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

E4orf1: The triple agent of adenovirus - Unraveling its roles in oncogenesis, infectious obesity and immune responses in virus replication and vector therapy

Human Adenoviruses (HAdV) are nearly ubiquitous pathogens comprising numerous sub-types that infect various tissues and organs. Among many encoded proteins that facilitate viral replication and subversion of host cellular processes, the viral E4orf1 protein has emerged as an intriguing yet under-investigated player in the complex interplay between the virus and its host. E4orf1 has gained attention as a metabolism activator and oncogenic agent, while recent research is showing that E4orf1 may play a more important role in modulating cellular pathways such as PI3K-Akt-mTOR, Ras, the immune response and further HAdV replication stages than previously anticipated. In this review, we aim to explore the structure, molecular mechanisms, and biological functions of E4orf1, shedding light on its potentially multifaceted roles during HAdV infection, including metabolic diseases and oncogenesis. Furthermore, we discuss the role of functional E4orf1 in biotechnological applications such as Adenovirus (AdV) vaccine vectors and oncolytic AdV. By dissecting the intricate relationships between HAdV types and E4orf1 proteins, this review provides valuable insights into viral pathogenesis and points to promising areas of future research.

Deep splicing plasticity of the human adenovirus type 5 transcriptome drives virus evolution

Viral genomes have high gene densities and complex transcription strategies rendering transcriptome analysis through short-read RNA-seq approaches problematic. Adenovirus transcription and splicing is especially complex. We used long-read direct RNA sequencing to study adenovirus transcription and splicing during infection. This revealed a previously unappreciated complexity of alternative splicing and potential for secondary initiating codon usage. Moreover, we find that most viral transcripts tend to shorten polyadenylation lengths as infection progresses. Development of an open reading frame centric bioinformatics analysis pipeline provided a deeper quantitative and qualitative understanding of adenovirus's genetic potential. Across the viral genome adenovirus makes multiple distinctly spliced transcripts that code for the same protein. Over 11,000 different splicing patterns were recorded across the viral genome, most occurring at low levels. This low-level use of alternative splicing patterns potentially enables the virus to maximise its coding potential over evolutionary timescales.

Induction of cancer-specific cell death by the adenovirus E4orf4 protein

The adenovirus E4orf4 protein is a multifunctional viral regulator that contributes to temporal regulation of the progression of viral infection. When expressed alone, outside the context of the virus, E4orf4 induces p53-independent cell-death in transformed cells. Oncogenic transformation of primary cells in tissue culture sensitizes them to cell killing by E4orf4, indicating that E4orf4 research may have implications for cancer therapy. It has also been reported that E4orf4 induces a caspase-independent, non-classical apoptotic pathway, which maintains crosstalk with classical caspase-dependent pathways. Furthermore, several E4orf4 activities in the nucleus and in the cytoplasm and various protein partners contribute to cell killing by this viral protein. In the following chapter I summarize the current knowledge of the unique mode of E4orf4-induced cell death and its underlying mechanisms. Although several explanations for the cancer-specificity of E4orf4-induced toxicity have been proposed, a better grasp of the mechanisms responsible for E4orf4-induced cell death is required to elucidate the differential sensitivity of normal and cancer cells to E4orf4.

Transcription mapping and characterization of proteins produced from early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus 3 (PAdV-3) was characterized by Northern blot, rapid amplification of cDNA ends (RACE), RT-PCR and cDNA sequence analysis. Northern blot analysis revealed three different classes of transcripts, which appeared and peaked at different times post-infection. The RT-PCR, RACE and cDNA sequence analysis identified nine major E4 transcripts, all of which shared a 107-bp 5' leader sequence and a 126-bp 3' terminus. These transcripts have one to three introns removed. Interestingly, of the nine major transcripts, there was one fusion transcript of ORFp1 and ORFp7 (ORFp1/7), which codes for a protein of 119 amino acids. All transcripts initiated at nucleotide 33740 of the PAdV-3 genome. To identify proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding p2, p3, p4 or p7 proteins. Serum against p2, p3 and p4 immunoprecipitated proteins of 13.5, 13.6 and 15.3 kDa, respectively, in in-vitro transcribed and translated mRNA and in PAdV-3-infected cells. Serum against p7 immunoprecipitated a protein of 19.8 kDa in in-vitro transcription and translation analysis but recognized two proteins of 19.8 kDa (encoded by ORFp7) and 14 kDa (encoded by the fusion transcript ORF1/7) in PAdV-3-infected cells. The protein encoded by ORFp2 was localized in the nucleus of PAdV-3-infected cells. The proteins encoded by ORFp3 and ORFp7\ORFp1/7 were detected in the cytoplasm of PAdV-3-infected cells. However, the protein encoded by ORFp4 was observed both in the cytoplasm and nucleus of PAdV-3-infected cells.

Anti-tumor efficacy of a transcriptional replication-competent adenovirus, Ad-OC-E1a, for osteosarcoma pulmonary metastasis

BACKGROUND

Osteosarcoma (OSA) is the most frequent type of primary malignant bone tumor and is apt to occur in children and young adults. Pulmonary metastasis (OSPM) is the major reason for its fatal outcome. Osteocalcin (OC) is a major noncollagenous bone protein whose expression is limited almost exclusively to bone marrow and osteotropic tumors. OC is also known to express in cell lines with bone metastasis feathers. Gene therapy strategies with the OC promoter directing the replication of adenovirus in a tumor-specific manner are a potential modality for OSPM therapy.

