-kilodalton protein, which is required for down-regulation of the epidermal growth factor receptor and prevention of tumor necrosis factor cytolysis, is an integral membrane protein oriented with its C terminus in the cytoplasm

We previously reported that the adenovirus type 5 E3 14.5-kilodalton protein (14.5K) forms a complex with E3 10.4K and that both proteins are required to down-regulate the epidermal growth factor receptor in adenovirus-infected human cells. Both proteins are also required to prevent cytolysis by tumor necrosis factor of most mouse cell lines infected by adenovirus mutants that lack E3 14.7K. The E3 14.5K amino acid sequence suggests that 14.5K is an integral membrane protein with an N-terminal signal sequence for membrane insertion. Here we show that 14.5K was found exclusively in cytoplasmic membrane fractions. Radiochemical sequencing of 14.5K indicated that the N-terminal signal sequence is cleaved predominantly between Cys-18 and Ser-19. With a mutant that does not express 10.4K, cleavage occurs predominantly between Phe-17 and Cys-18, indicating that the presence or absence of 10.4K affects the signal cleavage site. 14.5K was extracted into the detergent phase with Triton X-114, it remained associated with membranes after extraction with Na2CO3 at pH 11.5, and it was partially protected by membranes from proteinase K digestion; these observations indicate that 14.5K is an integral membrane protein. Proteinase K digestion followed by immunoprecipitation with antipeptide antisera directed against the N or C terminus of mature 14.5K indicated that 14.5K is oriented in the membrane with its N terminus in the lumen and its C terminus in the cytoplasm. Thus, 14.5K is a type I bitopic membrane protein. Previous studies indicated that 10.4K is also an integral membrane protein oriented with its C terminus in the cytoplasm. Altogether, these findings suggest that cytoplasmic membranes are the site of action when 10.4K and 14.5K down-regulate the epidermal growth factor receptor and prevent tumor necrosis factor cytolysis.

Adenovirus E4-dependent activation of the early E2 promoter is insufficient to promote the early-to-late-phase transition

The adenovirus E4 ORF6/7 protein has been shown to activate the cellular transcription factor E2F. E2F activation leads to activation of the adenovirus early E2 promoter which controls the production of viral DNA replication proteins. In the present study an adenovirus type 5 cDNA mutant, H5ilE4L, was constructed. This mutant is capable of making the ORF6/7 polypeptide but lacks the coding sequences for all other E4 products. H5ilE4L trans activates the early E2 promoter to wild-type levels, but still it is defective for viral DNA replication. A mutant expressing ORF6 in addition to ORF6/7, H5ilE4I, is normal for viral DNA replication. This indicates that activation of the early E2 promoter is insufficient to promote efficient viral DNA replication and that another E4-encoded function is necessary. The ORF6 protein seems to provide this function. We suggest that ORF6/7-induced activation of E2F is not necessary for adenovirus growth in HeLa cells. Rather, this activation might be of importance in the normal, growth-arrested host cell, since E2F has been shown to bind to the promoter regions of a number of immediate-early genes involved in regulation of cell proliferation (M. Mudryj, S. W. Hiebert, and J. R. Nevins, EMBO J. 9:2179-2184, 1990).

The endoplasmic reticulum retention signal of the E3/19K protein of adenovirus type 2 consists of three separate amino acid segments at the carboxy terminus

The E3/19K protein of adenovirus type 2 is a resident of the ER. Immediately after synthesis it binds to human major histocompatibility complex class I antigens and prevents their departure from the ER compartment. The ER retention signal of the E3/19K protein is contained within the 15 amino acids that protrude on the cytoplasmic side at the carboxy terminus of the protein. To define the ER retention sequence in more detail, we have generated 10 mutants of the E3/19K protein that differ only within this segment. Analysis of the rate of intracellular transport and cell surface expression of HLA antigens associated to these mutants, show that the sequences Ser-Phe-Ile, located in the middle of the 15-residue segment and Met-Pro, at the extreme carboxy terminus, are crucial for retention. Four charged residues, Asp-Glu-Lys-Lys, are located between these two retention elements but are of little or no importance. The basic cluster of amino acids close to the membrane also has some effect on retention. Thus, the retention signal of the E3/19K protein is not a contiguous sequence of amino acids but has a complex spatial arrangement.

A 6700 MW membrane protein is encoded by region E3 of adenovirus type 2

There is an open reading frame between ATG1022 and TGA1205 in the E3 transcription unit of adenovirus 2 that could encode a protein of MW 6700 (6.7K) (61 amino acids). To address whether this protein is expressed, we prepared an antiserum against a synthetic peptide corresponding to residues 47-61 in the 6.7K protein. This antiserum immunoprecipitated two series of protein bands, a 7K-8K doublet and a 15K-16K doublet or triplet, as observed by electrophoresis on 10-18% gradient SDS-polyacrylamide gels. These bands were not obtained from cells infected with mutants that lack the 6.7K gene. Most, if not all, of the 7K-8K and 15K-16K bands were detected by immunoblot, indicating that they are modified versions of the 6.7K protein. Only an 8K band was observed after cell-free translation of hybridization-purified mRNA, suggesting that this may be the primary translation product. As judged by DNA sequence, the 6.7K protein has a hydrophobic domain of at least 22 residues (residues 16-37), suggesting that 6.7K may be a membrane protein. Consistent with this, the 7K-8K and 15K-16K bands were observed in the crude membrane but not the cytosol or nuclear fractions of biochemically fractionated cells. The 6.7K protein was underproduced by mutants which underproduce E3 mRNAs a and c, indicating that 6.7K is translated from these mRNAs. Since the E3-gp 19K protein is also translated from mRNAs a and c, these mRNAs are bicistronic. The 6.7K protein is well-conserved in Ad5 (Ad2 and Ad5 are group C adenoviruses), and appears to be marginally conserved in Ad3 (group B).

