Deletion mutants that alter differential RNA processing in the E3 complex transcription unit of adenovirus

Genomic characterization of human adenovirus type 4 strains isolated worldwide since 1953 identifies two separable phylogroups evolving at different rates from their most recent common ancestor

Species Human mastadenovirus E (HAdV-E) comprises several simian types and a single human type: HAdV-E4, a respiratory and ocular pathogen. RFLP analysis for the characterization of intratypic genetic variability has previously distinguished two HAdV-E4 clusters: prototype (p)-like and a-like. Our analysis of whole genome sequences confirmed two distinct lineages, which we refer to as phylogroups (PGs). PGs I and II comprise the p- and a-like genomes, respectively, and differ significantly in their G + C content (57.7% ± 0.013 vs 56.3% ± 0.015). Sequence differences distinguishing the two clades map to several regions of the genome including E3 and ITR. Bayesian analyses showed that the two phylogroups diverged approximately 602 years before the present. A relatively faster evolutionary rate was identified for PG II. Our data provide a rationale for the incorporation of phylogroup identity to HAdV-E4 strain designation to reflect the identified unique genetic characteristics that distinguish PGs I and II.

Gene delivery from the E3 region of replicating human adenovirus: evaluation of the 6.7 K/gp19 K region

The use of genetically engineered, replication-selective viruses to treat cancer is being realized with viruses such as ONYX-015, a human adenovirus that selectively destroys p53 mutant cancer cells. To enhance further the clinical efficacy of ONYX-015 and viruses like it, we have developed a novel gene delivery system for replicating adenoviruses. This system has two unique features. First, it uses the endogenous adenoviral gene expression machinery (promoter, splicing, polyadenylation) to drive transgene expression. Second, a single region or gene in the multi-gene E3 transcription unit is selectively substituted for by the therapeutic transgene(s). Analyzing various transgene substitutions for the 6.7 K/gp19 K region of E3, we demonstrate the following: (1) transgene expression in this system is predictable and mimics the substituted endogenous gene expression pattern, (2) expression of surrounding E3 genes can be retained, (3) the insertion site choice can effect both the transgene expression level and the viral life cycle, and, (4) expression levels from this system are superior to those generated from a replication-defective virus using the HCMV enhancer-promoter and this is dependent on viral DNA replication. This unique methodology has broad application to the rapidly evolving field of replicating virus-based therapies.

The 5' and 3' splice sites come together via a three dimensional diffusion mechanism

We present evidence that the splice sites in mammalian pre-mRNAs are brought together via a three dimensional diffusion mechanism. We tested two mechanisms for splice site pairing: a lateral diffusion ('scanning') model and the currently favored three dimensional diffusion ('jumping') model. Two lines of evidence that distinguish between these two models are presented. The first utilized bipartite splicing substrates tethered by double-stranded RNA stems predicted to provide either a moderate or severe block to splice site pairing via a scanning mechanism. Splice site pairing via a jumping mechanism was expected to be unaffected or affected minimally. The second approach utilized a flexible poly(ethylene glycol) moiety within the intron. This insertion was predicted to reduce scanning efficiency but not the efficiency of a three dimensional diffusion mechanism. The best explanation for the data with the bipartite RNAs is that splice site pairing occurs through three dimensional diffusion. Kinetic analysis of the poly(ethylene glycol) containing substrate showed that neither the lag phase nor the initial rates of mRNA production and spliceosome assembly were affected by this insertion. Therefore, both experimental approaches supported the three dimensional diffusion model of splice site pairing.

Upstream and downstream cis-acting elements for cleavage at the L4 polyadenylation site of adenovirus-2

A study of the cis-acting elements involved in the 3' end formation of the RNAs from the major late L4 family of adenovirus-2 was undertaken. Series of 5' or 3' end deletion mutants and mutants harboring either internal deletions or substitutions were prepared and assayed for in vitro cleavage. This first allowed the demonstration of a sequence, located at -6 to -29, relative to AAUAAA, whose deletion or substitution reduces cleavage efficiency at the L4 polyadenylation site two to three fold. This upstream efficiency element 5' AUCUUUGUUGUC/AUCUCUGUGCUG 3' is constituted of a partially repeated 12 nucleotide long, UCG rich sequence. The activities of the 2 sequence elements in cleavage are additive. We also searched for regulatory sequences downstream of the L4 polyadenylation site. We found that the deletion or substitution of a 30 nucleotide long UCG rich sequence, between nucleotides +7 and +35 relative to the cleavage site and harboring a UCCUGU repeat reduces cleavage efficiency at least ten fold. A GUUUUU sequence, starting at +35 had no influence. Thus, the usage of the L4 polyadenylation site requires down-stream sequences different from the canonical GU or U boxes and is regulated by upstream sequence elements.