METHODS

We detected OC mRNA expression by RNA in situ hybridization in OSA and OSPM samples from patients, and tested OC promoter transcriptional activity in OSA and non-OSA cell lines. Then we used a transcriptional replication-competent adenovirus, Ad-OC-E1a, to treat OSPM, and evaluated its tumor-specific replication and killing activities in vitro as well as anti-OSPM efficacy in vivo via systemic delivery.

RESULTS

OC mRNA was detected in all types of OSA tissues, including OSPM tissues. The transcriptional activity of the OC promoter was much higher in a OSPM cell line SAOS-2LM7 and primary OSA cell line MG63 than in non-OSA cell lines, including cell lines from breast cancer, colon cancer, and liver cancer. Ad-OC-E1a expressed E1a protein only in MG63 and SAOS-2LM7, which indicated that adenovirus E1a was under strict control by the OC promoter. Ad-OC-E1a demonstrated killing and viral replication activity close to wild-type adenovirus levels in MG63 and SAOS-2LM7, but the killing and viral replication activities were attenuated significantly in cells expressing low OC transcriptional activity. To test whether Ad-OC-E1a could be used to target human OSPM in vivo, SAOS-2LM7 pulmonary metastasis models in nude mice were induced and treated by tail-vein injection with Ad-OC-E1a. Compared to tumor nodules in the lung in groups treated with PBS or control virus, the quantity of metastasized tumor nodules decreased significantly. Adenovirus-infected cells were stained immunohistochemically only inside and around the OSPM nodules but spared normal lung tissue and other organs.

CONCLUSIONS

These data demonstrated that OC promoter could direct adenovirus replication by controlling the E1a gene to target human OSPM in a tumor-specific manner, providing an efficient tool to develop a feasible therapeutic modality for OSPM.

Gene Therapy for Prostate Cancer by Controlling Adenovirus E1a and E4 Gene Expression with PSES Enhancer

PSES is a chimeric enhancer containing enhancer elements from prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) genes that are prevalently expressed in androgen-independent prostate cancers. PSES shows strong activity equivalent to cytomegalovirus (CMV) promoter, specifically in PSA/PSMA-positive prostate cancer cells, the major cell types in prostate cancer in the absence of androgen. We developed a recombinant adenovirus (AdE4PSESE1a) by placing adenoviral E1a and E4 genes under the control of the bidirectional enhancer PSES and enhanced green fluorescent protein gene for the purpose of intratumoral virus tracking under the control of CMV promoter. Because of PSES being very weak in nonprostatic cells, including HEK293 and HER911 that are frequently used to produce recombinant adenovirus, AdE4PSESE1a can only be produced in the HER911E4 cell line which expresses both E1 and E4 genes. AdE4PSESE1a showed similar viral replication and tumor cell killing activities to wild-type adenovirus in PSA/PSMA-positive prostate cancer cells. The viral replication and tumor cell killing activities were dramatically attenuated in PSA/PSMA-negative cells. To test whether AdE4PSESE1a could be used to target prostate tumors in vivo, CWR22rv s.c. tumors were induced in nude mice and treated with AdE4PSESE1a via intratumoral and tail vein injection. Compared to tumors treated with control virus, the growth of CWR22rv tumors was dramatically inhibited by AdE4PSESE1a via tail vein injection or intratumoral injection. These data show that adenoviral replication can be tightly controlled in a novel fashion by controlling adenoviral E1a and E4 genes simultaneously with a single enhancer.

Analysis of early region 4 of porcine adenovirus type 3

The early region 4 (E4) of porcine adenovirus (PAdV)-3, located at the right-hand end of the genome is transcribed in a leftward direction and has the potential to encode seven (p1-p7) open reading frames (ORFs). To determine the role of each protein in viral replication, we constructed full-length PAdV-3 genomic clones containing deletions of individual E4 ORF or combined deletions of the neighboring ORFs. Transfection of swine testicular (ST) cells with individual E4 mutant plasmid DNAs generated PAdV-3 E4 mutant viruses except with plasmids containing a deletion of ORF p3, ORF p2+ p3 or ORF p3+ p4. Each of the mutants was further analyzed for growth kinetics, and early/late protein synthesis. Mutant viruses carrying deletions in ORF p1, ORF p2 or ORF p4 showed growth characteristics similar to that of wild-type PAdV-3. Early/late protein synthesis was also indistinguishable from that of wild-type PAdV-3. However, mutant viruses carrying deletions in ORF p5, ORF p6 or ORF p7 showed a modest effect in their ability to grow in porcine cells and express early proteins. These results suggest that the E4 ORF p3 (showing low homology with non-essential human adenovirus (HAdV)-9-E4 ORF1 encoded proteins) is essential for the replication of PAdV-3 in vitro. In contrast, the E4 ORF p7 (showing homology to essential HAdV-2 34 kDa protein) is not essential for replication of PAdV-3 in vitro. Moreover, successful deletion of 1.957 kb fragment in E4 region increased the available capacity of replication-competent PAdV-3 (E3 + E4 deleted) to approximately 4.3 kb and that of replication-defective PAdV-3 (E1 + E3 + E4 deleted) to approximately 7 kb. This is extremely useful for the construction of PAdV-3 vectors that express multiple genes and/or regulatory elements for gene therapy and vaccination.