Requirements for the association of adenovirus type 2 E3/19K wild-type and mutant proteins with HLA antigens

The E3/19K protein of human adenovirus type 2 is a resident of the endoplasmic reticulum (ER). Immediately after synthesis, it associates with major histocompatibility complex class I antigens and prevents their intracellular transport and cell surface expression. We have generated several C-terminal deletion mutants of the E3/19K protein that are preterminated at various positions on both sides of the membrane-spanning segment of the protein. One of these mutants is terminated at the luminal side of the membrane (M310), and two are terminated in the hydrophobic segment (M374 and M392), whereas mutant M621 is terminated on the cytoplasmic side of the ER membrane. The M310, M374, and M392 mutants are soluble proteins. They do not associate with HLA antigens in transfected 293 cells, and they are, to some extent, secreted into the medium. The M621 mutant protein is integrated in the ER membrane, associates immediately after its synthesis with HLA antigens, and exits from the ER. By using either an in vitro translation system supplemented with microsomes or overexpression in insect cells, we showed that M374 and E3/19K are able to associate with HLA antigens. These results indicate that the conformation of the luminal part of the E3/19K protein is not grossly altered by the mutations. Rapid transport of the M374 mutant out of the ER and partial degradation of this protein may prevent the interaction with HLA class I antigens in transfected 293 cells.

A 14,500 MW protein is coded by region E3 of group C human adenoviruses

There is an ORF in the early region E3 transcription unit of human adenovirus 5 (Ad5) which could encode a protein of 14,500 MW (14.5K). This ORF is conserved in Ad5 and Ad2, both group C adenoviruses, and also in Ad3 and Ad7, both group B adenoviruses. To address whether the 14.5K protein is synthesized, we prepared antisera against synthetic peptides corresponding to residues 19-34 or 118-132 in the Ad5 version of 14.5K, and also against a TrpE-14.5K fusion protein expressed in Escherichia coli. These antisera immunoprecipitated the [35S]Met-labeled 14.5K protein from KB cells infected with rec700 (an Ad5-Ad2-Ad5 recombinant), Ad2, and a variety of E3 mutants. Mutants in the 14.5K ORF did not produce the 14.5K protein. The 14.5K is coded in large part, although probably not exclusively, by E3 mRNA f, as indicated by immunoprecipitation of 14.5K from cells infected with mutants that overproduce or underproduce mRNA f. The 14.5K migrated as five to six bands on SDS-PAGE after immunoprecipitation or Western blot, suggesting that it undergoes post-translational modification. Two bands of 14.5K were obtained by cell-free translation of 14.5K from mRNA purified by hybridization from infected cells.

A 10,400-molecular-weight membrane protein is coded by region E3 of adenovirus

Previous studies with adenovirus mutants have indicated that a 10,400-molecular-weight (10.4K) protein predicted to be coded by an open reading frame in region E3 of adenovirus functions to down regulate the epidermal growth factor receptor (C. R. Carlin, A. E. Tollefson, H. A. Brady, B. L. Hoffman, and W. S. M. Wold, Cell 57:135-144, 1989). We now demonstrate that the 10.4K protein is in fact synthesized in cells infected by group C adenoviruses. This was done by immunoprecipitation of 10.4K from cells infected by a variety of E3 mutants, using antisera against three different synthetic peptides corresponding to the predicted 10.4K sequence. The 10.4K protein was translated primarily from E3 mRNA f, as indicated by cell-free translation of mRNA purified by hybridization from cells infected with an RNA processing mutant that synthesizes predominantly mRNA f. The 10.4K protein was overproduced or underproduced in vivo, respectively, by mutants that overproduce or underproduce E3 mRNA f, also indicating that the 10.4K protein is translated primarily from mRNA f. The 10.4K protein migrated as two bands with apparent molecular weights of 16,000 and 11,000 (10 to 18% gradient gels); both bands contained 10.4K epitopes, as shown by Western blot (immunoblot). Only the 16K band was obtained by cell-free translation, suggesting that the 16K protein is the precursor to the 11K protein. The 10.4K protein is a membrane protein, as shown by cell fractionation experiments and as predicted from its sequence. The predicted 10.4K sequence as well as a putative N-terminal signal sequence and 30-residue transmembrane domain are conserved in adenovirus types 2 and 5 (group C) and in types 3, 7, and 35 (group B).

Immunogenicity and efficacy testing in chimpanzees of an oral hepatitis B vaccine based on live recombinant adenovirus

As a major cause of acute and chronic liver disease as well as hepatocellular carcinoma, hepatitis B virus (HBV) continues to pose significant health problems world-wide. Recombinant hepatitis B vaccines based on adenovirus vectors have been developed to address global needs for effective control of hepatitis B infection. Although considerable progress has been made in the construction of recombinant adenoviruses that express large amounts of HBV gene products, preclinical immunogenicity and efficacy testing of candidate vaccines has remained difficult due to the lack of a suitable animal model. We demonstrate here that chimpanzees are susceptible to enteric infection by human adenoviruses type 7 (Ad7) and type 4 (Ad4) following oral administration of live virus. Moreover, after sequential oral immunization with Ad7- and Ad4-vectored vaccines containing the hepatitis B surface antigen (HBsAg) gene, significant antibody responses to HBsAg (anti-HBs) were induced in two chimpanzees. After challenge with heterologous HBV, one chimpanzee was protected from acute hepatitis and the other chimpanzee experienced modified HBV-induced disease. These data demonstrate the feasibility of using orally administered recombinant adenoviruses as a general approach to vaccination.