Characterization of mutants within the gene for the adenovirus E3 14.7-kilodalton protein which prevents cytolysis by tumor necrosis factor

The 14,700-Da protein (14.7K protein) encoded by the E3 region of adenovirus has previously been shown to protect mouse cells from cytolysis by tumor necrosis factor (TNF). Delineating the sequences in the 14.7K protein that are required for this activity may provide insight into the mechanism of protection from TNF by 14.7K as well as the mechanism of TNF cytolysis. In the present study, we examined the ability of 14.7K mutants to protect cells from lysis by TNF. In-frame deletions as well as Cys-to-Ser mutations in the 14.7K gene were generated by site-directed mutagenesis and then built into the genome of a modified adenovirus type 5 (dl7001) that lacks all E3 genes. dl7001, which replicates to the same titers as does adenovirus type 5 in cultured cells, has the largest E3 deletion analyzed to date. 51Cr release was used to assay TNF cytolysis. Our results indicate that most mutations in the 14.7K gene result in a loss of function, suggesting that nearly the entire protein rather than a specific domain functions to prevent TNF cytolysis.

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.

Map of cis-acting sequences that determine alternative pre-mRNA processing in the E3 complex transcription unit of adenovirus

The E3 complex transcription unit of adenovirus encodes four major mRNAs (a, c, f, and h) and two minor (d and e) mRNAs with overlapping exons, alternative splice sites, and two polyadenylation sites, termed E3A (upstream) and E3B (downstream). mRNAs a and d use the E3A polyadenylation site, and mRNAs c, e, f, and h use the E3B site. We have analyzed virus mutants with deletions throughout the E3 region in order to identify cis-acting sequences that function in E3 pre-mRNA processing. The results presented in this report as well as previous results are summarized as follows. (i) Deletions in the first (5') intron at nucleotides (nt) 372 to 768 in E3 had no effect unless they removed the consensus sequence for the nt 372 5' splice site; however, the overall pattern of E3 mRNAs did not change significantly. (ii) Deletions in region I (nt 1441 to 2044) eliminated mRNAs a and c and resulted in corresponding increases in mRNAs f and h; we propose that region I contains sequences that suppress splicing. (iii) Mutations in region II (nt 2161 to 2243) resulted in nearly exclusive synthesis of mRNA f; this phenotype is understood and is discussed. (iv) Changing the AUUAAA component of the E3A poly(A) addition signal to AAUAAA resulted in increased mRNA a levels, suggesting that the E3A poly(A) addition signal is intrinsically inefficient. (v) Deletions in region III (nt 2488 to 3002) decreased mRNA a levels about two- to threefold and specifically increased mRNA f levels; we suggest that region III facilitates use of the E3A polyadenylation site. (vi) Deletions in region IV (nt 2904 to 3251) increased mRNA a levels about two- to threefold; we suggest that region IV may contain sequences that facilitate use of the E3B polyadenylation site. A map of sequences that determine alternative pre-mRNA processing in region E3 is now nearly complete.

The 11,600-MW protein encoded by region E3 of adenovirus is expressed early but is greatly amplified at late stages of infection

We have reported that an 11,600-MW (11.6K) protein is coded by region E3 of adenovirus. We have now prepared two new antipeptide antisera that have allowed us to characterize this protein further. The 11.6K protein migrates as multiple diffuse bands having apparent Mws of about 14,000, 21,000, and 31,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoblotting as well as virus mutants with deletions in the 11.6K gene were used to show that the various gel bands represent forms of 11.6K. The 11.6K protein was synthesized in very low amounts during early stages of infection, from the scarce E3 mRNAs d and e which initiate from the E3 promoter. However, 11.6K was synthesized very abundantly at late stages of infection, approximately 400 times the rate at early stages, from new mRNAs termed d' and e'. Reverse transcriptase-polymerase chain reaction and RNA blot experiments indicated that mRNAs d' and e' had the same body (the coding portion) and the same middle exon (the y leader) as early E3 mRNAs d and e, but mRNAs d' and e' were spliced at their 5' termini to the major late tripartite leader which is found in all mRNAs in the major late transcription unit. mRNAs d' and e' and the 11.6K protein were the only E3 mRNAs and protein that were scarce early and were greatly amplified at late stages of infection. This suggests that specific cis- or trans-acting sequences may function to enhance the splicing of mRNAs d' and e' at late stages of infection and perhaps to suppress the splicing of mRNAs d and e at early stages of infection. We propose that the 11.6K gene be considered not only a member of region E3 but also a member of the major late transcription unit.