Cell transformation by human adenoviruses

The last 40 years of molecular biological investigations into human adenoviruses have contributed enormously to our understanding of the basic principles of normal and malignant cell growth. Much of this knowledge stems from analyses of their productive infection cycle in permissive host cells. Also, initial observations concerning the carcinogenic potential of human adenoviruses subsequently revealed decisive insights into the molecular mechanisms of the origins of cancer, and established adenoviruses as a model system for explaining virus-mediated transformation processes. Today it is well established that cell transformation by human adenoviruses is a multistep process involving several gene products encoded in early transcription units 1A (E1A) and 1B (E1B). Moreover, a large body of evidence now indicates that alternative or additional mechanisms are engaged in adenovirus-mediated oncogenic transformation involving gene products encoded in early region 4 (E4) as well as epigenetic changes resulting from viral DNA integration. In particular, detailed studies on the tumorigenic potential of subgroup D adenovirus type 9 (Ad9) E4 have now revealed a new pathway that points to a novel, general mechanism of virus-mediated oncogenesis. In this chapter, we summarize the current state of knowledge about the oncogenes and oncogene products of human adenoviruses, focusing particularly on recent findings concerning the transforming and oncogenic properties of viral proteins encoded in the E1B and E4 transcription units.

Genetic content and evolution of adenoviruses

This review provides an update of the genetic content, phylogeny and evolution of the family Adenoviridae. An appraisal of the condition of adenovirus genomics highlights the need to ensure that public sequence information is interpreted accurately. To this end, all complete genome sequences available have been reannotated. Adenoviruses fall into four recognized genera, plus possibly a fifth, which have apparently evolved with their vertebrate hosts, but have also engaged in a number of interspecies transmission events. Genes inherited by all modern adenoviruses from their common ancestor are located centrally in the genome and are involved in replication and packaging of viral DNA and formation and structure of the virion. Additional niche-specific genes have accumulated in each lineage, mostly near the genome termini. Capture and duplication of genes in the setting of a 'leader-exon structure', which results from widespread use of splicing, appear to have been central to adenovirus evolution. The antiquity of the pre-vertebrate lineages that ultimately gave rise to the Adenoviridae is illustrated by morphological similarities between adenoviruses and bacteriophages, and by use of a protein-primed DNA replication strategy by adenoviruses, certain bacteria and bacteriophages, and linear plasmids of fungi and plants.

The complete nucleotide sequence, genome organization, and origin of human adenovirus type 11

The complete DNA sequence and transcription map of human adenovirus type 11 are reported here. This is the first published sequence for a subgenera B human adenovirus and demonstrates a genome organization highly similar to those of other human adenoviruses. All of the genes from the early, intermediate, and late regions are present in the expected locations of the genome for a human adenovirus. The genome size is 34,794 bp in length and has a GC content of 48.9%. Sequence alignment with genomes of groups A (Ad12), C (Ad5), D (Ad17), E (Simian adenovirus 25), and F (Ad40) revealed homologies of 64, 54, 68, 75, and 52%, respectively. Detailed genomic analysis demonstrated that Ads 11 and 35 are highly conserved in all areas except the hexon hypervariable regions and fiber. Similarly, comparison of Ad11 with subgroup E SAV25 revealed poor homology between fibers but high homology in proteins encoded by all other areas of the genome. We propose an evolutionary model in which functional viruses can be reconstituted following fiber substitution from one serotype to another. According to this model either the Ad11 genome is a derivative of Ad35, from which the fiber was substituted with Ad7, or the Ad35 genome is the product of a fiber substitution from Ad21 into the Ad11 genome. This model also provides a possible explanation for the origin of group E Ads, which are evolutionarily derived from a group C fiber substitution into a group B genome.

Mutational analysis of early region 4 of bovine adenovirus type 3

The primary objective of characterizing bovine adenovirus type 3 (BAV3) in greater detail is to develop it as a vector for gene therapy and vaccination of humans and animals. A series of BAV3 early region 4 (E4) deletion-mutant viruses, containing deletions in individual E4 open reading frames (Orf) or combinations of Orfs, were generated by transfecting primary fetal bovine retinal cells with E4-modified genomic DNA. Each of these mutants was further analyzed for growth kinetics, viral DNA accumulation, and early-late protein synthesis. Mutant viruses carrying deletions in Orf1, Orf2, Orf3, or Orf4 showed growth characteristics similar to those of the E3-deleted BAV3 (BAV302). DNA accumulation and early/late protein synthesis were also indistinguishable from those of BAV302. However, mutant viruses carrying a deletion in Orf5, Orfs 1-3 (BAV429), or Orfs 3-5 (BAV430) were modestly compromised in their ability to grow in bovine cells and express early/late proteins. E4 mutants containing larger deletions, Orfs 1-3 (BAV429) and Orfs 3-5 (BAV430), were further tested in a cotton rat model. Both mutants replicated as efficiently as BAV3 or BAV302 in the lungs of cotton rats. BAV3-specific IgA and IgG responses were detected in serum and at the mucosal surfaces in cotton rats inoculated with mutant viruses. In vitro and in vivo characterization of these E4 mutants suggests that none of the individual E4 Orfs are essential for viral replication. Moreover, successful deletion of a 1.5-kb fragment in the BAV3 E4 region increased the available insertion capacity of replication-competent BAV3 vector (E3-E4 deleted) to approximately 4.5 kb and that of replication-defective BAV3 vector (E1a-E3-E4 deleted) to approximately 5.0 kb. This is extremely useful for the construction of BAV3 vectors that express multiple genes and/or regulatory elements for gene therapy and vaccination.