Role of early region 3 (E3) in pathogenesis of adenovirus disease

The cotton rat Sigmodon hispidus has provided an animal model of adenovirus pneumonia that permits investigation of the viral gene products required to produce the disease and the molecular mechanisms effecting the damage. This study was carried out to test the hypothesis that early region 3 (E3) of the adenovirus genome plays a critical role in pathogenesis of the virus's disease process even though none of its gene products are essential for its replication. Mutants whose E3 region is largely deleted (i.e., H2dl801 and H5dl327) replicated like wild-type virus in the cotton rats' lungs, but the lymphocyte and macrophage/monocyte inflammatory response was markedly increased. Viruses containing mutations that ablated production of the 19-kDa glycoprotein had the same effect as H2dl801 and H5dl327. However, mutants with deletions in the other E3 open reading frames, some of which encode known proteins, did not differ from wild-type virus in their pathogenic properties. The 19-kDa glycoprotein markedly reduces expression of the class I major histocompatibility complex antigens on the surface of infected cells. A complete correlation was found between those mutants that had increased pathogenic effects and those that lost the ability to reduce transport of the class I major histocompatibility complex antigens to surface of infected cells (i.e., all mutants unable to express the 19-kDa glycoprotein). H5sub304, which has a deletion between 83.2 and 85.1 map units in the E3B region and expresses the 19-kDa glycoprotein, did not increase the extent of pneumonia but qualitatively changed the inflammatory response in that increased numbers of polymorphonuclear leukocytes accumulated, often in small foci.

Gene product of region E4 of adenovirus type 5 modulates accumulation of certain viral polypeptides

An adenovirus type 5 mutant, designated H5ilE4I, was constructed in which region E4 was replaced by a cloned cDNA. The cDNA was a copy of an mRNA which exclusively contains open translational reading frames 6 and 7. The phenotype of the mutant was compared with that of the previously characterized E4 mutant H2dl808 and wild-type adenovirus 5. Although the H5ilE4I mutant lacked at least five E4 genes, it was nondefective for growth in HeLa cells. The defects in viral DNA replication, late protein synthesis, and shutoff of host cell protein synthesis associated with the phenotype of the H2dl808 mutant were not observed in HeLa cells infected with the H5ilE4I mutant. However, differences were observed regarding the time of onset of viral DNA replication and the accumulation of the hexon polypeptide as well as the 72-kilodalton adenovirus-specific DNA-binding protein. The results thus indicate that open reading frame 6 or 7 or both contain all genetic information required for viral replication in tissue culture cells, whereas another E4 gene modulates the accumulation of certain viral polypeptides. The early onset of viral DNA replication in H5ilE4I-infected cells may be an indirect effect of the enhanced expression of the 72-kilodalton DNA-binding protein.

A 14,700 MW protein from the E3 region of adenovirus inhibits cytolysis by tumor necrosis factor

We find that cells infected with wild-type group C human adenoviruses are not killed by exposure to tumor necrosis factor (TNF), but cells infected with adenoviruses that delete the E3 transcription unit are highly sensitive to TNF lysis. Mock-infected cells are resistant to TNF. Thus, adenovirus infection induces cellular susceptibility to lysis by TNF, and a product of E3 protects against lysis by TNF. The E3-dependent resistance to TNF was investigated using virus mutants that delete different segments of E3. Resistance was found to depend on the presence of a 14,700 MW protein, which has only recently been identified and for which there was no known function. Our results support the hypothesis that one of the functions of TNF in vivo is to combat virus infections, and that the 14,700 MW protein evolved in adenovirus to counteract the antiviral effects of TNF.

Identification and gene mapping of a 14,700-molecular-weight protein encoded by region E3 of group C adenoviruses

Early region E3 of adenovirus type 5 should encode at least nine proteins as judged by the DNA sequence and the spliced structures of the known mRNAs. Only two E3 proteins have been proved to exist, a glycoprotein (gp19K) and an 11,600-molecular-weight protein (11.6K protein). Here we describe an abundant 14.7K protein coded by a gene in the extreme 3' portion of E3. To identify this 14.7K protein, we constructed a bacterial vector which synthesized a TrpE-14.7K fusion protein, then we prepared antiserum against the fusion protein. This antiserum immunoprecipitated the 14.7K protein from cells infected with adenovirus types 5 and 2, as well as with a variety of E3 deletion mutants. Synthesis of the 14.7K protein correlated precisely with the presence or absence of the 14.7K gene and with the synthesis of the mRNA (mRNA h) which encodes the 14.7K protein. The 14.7K protein appeared as a triplet on immunoprecipitation gels and Western blots (immunoblots).