Adenovirus E3 14.7K protein functions in the absence of other adenovirus proteins to protect transfected cells from tumor necrosis factor cytolysis

A 14,700-kDa protein (14.7K) encoded by the E3 region of adenovirus has been shown to protect adenovirus-infected mouse C3HA cells from lysis by tumor necrosis factor (TNF) (L. R. Gooding, L. W. Elmore, A. E. Tollefson, H. A. Brady, and W. S. M. Wold, Cell 53:341-346, 1988). These infected cells are sensitized to TNF by expression of the adenovirus E1A proteins (P. Duerksen-Hughes, W. S. M. Wold, and L. R. Gooding, J. Immunol. 143:4193-4200, 1989). In this study we show that 14.7K suppresses TNF cytolysis independently of adenovirus infection. Mouse C3HA and C127 cells were transfected with the 14.7K gene controlled by the mouse metallothionein promoter, and permanent 14.7K-expressing cell lines were tested for sensitivity to TNF cytolysis. Transfected cells which were sensitized to TNF either by inhibitors of protein synthesis, microfilament-destabilizing agents, or adenovirus infection were found to be resistant to TNF cytolysis. Two monoclonal antibodies were isolated and used to quantitate 14.7K in transfected and infected cells. Enzyme-linked immunosorbent assay (ELISA) analysis with these monoclonal antibodies and 14.7K immunoblots showed that 14.7K expression can be induced with cadmium in C3HA and C127 transfectants. The 14.7K induction correlated with a dose-dependent decrease in sensitivity to TNF cytotoxicity. The 14.7K protein does not substantially alter cell surface TNF receptor numbers or affinity on C3HA mouse fibroblasts, as determined by Scatchard analysis of 125I-TNF binding. The 14.7K protein also does not alter TNF signal transduction in general, because TNF induction of cell surface class I major histocompatibility complex molecules on 14.7K transfectants was unmodified. Our findings indicate that the adenovirus 14.7K protein functions as a specific inhibitor of TNF cytolysis in the absence of other adenovirus proteins and thus is a unique tool to study the mechanism of TNF cytotoxicity.

Adenovirus E1A and E1B genes are regulated posttranscriptionally in human lymphoid cells

The interactions of adenovirus with differentiated human cells have been investigated in human myeloma cells. Relative to HeLa cells, the E1A and E1B genes, but not other viral genes, were markedly repressed by differential RNA stabilization, resulting in 20- to 50-fold less E1A and E1B mRNAs at steady state late in infection. The reduced E1A level corresponded to an approximately 200-fold-lower abundance of E1A polypeptides, which were nonetheless capable of efficient transactivation of E1A-dependent viral genes and were necessary for productive infection. The E1B gene was further regulated posttranscriptionally, yielding altered molar representation of alternatively spliced 22S and 13S mRNAs early in infection of myeloma cells. Taken together, these results suggested that repression and altered expression of E1A and E1B genes may provide a molecular basis of delayed kinetics of infection of lymphoid cells with adenovirus (D. Lavery, S. M. Fu, T. Lufkin, and S. Chen-Kiang, J. Virol. 61:1466-1472, 1987). The molecular mechanisms by which E1A and E1B are regulated and by which E1A transactivates viral genes in lymphoid cells are discussed.

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).

Adenovirus vaccine vectors expressing hepatitis B surface antigen: importance of regulatory elements in the adenovirus major late intron

Adenovirus types 4 and 7 are currently used as live oral vaccines for prevention of acute respiratory disease caused by these adenovirus serotypes. To investigate the concept of producing live recombinant vaccines using these serotypes, adenovirus types 4 (Ad4) and 7 (Ad7) were constructed that produce HBsAg upon infection of cell cultures. Ad4 recombinants were constructed that express HBsAg from a cassette inserted 135 bp from the right-hand terminus of the viral genome. The cassette contained the Ad4 major late promoter followed by leader 1 of the tripartite leader, the first intervening sequence between leaders 1 and 2, leaders 2 and 3, the HBsAg gene, and tandem polyadenylation signals from the Ad4 E3B and hexon genes. Using this same cassette, a series of Ad4 recombinants expressing HBsAg were constructed with deletions in the intervening sequence between leaders 1 and 2 to evaluate the contribution of the downstream control elements more precisely. Inclusion of regions located between +82 and +148 as well as +148 and +232 resulted in increases in expression levels of HBsAg in A549-infected cells by 22-fold and 44-fold, respectively, over the levels attained by an adenovirus recombinant retaining only sequences from +1 to +82, showing the importance of these elements in the activation of the major late promoter during the course of a natural Ad4 viral infection. Parallel increases were also observed in steady-state levels of cytoplasmic HBsAg-specific mRNA. When similar Ad7 recombinant viruses were constructed, these viruses also expressed 20-fold more HBsAg due to the presence of the intron. All Ad4 and Ad7 recombinants produced HBsAg particles containing gp27 and p24 which were secreted in the medium. When dogs were immunized intratracheally with one of these Ad7 recombinants, they seroconverted to both Ad7 and HBsAg to a high level.