Molecular regulation and biological function of adenovirus early genes: the E4 ORFs

Over the past few years there have been a number of interesting advances in our understanding of the functions encoded by the adenovirus early transcription unit 4 (Ad E4). A large body of recent data demonstrates that E4 proteins encompass an unexpectedly diverse collection of functions required for efficient viral replication. E4 gene products operate through a complex network of protein interactions with key viral and cellular regulatory components involved in transcription, apoptosis, cell cycle control and DNA repair, as well as host cell factors that regulate cell signaling, posttranslational modifications and the integrity of nuclear multiprotein complexes known as nuclear bodies (NBs) or PML oncogenic domains (PODs). As understood at present, some of the lytic functions overlap with roles in oncogenic transformation of primary mammalian cells. These observations, together with findings that E4 proteins substantially affect cell toxicity and the immune response of the host have profound implications for the development of Ad vectors for gene therapy. In this article we will summarize recent findings regarding the diverse functions of E4 gene products in the context of earlier work. We will emphasize the interaction of E4 proteins with cellular and viral interaction partners, the role of these interactions for lytic virus growth and how these interactions may contribute to viral oncogenesis. Finally, we will discuss their role in Ad vector and adeno-associated virus infections.

Analysis of synthesis, stability, phosphorylation, and interacting polypeptides of the 34-kilodalton product of open reading frame 6 of the early region 4 protein of human adenovirus type 5

The 34-kDa early-region 4 open reading frame 6 (E4orf6) product of human adenovirus type 5 forms complexes with both the cellular tumor suppressor p53 and the viral E1B 55-kDa protein (E1B-55kDa). E4orf6 can inhibit p53 transactivation activity, as can E1B-55kDa, and in combination these viral proteins cause the rapid turnover of p53. In addition, E4orf6-55kDa complexes play a critical role at later times in the regulation of viral mRNA transport and shutoff of host cell protein synthesis. In the present study, we have further characterized some of the biological properties of E4orf6. Analysis of extracts from infected cells by Western blotting indicated that E4orf6, like E1A and E1B products, is present at high levels until very late times, suggesting that it is available to act throughout the infectious cycle. This pattern is similar to that of E4orf4 but differs markedly from that of another E4 product, E4orf6/7, which is present only transiently. Synthesis of E4orf6 is maximal at early stages but ceases completely with the onset of shutoff of host protein synthesis; however, it was found that unlike E4orf6/7, E4orf6 is very stable, thus allowing high levels to be maintained even at late times. E4orf6 was shown to be phosphorylated at low levels. Coimmunoprecipitation studies in cells lacking p53 indicated that E4orf6 interacts with a number of other proteins. Five of these were shown to be viral or virally induced proteins ranging in size from 102 to 27 kDa, including E1B-55kDa. One such species, of 72 kDa, was shown not to represent the E2 DNA-binding protein and thus remains to be identified. Another appeared to be the L4 100-kDa nonstructural adenovirus late product, but it appeared to be present nonspecifically and not as part of an E4orf6 complex. Apart from p53, three additional cellular proteins, of 84, 19, and 14 kDa were detected by using an adenovirus vector that expresses only E4orf6. The 19-kDa species and a 16-kDa cellular protein were also shown to interact with E4orf6/7. It is possible that complex formation with these viral and cellular proteins plays a role in one or more of the biological activities associated with E4orf6 and E4orf6/7.

Characterization of the early region 4 of porcine adenovirus type 3

The nucleotide sequence of a 3028 bp DNA segment, located between map co-ordinates 100 and 92 in the genome of porcine adenovirus type 3 (PAV-3), was determined. The segment includes the entire early region 4 (E-4) and the right inverted terminal repeat sequences. There were two TATA boxes and one canonical polyadenylation signal on the 1 strand. Homology searches of the GenBank data base for the predicted amino acid sequences revealed that, of the eight open reading frames (ORFs) on the 1 strand, and four ORFs on the r strand, only ORF 8 on the 1 strand showed homology with the 34 kDa E-4 protein of human adenovirus types 2, 12 and 34. Northern blot analysis showed that transcription from the E-4 region of PAV-3 began 4 h after infection, peaked at 8 h and declined after 10 h, before DNA replication began 16 h after infection. The E-4 region of PAV-3 was further characterized by 5' and 3' end mapping of the transcription unit.

A gene transfer vector-cell line system for complete functional complementation of adenovirus early regions E1 and E4

The improvements to adenovirus necessary for an optimal gene transfer vector include the removal of virus gene expression in transduced cells, increased transgene capacity, complete replication incompetence, and elimination of replication-competent virus that can be produced during the growth of first-generation adenovirus vectors. To achieve these aims, we have developed a vector-cell line system for complete functional complementation of both adenovirus early region 1 (E1) and E4. A library of cell lines that efficiently complement both E1 and E4 was constructed by transforming 293 cells with an inducible E4-ORF6 expression cassette. These 293-ORF6 cell lines were used to construct and propagate viruses with E1 and E4 deleted. While the construction and propagation of AdRSV beta gal.11 (an E1-/E4- vector engineered to contain a deletion of the entire E4 coding region) were possible in 293-ORF6 cells, the yield of purified virus was depressed approximately 30-fold compared with that of E1- vectors. The debilitation in AdRSV beta gal.11 vector growth was found to correlate with reduced fiber protein and mRNA accumulation. AdCFTR.11A, a modified E1-/E4- vector with a spacer sequence placed between late region 5 and the right inverted terminal repeat, efficiently expressed fiber and grew with the same kinetic profile and virus yield as did E1- vectors. Moreover, purified AdCFTR.11A yields were equivalent to E1- vector levels. Since no overlapping sequences exist in the E4 regions of E1-/E4- vectors and 293-ORF6 cell lines, replication-competent virus cannot be generated by homologous recombination. In addition, these second-generation E1-/E4- vectors have increased transgene capacity and have been rendered virus replication incompetent outside of the new complementing cell lines.