"E3/19K" protein of adenovirus type 2 inhibits lysis of cytolytic T lymphocytes by blocking cell-surface expression of histocompatibility class I antigens

The E3 19,000-dalton protein termed "E3/19K" of adenovirus type 2 binds to human class I histocompatibility antigens (HLA antigens). Human 293.12 cultured cells that express a cloned gene for the E3/19K protein show reduced levels of HLA antigens on the cell surface compared to parental 293 cells. We have transfected these cell lines with plasmid DNA containing the murine histocompatibility H-2Kd allele to demonstrate that this antigen binds also to the E3/19K protein. The resulting association prevents the H-2Kd antigen from being terminally glycosylated and inhibits its cell-surface expression. Two murine cytolytic T-lymphocyte clones specific for HLA antigens and restricted by the H-2Kd antigen lyse the human 293Kd cells. In the presence of the E3/19K protein, a dramatically reduced cell surface density of both HLA and H-2Kd antigens was shown. This decreased amount of cell-surface HLA/H-2Kd antigens correlated with a reduction in susceptibility to lysis of the target cells. In particular, the cell-surface level of the H-2Kd antigen, which is the restricting element, was crucial for efficient lysis. Thus, the E3/19K protein of adenovirus type 2 indirectly reduces the cellular immune recognition in the in vitro system. This might be the mechanism involved in latent and persistent infections caused by adenoviruses in vivo.

Adenoviruses of subgenera B, C, D, and E modulate cell-surface expression of major histocompatibility complex class I antigens

The immune defense against viral infections involves cytotoxic T lymphocytes that recognize viral products in the context of class I major histocompatibility complex (MHC) antigens. To evade such immune surveillance viruses may have evolved various strategies to manipulate the expression of class I antigens. Adenovirus 2 manufactures an early glycoprotein, E19, that binds to nascent class I antigens in the endoplasmic reticulum and impedes their transport to the cell surface. We now show that adenoviruses typical of all viral subgenera except the highly oncogenic subgenus A dramatically reduce the cell-surface expression of class I antigens. It has been shown that subgenus A viruses abolish class I antigen expression in transformed cells by reducing mRNA levels. Thus, all adenoviruses can modulate the cell-surface expression of class I antigens.

Evidence that AGUAUAUGA and CCAAGAUGA initiate translation in the same mRNA region E3 of adenovirus

We described a simple method to introduce site-specific mutations into region E3 of adenovirus (Ad). Mutations are made in cloned Ad2 EcoRI-D (map position 76-83), then ligated between Ad5 EcoRI-A (map position 0-76) and EcoRI-B (map position 83-100) to complete the viral genome. We have used this method to isolate a viable virus mutant (dl702) that is relevant to the problems of translation initiation and gene organization in the E3 complex transcription unit. mRNA a in region E3 encodes an abundant glycoprotein termed gp19K. There are two AUGs in mRNA a that are 5' to AUG1204 which initiates gp19K. One of these, AUG1022, could initiate a 6.7K protein, although this protein has not been identified in infected cells. Mutant dl702 has a deletion such that the 6.7K gene is fused in-frame to the gp19K gene. We report that the 6.7K-gp19K fusion protein is synthesized both in dl702-infected cells and after cell free translation of infected cell RNA. The quantity of fusion protein made is much less than that of wild type gp19K. The sequence context of AUG1022 for 6.7K is AGUAUAUGA, and that of AUG1204 for gp19K is CCAAGAUGA. The consensus sequence of eukaryotic initiation codons is CCPuCCAUGG, with the Pu at -3 being important (M. Kozak, Nucleic Acids Res. 12, 857-872, 1984). Our results suggest that (i) AUG1022 can initiate translation in vivo and therefore the 6.7K protein probably is made in infected cells, (ii) that mRNA a is a dicistronic mRNA encoding the 6.7K and gp19K proteins, and (iii) that the initiation codon for 6.7K may be much less efficient than that for gp19K. Thus, the E3 genes may be organized such that the relative abundance of the 6.7K and gp19K proteins is controlled by the efficiency of their initiation codons in the same mRNA.

Novel deletion mutants that enhance a distant upstream 5' splice in the E3 transcription unit of adenovirus 2

Region E3 of adenovirus is a "complex" transcription unit: i.e. different mRNAs and proteins arise by differential RNA 3' end selection and differential splicing of the primary transcript. We are using viable virus mutants to understand the controls that dictate the specificity and efficiency of the RNA processing signals. We describe a novel class of deletion mutations that enhance a natural 5' splice site located approximately 740 nucleotides (nt) upstream. In particular, deletions within nt 1691-2044 in the E3 transcription unit result in a 5-fold enhancement of the 5' splice site at nt 951 (as reflected in steady-state mRNA). The effect is specific, because the deletions do not affect the 5' splice site at nt 372, and because deletions within nt 2044-2214 do not affect either the 951 or the 372 5' splice sites. As a consequence of the enhanced splicing at the 951 5' site, synthesis of the major E3 mRNA and the major E3 protein (gp19K) are dramatically reduced. At least one of the natural 3' splice sites, located at nt 2157, is the recipient of the enhanced splicing at the 951 5' splice site. We conclude that sequences located within nt 1691-2044 affect (probably in cis) splicing at the 951 5' splice site. We speculate that nt 1691-2044 includes a splicing control region which functions to suppress splicing at the 951 5' splice site.