Role of early genes in pathogenesis of adenovirus pneumonia

Intranasal inoculation of type 5 adenovirus into the cotton rat Sigmodon hispidus produces a pneumonia pathologically similar to that in humans, and it, therefore, provides an excellent animal model to investigate the pathogenesis of this disease. The goal of this study was to test the hypothesis that accumulation of viral structural proteins is responsible for a major portion of the cell-damage-producing disease. Since viral DNA replication is essential for synthesis of the viral structural proteins, which are products of late genes, the hypothesis was tested using mutants defective in genes required for DNA synthesis. Most experiments were done with the conditionally lethal temperature-sensitive (ts) mutant H5ts125, which contains a mutation in the early region 2A (E2A) gene encoding the DNA-binding protein. The data show that infection with 1 x 10(9.0) plaque-forming units of H5ts125 induced a pneumonia that was as extensive and qualitatively the same as that after wild-type adenovirus type 5 infection, although H5ts125 did not replicate to produce infectious virus. When cotton rats were infected with 1 x 10(8.0) plaque-forming units of wild-type adenovirus type 5 or H5ts125, the pneumonias that followed were pathologically similar; in the latter phases, however, wild-type virus produced slightly more extensive pneumonia than did H5ts125, probably because its replication permitted infection of more susceptible 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.

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.

Multiple cis-acting sequence elements are required for efficient splicing of simian virus 40 small-t antigen pre-mRNA

We have determined the effects of a number of mutations in the small-t antigen mRNA intron on the alternative splicing pattern of the simian virus 40 early transcript. Expansion of the distance separating the small-t pre-mRNA lariat branch point and the shared large T-small t 3' splice site from 18 to 29 nucleotides (nt) resulted in a relative enhancement of small-t splicing in vivo. This finding, coupled with the observation that large-T pre-RNA splicing in vitro was not affected by this expansion, suggests that small-t splicing is specifically constrained by a short branch point-3' splice site distance. Similarly, the distance separating the 5' splice site and branch point (48 nt) was found to be at or near a minimum for small-t splicing, because deletions in this region as small as 2 nt dramatically reduced the ratio of small-t to large-T mRNA that accumulated in transfected cells. Finally, a specific sequence within the small-t intron, encompassing the upstream branch sites used in large-T splicing, was found to be an important element in the cell-specific pattern of early alternative splicing. Substitutions within this region reduced the ratio of small-t to large-T mRNA produced in HeLa cells but had only minor effects in human 293 cells.

Competition between splicing and polyadenylation reactions determines which adenovirus region E3 mRNAs are synthesized

Complex transcription units encode multiple mRNAs which arise by alternative processing of a common pre-mRNA precursor. It is not known how the pre-mRNA processing pathways are determined or controlled. We are investigating this problem by using the E3 complex transcription unit of adenovirus as a model. Our approach is to construct virus mutants with lesions in E3 and then determine how the mutation affects the accumulation of E3 mRNAs in vivo. We report results which indicate that competition between splicing reactions and polyadenylation reactions occurs in vivo and that this plays an important role in alternative pre-mRNA processing.

Multiple cis-acting sequence elements are required for efficient splicing of simian virus 40 small-t antigen pre-mRNA

We have determined the effects of a number of mutations in the small-t antigen mRNA intron on the alternative splicing pattern of the simian virus 40 early transcript. Expansion of the distance separating the small-t pre-mRNA lariat branch point and the shared large T-small t 3' splice site from 18 to 29 nucleotides (nt) resulted in a relative enhancement of small-t splicing in vivo. This finding, coupled with the observation that large-T pre-RNA splicing in vitro was not affected by this expansion, suggests that small-t splicing is specifically constrained by a short branch point-3' splice site distance. Similarly, the distance separating the 5' splice site and branch point (48 nt) was found to be at or near a minimum for small-t splicing, because deletions in this region as small as 2 nt dramatically reduced the ratio of small-t to large-T mRNA that accumulated in transfected cells. Finally, a specific sequence within the small-t intron, encompassing the upstream branch sites used in large-T splicing, was found to be an important element in the cell-specific pattern of early alternative splicing. Substitutions within this region reduced the ratio of small-t to large-T mRNA produced in HeLa cells but had only minor effects in human 293 cells.