Adenovirus replication is coupled with the dynamic properties of the PML nuclear structure

Wild-type PML and at least four other novel proteins are localized within discrete nuclear structures known as PODs. We demonstrate here that during adenovirus infection, immediate early viral proteins from the E1 and E4 transcription units associate with the POD, which in turn undergoes a dramatic morphological change. During this process, the auto-antigen Sp-100 and NDP55 but not PML, relocate from the POD to the viral inclusion bodies, the sites of adenovirus DNA replication and late RNA transcription. The E4-ORF3 11-kD protein alone will induce this reorganization and reciprocally, viruses carrying mutations in the E4-domain fail to do so. These same viral mutants are defective in viral replication as well as the accumulation of late viral mRNAs and host cell transcription shutoff. We show that interferon (INF) treatment enhances the expression of PML, reduces or blocks PODs reorganization, and inhibits BrdU incorporation into viral inclusion bodies. In addition, cell lines engineered to overexpress PML prevent PODs from viral-induced reorganization and block or severely delay adenovirus replication. These results suggest that viral replication relies on components of the POD and that the structure is a target of early viral proteins.

Efficient dual transcomplementation of adenovirus E1 and E4 regions from a 293-derived cell line expressing a minimal E4 functional unit

Transgene expression after the administration of recombinant adenovirus with E1 deleted is constantly transient. It is admitted that E1A-substituting activities of cellular or viral origin allow viral antigen synthesis and trigger cytotoxic lymphocyte-mediated clearance of the recipient cells. Our approach to solving this problem relies on the additional deletion of the E4 region from the vector backbone as this region upregulates viral gene expression at both transcriptional and posttranscriptional levels. As a prerequisite to the construction of E1 E4 doubly defective adenoviruses, we investigated the possibility of transcomplementing both functions within a single cell. In particular, the distal ORF6+ORF7 segment from the E4 locus of adenovirus type 5 was cloned under the control of the dexamethasone-inducible mouse mammary tumor virus long terminal repeat. Following transfection into 293 cells, clone IGRP2 was retained and characterized as it can rescue the growth defect of all E1+ E4- adenoviral deletants tested. DNA and RNA analysis experiments verified that the mouse mammary tumor virus promoter drives the expression of the ORF6+ORF7 unit and permits its bona fide alternative splicing, generating ORF6/7 mRNA in addition to the ORF6-expressing primary transcript. Importantly, IGRP2 cells sustain cell confluence for a period longer than that of 293 parental cells and allow the plaque purification of E1- or E4- defective viruses. The dual expression of E1 and E4 regulatory genes within IGRP2 cells is demonstrated by the construction, plaque purification, and helper-free propagation of recombinant lacZ-encoding doubly defective adenoviruses harboring different E4 deletions. In addition, the emergence, if any, of replicative particles during viral propagation in this novel packaging cell line will be drastically impaired as only a limited segment of E4 has been integrated.

Development of cell lines capable of complementing E1, E4, and protein IX defective adenovirus type 5 mutants

The cloning capacity of currently available E1- and E3-deleted adenovirus (Ad) vectors does not exceed 8 kb. To increase capacity and improve vector safety further, we have explored the possibility that Early Region 4 (E4) and the gene encoding protein IX (pIX) might also be deleted. To generate cell lines expressing sufficient levels of E4 and pIX proteins in trans in addition to E1-encoded proteins to complement mutations in these genes, we transformed 293 cells with constructs containing the E4 transcription unit and pIX coding sequences under the control of inducible mouse mammary tumor virus (MMTV) and metallothionein promoters, respectively. We obtained two lines, VK2-20 and VK10-9, that express both E4 and pIX proteins as well as E1. The lines could be efficiently transfected with DNA, and allowed the rescue and propagation of an adenovirus; recombinant, Ad5dlE3,4, containing a 2.7-kb E3 deletion and a 2.8-kb E4 deletion in addition to an insertion of plasmid DNA sequences in E1A. Because the E4 sequences within VK2-20 and VK10-9 cells do not overlap with the DNA sequence of Ad5dlE3,E4, the probability of regeneration of the wild-type E4 during virus propagation should be very low. Using the cell lines described in this study, it should be possible to generate Ad vectors lacking E1, pIX, E3, and E4. This would not only increase capacity over that of currently available vectors (to approximately 11 kb) but would also result in more severely attenuated vectors than those with deletions only of E1 or of E1 and E3 and, hence, safer for use in gene therapy protocols.

Human adenovirus encodes two proteins which have opposite effects on accumulation of alternatively spliced mRNAs

All mRNAs expressed from the adenovirus major late transcription unit have a common, 201-nucleotide-long 5' leader sequence, which consists of three short exons (the tripartite leader). This leader has two variants, either with or without the i-leader exon, which, when present, is spliced between the second and the third exons of the tripartite leader. Previous studies have shown that adenovirus early region 4 (E4) encodes two proteins, E4 open reading frame 3 (E4-ORF3) and E4-ORF6, which are required for efficient expression of mRNAs from the major late transcription unit. These two E4 proteins appear to have redundant activities, and expression of one has been shown to be sufficient for efficient major late mRNA accumulation during a lytic virus infection. In this report, we provide evidence that E4-ORF3 and E4-ORF6 both regulate major late mRNA accumulation by stimulating constitutive splicing. Moreover, we show that the two proteins have different effects on accumulation of alternatively spliced tripartite leader exons. In a DNA transfection assay, E4-ORF3 was shown to facilitate i-leader exon inclusion, while E4-ORF6 preferentially favored i-leader exon skipping. In addition, E4-ORF3 and E4-ORF6 had the same effects on accumulation of alternatively spliced chimeric beta-globin transcripts. This finding suggests that the activities of the two proteins may be of more general relevance and not restricted to splicing of major late tripartite leader-containing pre-mRNAs. Interestingly, E4-ORF6 expression was also shown to stimulate i-leader exon skipping during a lytic virus infection.