Abrogation of cell surface expression of human class I transplantation antigens by an adenovirus protein in Xenopus laevis oocytes

Class I transplantation antigens form complexes with a virus protein encoded in the early region E3 of the adenovirus-2 genome. The interaction between this viral glycoprotein, E19, and nascent human class I antigens has been examined by microinjecting purified mRNA into Xenopus laevis oocytes. Both E19 and the two class I antigen subunits, the heavy chain and beta 2-microglobulin (beta 2M), were efficiently translated. The heavy chains did not become terminally glycosylated, as monitored by endoglycosidase H digestion, and were not expressed on the oocyte surface unless they were associated with beta 2M. The E19 protein did not become terminally glycosylated, and we failed to detect this viral protein on the surface of the oocytes. Co-translation of heavy chain and E19 mRNA demonstrated that the two proteins associate intracellularly. However, neither protein appeared to be transported to the trans-Golgi compartment. Similar observations were made in adenovirus-infected HeLa cells. Heavy chains bound to beta 2M became terminally glycosylated in oocytes in the presence of low concentrations of E19. At high concentrations of the viral protein, no carbohydrate modifications and no cell surface expression of class I antigens were apparent. Thus, beta 2M and E19 have opposite effects on the intracellular transport of the heavy chains. These data suggest that adenovirus-2 may impede the cell surface expression of class I antigens to escape immune surveillance.

An adenovirus type 2 glycoprotein blocks cell surface expression of human histocompatibility class I antigens

The adenovirus type 2 encoded protein E3/19K binds to human histocompatibility class I antigens (HLA). This association occurs both in adenovirus-infected cells and in cells that have been transfected with the gene encoding the E3/19K protein. The formation of the HLA-E3/19K complex prevents the HLA antigens from being correctly processed by inhibiting their terminal glycosylation. This effect is specific for HLA antigens and does not generally involve the glycosyltransferases. Furthermore, the HLA-E3/19K association dramatically reduces the cell surface expression of the HLA antigens. This reduced level of antigens might influence the cytotoxic T cell response. Therefore, our results show a possible molecular mechanism whereby adenoviruses, and perhaps other viruses, delay or escape the cellular immune system of the host.

Mapping the 5' ends, 3' ends, and splice sites of mRNAs from the early E3 transcription unit of adenovirus 5

Using nuclease gel analyses, the sites in the approximately 4000 nucleotide (nt) E3 transcription unit of adenovirus 5 (Ad5) that encode the 5' ends, 3' ends, and 5' and 3' splice sites of the approximately 10 E3 mRNAs were determined. Transcription initiation of all mRNAs occurs at two major (nt 1 and 8) and approximately two minor sites, situated 20-30 nt 3' to a TATA box. There are two major 3' end sites (nt 2227 and 3308), located approximately 20 nt downstream from ATTAAA and an AATAAA sequences, respectively. Thus, ATTAAA, as well as the usual AATAAA, apparently can function as a 3' end signal. There are two 5' splice sites (nt 372 and 923), both with GT at the intron boundary. There are four 3' splice sites (nt 766, 1817, 2201, and 2880), all with AG at the intron boundary. The nt 1817, 2201, and 2880 3' splice sites are located immediately upstream from open reading frames, such that splicing at the different sites allows synthesis of completely different proteins.

DNA sequence of the early E3 transcription unit of adenovirus 5

The DNA sequence of the early E3 transcription unit of adenovirus 5 (Ad5) has been determined and it has been compared to Ad2, as published previously [J. Hérissé, G. Courtois, and F. Galibert (1980), Nucl. Acids Res. 8, 2173-2192; J. Hérissé and F. Galibert (1981), Nucl. Acids Res. 9, 1229-1240]. The E3 regions of Ad5 and Ad2 are quite homologous despite being nonessential for Ad growth in cultured cells. The major differences are "gaps" that exist either in Ad5 or Ad2 in intergenic regions. The conservation of sequences suggests that E3 plays a beneficial role in natural infection of humans. E3 appears to encode about seven to nine proteins; based on sequence, seven of these may be membrane proteins. Thus, E3 may be a transcription unit devoted to the synthesis of membrane proteins. The E3 genes lie essentially one after the other along the genome, and which gene is expressed from a given primary transcript is determined by the choice of the 3' end site and the 5' and 3' splice sites. Almost all E3 mRNAs contain nonfunctional AUGs that are 5' to the initiation codon. Codon usage is nonrandom. Although the CG dinucleotide frequency is low, CG clusters exist in the promoter and other regions.

Mapping a new gene that encodes an 11,600-molecular-weight protein in the E3 transcription unit of adenovirus 2

The DNA sequence of the early E3 transcription unit of adenovirus 2 (Ad2) (J. Hérissé et al., Nucleic Acids Res. 8:2173-2192, 1980), indicates that an open reading frame exists between nucleotides 1860 and 2163 that could encode a protein of Mr 11,600 (11.6K). We have determined the DNA sequence of the corresponding region in Ad5 (closely related to Ad2) and have established that this putative gene is conserved in Ad5 (a 10.5K protein). To determine whether this protein is expressed, we prepared an antiserum in rabbits against a synthetic peptide corresponding to amino acids 66 to 74 in the 11.6K protein of Ad2. The peptide antiserum immunoprecipitated a ca. 13K-14K protein doublet, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, from [35S]methionine-labeled Ad2- or Ad5-early-infected KB cells. The antiserum also immunoprecipitated a 13K-14K protein doublet translated in vitro from Ad2 or Ad5 early E3-specific mRNA purified by hybridization to Ad2 EcoRI-D (nucleotides -236 to 2437). The synthetic peptide successfully competed with the 13K-14K protein doublet in immunoprecipitation experiments, thereby confirming the specificity of the antiserum. As deduced from the DNA sequence, the 11.6K protein (and the corresponding 10.5K Ad5 protein) has a conserved 22-amino-acid hydrophobic domain, suggesting that the protein may be associated with membranes. We conclude that a gene located at nucleotides 1860 to 2143 in the Ad2 E3 transcription unit (nucleotides 1924 to 2203) in the Ad5 E3 transcription unit) encodes an 11.6K protein (10.5K in Ad5).