Competition between splicing and polyadenylation reactions determines which adenovirus region E3 mRNAs are synthesized

Complex transcription units encode multiple mRNAs which arise by alternative processing of a common pre-mRNA precursor. It is not known how the pre-mRNA processing pathways are determined or controlled. We are investigating this problem by using the E3 complex transcription unit of adenovirus as a model. Our approach is to construct virus mutants with lesions in E3 and then determine how the mutation affects the accumulation of E3 mRNAs in vivo. We report results which indicate that competition between splicing reactions and polyadenylation reactions occurs in vivo and that this plays an important role in alternative pre-mRNA processing.

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).

A small deletion distant from a splice or polyadenylation site dramatically alters pre-mRNA processing in region E3 of adenovirus

The E3 complex transcription unit of adenovirus encodes overlapping mRNAs (a to i) with different exon structures. The major mRNAs are a (approximately 40% of the total) and c (approximately 15%), which are spliced once, and f (approximately 15%) and h (approximately 25%), which are spliced twice. mRNA a uses the upstream E3A polyadenylation site, and the other mRNAs use the downstream E3B polyadenylation site. We analyzed virus deletion mutants to identify sequences important in alternative pre-mRNA processing in region E3. Our main finding is that a 64-base-pair deletion in dl742 causes mainly mRNAs f and h to be formed. mRNAs a and c are barely made. dl742 does not delete either a splice site or a polyadenylation site. Thus, the sequences deleted must function in alternative pre-mRNA processing independently of the signals at the actual splice and polyadenylation sites. The lack of synthesis of mRNA a by dl742 does not appear to result from a defect in the E3A polyadenylation signal but rather from an increase in splicing activity which results in the synthesis of doubly spliced mRNAs f and h at the expense of singly spliced mRNAs a and c. This suggests, in the wild-type situation, that the frequency of use of the E3A versus the E3B polyadenylation site may be determined by the rate of splicing, as well as, presumably, the rate of cleavage-polyadenylation at the E3A site.

Identification of a novel sequence that governs both polyadenylation and alternative splicing in region E3 of adenovirus

Region E3 encodes four major overlapping mRNAs with different splicing patterns. There are two poly(A) sites, an upstream site called E3A and a downstream site called E3B. We have analyzed virus mutants with deletions or insertions in E3 in order to identify sequences that function in the alternative processing of E3 pre-mRNAs, and to understand what determines which poly(A) sites and which splice sites are used. In previous studies we established that the 5' boundary of the E3A poly(A) signal is at an ATTAAA sequence. We now show, using viable virus mutants, that the 3' boundary of the E3A signal is located within 47-62 nucleotides (nt) downstream of the ATTAAA (17-32 nt downstream of the last microheterogenous poly(A) addition site). Our data further suggest that the spacing between the ATTAAA, the cleavage sites, and the essential downstream sequences may be important in E3A 3' end formation. Of particular interest, these mutants suggest a novel mechanism for the control of alternative pre-mRNA processing. Mutants which are almost completely defective in E3A 3' end formation display greatly increased use of a 3' splice site located 4 nt upstream of the ATTAAA. The mRNA that uses this 3' splice site is polyadenylated at the E3B poly(A) site. We suggest, for this particular case, that alternative pre-mRNA processing could be determined by a competition between trans-acting factors that function in E3A 3' end formation or in splicing. These factors could compete for overlapping sequences in pre-mRNA.

A short sequence in the COOH-terminus makes an adenovirus membrane glycoprotein a resident of the endoplasmic reticulum

The E19 protein of adenoviruses is a transmembrane protein that abrogates the intracellular transport of class I antigens by forming complexes with them in the ER. We show here that the E19 protein is retained in the ER even in the absence of class I antigens. To define the region conferring residency in the ER, we examined two mutant forms of the viral protein. A 5 amino acid extension of the 15-membered cytoplasmic tail of the protein reduces its interaction with class I antigens but does not change its intracellular distribution. Shortening the tail to 7 amino acids also diminishes the affinity for class I antigens; however, this mutant E19 protein becomes transported to the cell surface. Thus, we concluded that a small stretch of amino acids exposed on the cytoplasmic side of the ER membrane is responsible for the retention of the E19 protein in the ER.