Human adenovirus encodes two proteins which have opposite effects on accumulation of alternatively spliced mRNAs

All mRNAs expressed from the adenovirus major late transcription unit have a common, 201-nucleotide-long 5' leader sequence, which consists of three short exons (the tripartite leader). This leader has two variants, either with or without the i-leader exon, which, when present, is spliced between the second and the third exons of the tripartite leader. Previous studies have shown that adenovirus early region 4 (E4) encodes two proteins, E4 open reading frame 3 (E4-ORF3) and E4-ORF6, which are required for efficient expression of mRNAs from the major late transcription unit. These two E4 proteins appear to have redundant activities, and expression of one has been shown to be sufficient for efficient major late mRNA accumulation during a lytic virus infection. In this report, we provide evidence that E4-ORF3 and E4-ORF6 both regulate major late mRNA accumulation by stimulating constitutive splicing. Moreover, we show that the two proteins have different effects on accumulation of alternatively spliced tripartite leader exons. In a DNA transfection assay, E4-ORF3 was shown to facilitate i-leader exon inclusion, while E4-ORF6 preferentially favored i-leader exon skipping. In addition, E4-ORF3 and E4-ORF6 had the same effects on accumulation of alternatively spliced chimeric beta-globin transcripts. This finding suggests that the activities of the two proteins may be of more general relevance and not restricted to splicing of major late tripartite leader-containing pre-mRNAs. Interestingly, E4-ORF6 expression was also shown to stimulate i-leader exon skipping during a lytic virus infection.

Activation of RNA polymerase III transcription of human Alu repetitive elements by adenovirus type 5: requirement for the E1b 58-kilodalton protein and the products of E4 open reading frames 3 and 6

We found that transcription of endogenous human Alu elements by RNA polymerase III was strongly stimulated following infection of HeLa cells with adenovirus type 5, leading to the accumulation of high levels of Alu transcripts initiated from Alu polymerase III promoters. In contrast to previously reported cases of adenovirus-induced activation of polymerase III transcription, induction required the E1b 58-kDa protein and the products of E4 open reading frames 3 and 6 in addition to the 289-residue E1a protein. In addition, E1a function was not required at high multiplicities of infection, suggesting that E1a plays an indirect role in Alu activation. These results suggest previously unsuspected regulatory properties of the adenovirus E1b and E4 gene products and provide a novel approach to the study of the biology of the most abundant class of dispersed repetitive DNA in the human genome.

Regulated splicing of adenovirus type 5 E4 transcripts and regulated cytoplasmic accumulation of E4 mRNA

The E4 gene of human type C adenoviruses has been shown previously to give rise to an array of mRNAs via differential splicing. In this study, the pattern of expression of these mRNAs during lytic infection was examined, and two distinct temporal classes were defined. mRNAs of the early class were distinguished from those of the late class by the presence, in the early class, of a sequence in the 3' half of the mRNA that was removed as an intron in the late class. A single mRNA of the late class was found to show a strong dependence on the presence of the 55-kDa protein from region E1b and the open reading frame 6 protein from region E4 for its normal cytoplasmic accumulation. One feature of this mRNA that distinguishes it from other E4 mRNAs expressed at late times is the retention within it of an intron from the 5' half of E4; it may therefore be recognized as incompletely spliced by the host cell and retained in the nucleus. It is proposed that the E1b 55-kDa/E4 open reading frame 6 protein complex facilitates accumulation of this mRNA by overcoming this retention mechanism.

Activation of RNA polymerase III transcription of human Alu repetitive elements by adenovirus type 5: requirement for the E1b 58-kilodalton protein and the products of E4 open reading frames 3 and 6

We found that transcription of endogenous human Alu elements by RNA polymerase III was strongly stimulated following infection of HeLa cells with adenovirus type 5, leading to the accumulation of high levels of Alu transcripts initiated from Alu polymerase III promoters. In contrast to previously reported cases of adenovirus-induced activation of polymerase III transcription, induction required the E1b 58-kDa protein and the products of E4 open reading frames 3 and 6 in addition to the 289-residue E1a protein. In addition, E1a function was not required at high multiplicities of infection, suggesting that E1a plays an indirect role in Alu activation. These results suggest previously unsuspected regulatory properties of the adenovirus E1b and E4 gene products and provide a novel approach to the study of the biology of the most abundant class of dispersed repetitive DNA in the human genome.

Transcription mapping of mouse adenovirus type 1 early region 4

Early region 4 (E4) of mouse adenovirus type 1 was analyzed by Northern blotting, cDNA sequencing, and S1 nuclease protection and primer extension assays. The transcription map of this region was dissimilar to the consensus human adenovirus E4 transcription map in which all transcripts have identical 5' and 3'-terminal sequences. Seven classes of mouse adenovirus type 1 mRNAs were identified; all shared the same 3' end. Three classes of unspliced mRNAs differed at their 5' start sites, two classes of spliced transcripts differed in the locations of their splice acceptors, and two classes of spliced messages differed in their splice donors and acceptors. From the structure of the various transcripts, translational products were predicted. In addition to a predicted polypeptide with similarity to the human adenovirus 2 E4 34K protein previously identified (A. O. Ball, C. W. Beard, P. Villegas, and K. R. Spindler, 1991, Virology 180, 257-265), two open reading frames with similarity to human adenovirus 2 E4 open reading frames 2 and 3 were found.