Altered mRNA splicing in monkey cells abortively infected with human adenovirus may be responsible for inefficient synthesis of the virion fiber polypeptide

Messenger RNA encoding the fiber protein of the human adenovirus serotype 2 (Ad2) capsid is inefficiently translated in abortively infected African green monkey kidney cells. The amount of fiber mRNA present in the cytoplasm of abortively infected monkey cells is less than that in productively infected cells by a factor of 5-10 but synthesis of the fiber polypeptide is reduced by a factor of more than 100. Evidence from a variety of experiments indicates that the defect does not lie in the translational apparatus of the monkey cell but may best be explained by differences in the fiber messages made in abortively versus productively infected cells. Here we report that fiber mRNA isolated from abortively infected monkey cells is processed differently than that made in productively infected cells. Primer extension analysis of the 5' ends of fiber messages from several different productive and abortive infections shows a direct correlation between synthesis of the fiber polypeptide in vivo and the presence of the "x" and/or "y" ancillary leaders on messages encoding the fiber polypeptide. Of all the mRNAs encoded by the major late transcriptional unit of Ad2 only the fiber message can contain the x and y leaders, and the fiber protein is the only late Ad2 protein reported to be glycosylated. We speculate that these leader sequences play a role in the synthesis of this glycoprotein, as well as that of the Ad2 19-kilodalton glycoprotein encoded by early region 3, whose mRNA also contains the x and y leaders.

Generation of adenovirus by transfection of plasmids

Biologically active fragments of Adenovirus 5 (Ad5) DNA that span the entire genome have been cloned into plasmids. The covalently attached terminal protein was removed and Eco RI linkers added in a fashion that preserves the Ad5 terminal sequences. When plasmids containing overlapping fragments that represent the entire genome are cotransfected onto 293 cells, infectious virus is obtained. Generation of virus depends upon the release of the 0 or 100 mu Ad5 terminus from pBR322 DNA by Eco RI cleavage. During virus production the modified termini of the transfected fragments are corrected exactly to that of wt viral DNA. The above method for preparing adenovirus recombinants has been used to construct a mutant, Ad5 delta (78.9-84.3), lacking most of the non-essential EIII transcriptional unit. This mutant is phenotypically wild type with respect to burst size and kinetics of growth. Surprisingly, it inhibits wt viral growth upon mixed infections of HeLa or 293 cells, apparently at the level of DNA replication.

Association between transplantation antigens and a viral membrane protein synthesized from a mammalian expression vector

Cells infected or transformed by adenovirus-2 express complexes of the viral E19 protein and class I transplantation antigens. To eliminate the complication of the numerous metabolic changes in adenovirus-infected and -transformed cells, we detached the E19 gene from its viral background by constructing transient expression vectors where the E19 coding sequence is flanked by the SV40 early promoter and the 3' region of the rabbit beta-globulin gene. E19 production was assayed 2 days after transfection of monkey COS or human HeLa cells with vector DNA. Efficient E19 gene expression depends on the presence of an intron (the second beta-globin intron), and the E19 protein is properly processed and anchored in the plasma membrane of transfected cells. The vector-synthesized E19 protein is also associated with class I transplantation antigens in human cells, since a fraction of it is coprecipitated both with antiserum against HLA antigens and with monoclonal antibodies against beta 2-microglobulin.

On the insertion of proteins into membranes

Recent data concerning the primary structure and the interactions of proteins with membranes suggest the existence of two classes of integral membrane proteins. In the first class, the polypeptide chain crosses the membrane only once. The membrane penetrating fragment is markedly hydrophobic and contains several positive charges on its C-terminal border. In the second class, the protein is folded in a complex fashion within the membrane and the knowledge of its amino acid sequence is not sufficient to predict the manner in which the protein interacts with the membrane.

Control of adenovirus gene expression: cellular gene products restrict expression of adenovirus host range mutants in nonpermissive cells

Adenovirus type 5 (Ad5) host range mutants dl312 and hr-1, with lesions in region E1A (0 to 4.5 map units) of the viral genome, fail to accumulate virus-specific early RNA during infection in HeLa cells. In a recent report, we showed that the addition of anisomycin, a stringent inhibitor of protein synthesis, at 1 h after infection of HeLa cells with hr-1 virus resulted in the accumulation of properly spliced and translatable mRNA from all early regions (M. G. Katze, H. Persson, and L. Philipson, Mol. Cell. Biol. 1:807-813, 1981). Based on these results we proposed a model in which expression of early mutant RNA was achieved through inactivation of a cellular protein normally causing a reduction in the amount of viral RNA. These studies have been extended in the present report, which shows that early viral proteins can be detected in Ad5 dl312- and Ad5 hr-1-infected HeLa cells which have been treated for several hours with anisomycin either shortly after infection or before infection. A pulse of drug treatment also resulted in expression of substantial amounts of adenovirus structural proteins after infection with both Ad5 hr-1 and Ad5 dl312, whereas in drug-free controls no late proteins were detected. The Ad5 hr-1 virus previously reported to be DNA replication negative in nonpermissive HeLa cells was found to replicate its DNA, albeit at low levels, when anisomycin was present either from 1 to 5 h postinfection or for 5 h before infection. When infectious virus production was examined in mutant-infected cells the titer of Ad5 dl312 virus was found to increase at least 500-fold in anisomycin-treated HeLa cells. Taken together, these and our previous results suggest that the block in gene expression characteristic for complementation group I Ad5 host range mutants in HeLa cells can be overcome by inactivating cellular gene products serving as negative regulators of viral gene expression.