Defective synthesis of early region 4 mRNAs during abortive adenovirus infections in monkey cells

Human adenovirus 2 grows poorly in monkey cells, partly because of defects in late gene expression. Since deletions in early region 4 (E4) cause similar defects in late gene expression, we examined E4 mRNA expression in abortive infections. Processing of E4 mRNAs was defective during abortive infections, most likely at the level of splicing. At early times in productive infections in HeLa cells, the major E4 species produced is a 2-kb mRNA; at late times, a shift occurs so that smaller spliced E4 mRNAs are also produced. In CV-1 cells, a nonpermissive monkey cell line, this shift did not take place and only the 2-kb species was produced at late times, suggesting a defect in E4 mRNA splicing during abortive infections. The adenovirus DNA-binding protein (DBP) was required for normal processing of E4 mRNAs, since a host range mutant (hr602) containing an altered DBP gene showed a normal late E4 mRNA pattern in CV-1 cells; in addition, DBP was required during infections in HeLa cells for late E4 mRNA expression. DBP was not required for production of the late E4 pattern in transient expression assays in HeLa or 293 cells, suggesting that a second factor in addition to the DBP, present during infection but not transfection, modulates E4 mRNA processing.

Enteric adenovirus type 40: complementation of the E4 defect in Ad2 dl808

The enteric adenovirus type 40 cannot be passaged in HeLa cells, but will grow productively in cells that express the E1B region of adenovirus types 2 or 5. Even in such permissive cells, the lytic cycle is prolonged, there is an abnormal pattern of E1B early gene expression and a failure to switch off host cell functions, suggesting that other gene functions might be impaired in Ad40. For Ad2, E4 ORF 6 and ORF 3 proteins are known to have an essential role in progressing from the early to the late phase of lytic infection and the shutoff of host functions requires an interaction between the E4 ORF 6 34K protein and the E1B 55K protein. To test whether E4 functions of Ad40 are impaired, complementation tests have been made between Ad40 and the E4 deletion mutant Ad2 dl808, which lacks all but ORF 1 of the E4 region. In HeLa and Vero cells, Ad40 complements dl808 to levels equivalent to an Ad2 wild-type infection, as demonstrated by measuring virion packaged DNA, virus titration, and viral protein synthesis. Surprisingly, Ad2 dl808 fails to reciprocally complement Ad40. The results show that Ad40 produces functional E4 ORF 6 and/or ORF 3 activity, and that their expression precedes DNA replication.

Differential regulation of adenovirus late transcriptional units by the products of early region

Normal accumulation of adenoviral late mRNAs derived from the major late promoter, is mediated independently by either the ORF 6 or the ORF 3 product of E4. We have examined the role of E4 products in the expression of three late messages that are derived from outside the major late transcriptional unit: polypeptide IX mRNA, IVa2 mRNA, and messages from the E2 late promoter. We conclude that the late RNA accumulation phenotypes of E4 mutants can be broken down into two components. The effect on nuclear RNA accumulation, which is mediated independently by either the ORF 6 or ORF 3 products, is targeted to late messages derived from the major late transcriptional unit. A second small effect, that is mediated only by ORF 6, is needed for optimal accumulation of mRNA in the cytoplasm, and affects major late promoter derived messages as well as the message for polypeptide IX. The levels of IVa2 and E2 late messages are not reduced in cells infected with E4 negative mutants.

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).

Early region 4 sequence and biological comparison of two isolates of mouse adenovirus type 1

The DNA sequence of 88-100 map units of mouse adenovirus type 1 (MAV-1) was determined. One translational open reading frame showed 48% sequence similarity to a human adenovirus type 2 early region 4 protein. Based on the protein similarity, genome location, and transcriptional polarity, we concluded that this region of MAV-1 corresponds to early region 4. A 241-bp sequence consisting of 10 imperfect direct repeats with sequence similarity to minisatellite DNA was found in this region. Two virus isolates with different passage histories were examined and were found to have a sequence polymorphism within this region. The two viruses were compared for growth in cell culture and mice and small quantitative differences were observed only in vivo.

The adenovirus E4 17-kilodalton protein complexes with the cellular transcription factor E2F, altering its DNA-binding properties and stimulating E1A-independent accumulation of E2 mRNA

E2F is a cellular DNA-binding factor. Its binding activity is changed within adenovirus-infected cells so that it binds cooperatively to pairs of properly spaced and oriented E2F recognition sites. In the work described in this report, the conversion to cooperative binding was shown to require the adenovirus E4 17-kilodalton (kDa) polypeptide. Mutant viruses carrying alterations within the E4 17-kDa coding region failed to generate the infection-specific, cooperatively binding form of E2F. It was possible to alter E2F from uninfected cells so that it bound cooperatively by incubation with a partially purified fraction obtained from infected cells. The E4 17-kDa protein copurified with this activity and was also found to be present in a complex containing E2F. Consistent with its ability to alter the binding of E2F to its recognition sites within the E2 promoter, the E4 17-kDa polypeptide contributed to maximal expression of E2 mRNAs in some cell types. Its ability to enhance E2 transcription did not require expression of the E1A transactivator protein. These results are consistent with a model which proposes that the E4 17-kDa polypeptide binds to the cellular E2F factor, altering its binding behavior and thereby enhancing its ability to stimulate transcription.