Secretion of the hepatitis B virus surface antigen from mouse cells using an extra-chromosomal eucaryotic vector

Recombinant DNA molecules which contained a subgenomic fragment of the hepatitis B virus (HBV) genome, the pML2 vector and the bovine papillomavirus type 1 (BPV) genome were constructed. The HBV fragment includes the entire transcription unit for the hepatitis B surface antigen (HBsAg). After propagation in Escherichia coli, the recombinant plasmids were cleaved with endonucleases SalI and PvuI to eliminate most of the bacterial sequences before transfection of mouse C127 cells. Foci were observed 10--14 days after transfection. Cells from selected foci were cloned and the supernatants were assayed for the presence of HBsAg. Most of the clones tested were found to secrete HBsAg particles into the growth medium. These particles appear to be similar to the 22 nm particles present in the serum of HBV chronic carriers. SDS-polyacrylamide gel electrophoresis revealed that the particles contain two polypeptides, probably representing the glycosylated and unglycosylated forms of the HBsAg major polypeptide. An analysis of DNA from the transformed clones revealed that they contain multiple extra-chromosomal copies of the recombinant, which, however, had suffered rearrangement.

The nucleotide sequence of mRNA for the Mr 19 000 glycoprotein from early gene block III of adenovirus 2

The cytoplasmic poly(A)RNA from early stages of infection of HeLa cells by adenovirus 2 (Ad2), was used to synthesize cDNA. The resulting cDNA segments were inserted at the PstI site of the plasmid pBR322 after dG/dC tailing. The clones containing sequences corresponding to early region III transcripts were identified by hybridization against the SmaI-C fragment (76.5 to 91.9 map units) of the Ad2 genome. The clone pE22 which contained a copy of the mRNA coding for the Mr 19 000 glycoprotein was characterized in detail. A comparison with the genomic sequences allowed the identification of the splice between the x and y leaders. The nucleotide sequences CCGGTG and CAGTTT were found at the donor and acceptor sites of the splice junction, respectively. The coding region consisted of a continuous stretch of 159 amino acids with a hydrophobic N-terminus and two possible glycosylation sites. The triplet ATG was encountered twice in phase 3 before the actual site of initiation of translation, which was in phase 2. The 3'-untranslated region was 500 nucleotides long and contained an open, translational reading frame for a Mr 11 000 protein, following a potential initiator ATG. The sequence ATTAAA was observed 17 bp before the poly(A) tail.

An adenovirus glycoprotein binds heavy chains of class I transplantation antigens from man and mouse

The successful killing of virus-infected cells by cytotoxic T lymphocytes (CTL) is dependent on the recognition of both a viral product and class I antigens of the major histocompatibility complex (MHC) on the infected cell surface. Whether these two entities are found independently on the cell surface and therefore recognized by two different CTL receptors, or whether they are associated together and can therefore be recognized by a single receptor is not known. The association between an adenovirus-encoded glycoprotein expressed on the cell surface early after infection and class I antigens has been investigated and it has been found that antisera against class I antigens can co-precipitate the antigen and the viral glycoprotein from an adenovirus-transformed cell line from the Hooded-Lister rat strain. We show here by in vitro affinity chromatography and in vivo immunoprecipitation that the viral glycoprotein specifically binds to the heavy chain of class I antigens in both man and mouse.

Purification of a native membrane-associated adenovirus tumor antigen

A 15,000-dalton protein was purified from HeLa cells infected with adenovirus type 2. Proteins solubilized from a membrane fraction of lytically infected cells was used as the starting material for purification. Subsequent purification steps involved lentil-lectin, phosphocellulose, hydroxyapatite, DEAE-cellulose, and aminohexyl-Sepharose chromatographies. A monospecific antiserum, raised against the purified protein, immunoprecipitated a 15,000-dalton protein encoded in early-region E1B (E1B/15K protein) of the adenovirus type 2 DNA. Tryptic finger print analysis revealed that the purified protein was identical to the E1B/15K protein encoded in the transforming part of the viral genome. The antiserum immunoprecipitated the E1B/15K protein from a variety of viral transformed cell lines isolated from humans, rats, or hamsters. The E1B/15K protein was associated with the membrane fraction of both lytically and virus-transformed cell lines and could only be released by detergent treatment. Furthermore, a 11,000- to 12,000-dalton protein that could be precipitated with the anti-E1B/15K serum was recovered from membranes treated with trypsin or proteinase K, suggesting that a major part of the E1B/15K protein is protected in membrane vesicles. Translation of early viral mRNA in a cell-free system, supplemented with rough microsomes, showed that this protein was associated with the membrane fraction also in vitro.

Characterization and molecular cloning of the mRNA for the heavy (epsilon) chain of rat immunoglobulin E

We report a study of the mRNA for the heavy (epsilon) chain of rat IgE. Cytoplasmic RNA was prepared from the two rat immunocytomas IR2 and IR162 and fractionated by sucrose gradient centrifugation. An enriched fraction containing approximately 5% mRNA for the epsilon chain was obtained in this way. When translated in vitro, it produced a 59,000-dalton polypeptide, which in the presence of a membrane fraction yielded a 90,000-dalton polypeptide, presumably through posttranslational modification. Both polypeptides were precipitated by rabbit antisera that were monospecific for rat epsilon chains. The epsilon chain mRNA was estimated to be approximately 2200 nucleotides long and constitutes a minute fraction in the total mRNA both in the IR2 and the IR162 tumors, unlike the mRNA for light chains. Double-stranded cDNA copies prepared frm the RNA fraction, which was enriched for epsilon chain mRNA, were inserted into the Pst I cleavage site of the pBR322 vector. Twenty clones with inserts exceeding 1000 base pairs were used for selection of mRNA from the IR2 tumor. By in vitro translation of the selected mRNA, one clone was identified that yielded a polypeptide with the same size as the unprocessed epsilon chain. The nucleotide sequence was determined for part of the inserted cDNA in this candidate clone and was found to be homologous to a sequence in the constant region (C) of the human epsilon chain. In this communication we report a sequence from the C epsilon 3 domain of the rat IgE. When compared to the corresponding sequence of human IgE, 55% of the amino acids in the rat sequence were found to be conserved.

Control of adenovirus early gene expression: accumulation of viral mRNA after infection of transformed cells

We studied the accumulation of viral mRNA in the presence of inhibitors of protein synthesis in an adenovirus type 5-transformed cell line (line 293 cells). An analysis of the endogenous viral mRNA's and proteins revealed that only early regions 1A and 1B were expressed in uninfected 293 cells. However, viral mRNA's from early regions 2, 3, and 4, as well as mRNA's from early regions 1A and 1B, accumulated in 293 cells after infection with adenovirus type 2. Cells treated with anisomycin before infection showed a drastic enhancement of mRNA from early region 4 compared with drug-free controls, This increase in viral mRNA was detected by using filter hybridization, S1 endonuclease mapping, and in vitro translation. The rate of transcription of early region 4 nuclear RNA also increased significantly in the presence of anisomycin. In contrast to the results with early region 4, the levels of cytoplasmic mRNA's from early regions 2 and 3 did not increase in cells treated with inhibitors. These results suggested that multiple virus encoded controls operate on the early regions of the adenovirus genome.

Structures of the oligosaccharides of the glycoprotein coded by early region E3 of adenovirus 2

Early region E3 of adenovirus 2 encodes a glycoprotein, E3-gp25K, that is a good model with which to study structure-function relationships in transmembrane glycoproteins. We have determined the structures of the oligosaccharides linked to E3-gp25K. The oligosaccharides were labeled with [2-(3)H]mannose in adenovirus 2-early infected KB cells for 5.5h (pulse) or for 5.5 h followed by a 3-h chase (pulse-chase). E3-gp25K was extracted and purified by chromatography on DEAE-Sephacel in 7 M urea, followed by gel filtration on a column of Bio-Gel A-1.5m in 6 M guanidine hydrochloride. An analysis of the purified protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that it was >95% pure. The oligosaccharides were isolated by pronase digestion followed by gel filtration on a column of Bio-Gel P-6, then by digestion with endo-beta-N-acetylglucosaminidase H, followed by gel filtration on Bio-Gel P-6, and finally by paper chromatography. The pulse sample contained equal amounts of Man(9)GlcNAc and Man(8)GlcNAc and small amounts of Man(7)GlcNAc and Man(6)GlcNAc. The pulse-chase sample had predominantly Man(8)GlcNAc and much less Man(9)GlcNAc, indicating that processing of the Man(9)GlcNAc to Man(8)GlcNAc had occurred during the chase period. Thus, Man(8)GlcNAc is the major oligosaccharide on mature E3-gp25K. The structures of these oligosaccharides were established by digestion with alpha-mannosidase, methylation analysis, and acetolysis. The oligosaccharides found had typical high-mannose structures that have been observed in other membrane and soluble glycoproteins, and the branching patterns and linkages of the mannose residues of Man(9)GlcNAc were identical to those of the lipid-linked Glc(3)Man(9)GlcNAc(2) donor. Thus, adenovirus 2 infection (early stages) apparently does not affect the usual cellular high-mannose glycosylation pathways, and despite being virus coded, E3-gp25K is glycosylated in the same manner as a typical mammalian cell-coded glycoprotein.

Control of adenovirus early gene expression: posttranscriptional control mediated by both viral and cellular gene products

An adenovirus type 5 host range mutant (hr-1) located in region E1A and phenotypically defective in expressing viral messenger ribonucleic acid (RNA) from other early regions (Berk et al., Cell 17:935-944, 1979) was analyzed for accumulation of viral RNA in the presence of protein synthesis inhibitors. Nuclear RNA was transcribed from all early regions at the same rate, regardless of whether the drug was present or absent. As expected, low or undetectable levels of RNA were found in the cytoplasm of hr-1-infected cells compared with the wild-type adenovirus type 5 in the absence of drug. When anisomycin was added 30 min before hr-1 infection, cytoplasmic RNA was abundant from early regions E3 and E4 when assayed by filter hybridization. In accordance, early regions E3 and E4 viral messenger RNA species were detected by the S1 endonuclease mapping technique only in hr-1-infected cells that were treated with the drug. Similar results were obtained by in vitro translation studies. Together, these results suggest that this adenovirus type 5 mutant lacks a viral gene product necessary for accumulation of viral messenger RNA, but not for transcription. It is proposed that a cellular gene product serves as a negative regulator of viral messenger RNA accumulation at the posttranscriptional level.

Controls of RNA splicing and termination in the major late adenovirus transcription unit

The major late adenovirus promoter is active early after infection, selectively producing messenger RNAs coding for polypeptides with molecular weights of 55,000, 52,000 and 14,000. This selective expression suggests that a differential splicing pattern occurs at the transition from early to late viral gene expression. Activation of the late promoter and splicing of the 55, 52K mRNAs does not require newly synthesized virus polypeptides.