Inhibitory effect of interferon-gamma on adenovirus replication and late transcription

We have previously shown that human interferon-gamma inhibited adenovirus multiplication in vitro in a dose-dependent fashion. This action was previous to capsid proteins synthesis and did not involve virus adsorption nor penetration. In this report we have analysed viral mRNA levels at early (7 hr post infection (p.i.)) or late (20 hr p.i.) times, as well as DNA replication in Wish cells pretreated with interferon-gamma and infected with adenovirus 5. Controls included untreated cells as well as cells treated with interferon-alpha, to which adenovirus are reported to be resistant. Transcription of adenovirus regions E1, E4, L1 and L2 has been analysed by Northern blot. Adenovirus DNA replication was determined by DNA-DNA hybridization with total adenovirus 2 DNA. We have also searched for adenovirus E1A proteins by immunoblot with a specific monoclonal antibody. Although pretreatment of cells with either interferon-alpha or interferon-gamma resulted in reduced amounts of E1 and E4 mRNA in the early phase of infection (7 hr p.i.), the near complete inhibition of viral DNA and late transcription was only achieved by interferon-gamma. Immunoblot has shown the absence of the 48-kD E1A protein in cells pretreated with interferon-gamma. The lack of this regulatory adenovirus protein may be involved in the inhibitory mechanism of interferon-gamma on adenovirus.

Adenovirus early region 4 encodes two gene products with redundant effects in lytic infection

In order to assign specific functions to individual gene products encoded by adenovirus type 5 early region 4 (E4), we have constructed and analyzed a set of mutant viruses that express individual E4 open reading frames or combinations of open reading frames. The results of these analyses demonstrate that the gene products of E4 open reading frames 3 and 6 have redundant effects in viral lytic infection. These E4 products independently augment viral DNA replication, viral late protein synthesis, the shutoff of host cell protein synthesis, and the production of infectious virus. The product of open reading frame 6 is more efficient in the regulation of these processes than is the product of open reading frame 3. The regulation of viral DNA replication and the control of viral and cellular protein synthesis appear to be separable functions associated with both E4 gene products. The role of early region 4 in adeno-associated virus helper function, however, is mediated only by the product of open reading frame 6. Finally, we demonstrate that E4 mutant viruses display a multiplicity-leakiness phenotype which is consistent with the regulatory role that this region plays in viral infection.

Cloning of the herpes simplex virus ICP4 gene in an adenovirus vector: effects on adenovirus gene expression and replication

To assess the ability of the herpes simplex virus ICP4 protein to complement adenovirus E1a mutants we have constructed an adenovirus type 5 vector containing a temperature-sensitive ICP4 gene, under control of its own promoter, within the E1 region of the genome. The recombinant virus expresses ICP4 in cells which are permissive (293) or nonpermissive (KB and R970-5) for viral replication, and at levels which approximate those obtained in herpes simplex infection. The adenovirus-encoded protein is functional in that it complements an ICP4 deletion mutant of herpes simplex virus; however, it is incapable of complementing adenovirus E1a mutants for viral growth or DNA replication. At the level of activation of gene expression, ICP4 stimulates the expression of the adenovirus E2a gene but not that of other early genes. Our results indicate that ICP4 does not possess all of the functions of the E1a proteins and, furthermore, that adenovirus early genes differ in their susceptibility to heterologous trans-activators.

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.

Adenovirus early region 4 is required for efficient virus particle assembly

H2dl807, a defective deletion mutant of human adenovirus type 2 lacking parts of early regions 3 and 4 and all of late region 5, was severely defective for virus particle assembly on HeLa cells, producing about 1% of the normal yield of particles. On Vero cells, H2dl807 produced only 5% as many particles as wild type, while on W162 cells, a Vero cell derivative which supports the growth of early region 4 mutants, H2dl807 produced nearly 40% of the wild-type level of particles. Two other defective deletion mutants, H2dl802 and H5dl1021, which lack parts of early region 3 and which are incapable of making fiber, the product of late region 5, were wild type for virus assembly. These data suggest that the cause of the assembly defect of H2dl807 is the lack of a diffusible early region 4 product. H2dl807-infected Vero cells accumulated nearly wild-type amounts of viral late proteins in the nucleus and cytoplasm. Thus, the defect of the mutant in assembly on Vero cells is not due to a general lack of late proteins. Finally, the fact that H2dl802 and H5dl1021 make wild-type amounts of virus particles suggests that fiber is not essential for adenovirus assembly.

Analysis of adenovirus early region 4-encoded polypeptides synthesized in productively infected cells

Peptide-specific antisera were developed to analyze the products encoded by adenovirus type 5 early region 4 (E4) open reading frames 6 and 7. Reading frame 6 previously was shown to encode a 34-kilodalton polypeptide (34K polypeptide) that forms a complex with the early region 1B (E1B)-55K antigen and is required for efficient viral growth in lytic infection. Antisera that were generated recognized the E4-34K protein as well as a family of related polypeptides generated by the fusion of open reading frames 6 and 7. These polypeptides shared amino-terminal sequences with the 34K protein. Short-pulse analysis suggested that the heterogeneity observed with the 6/7 fusion products resulted from differential splicing patterns of related E4 mRNAs. An antiserum directed against the amino terminus of reading frame 6 recognized only the free form of the 34K antigen that was not associated with the E1B-55K protein. This observation allowed the determination of the stability of the free and complexed form of this polypeptide. Pulse-chase analyses demonstrated that both forms of the 34K protein had half-lives greater than 24 h, suggesting that complex formation did not result in stabilization of this gene product. The half-lives of the 6/7 fusion products were approximately 4 h. The 34K protein also was shown to have a nuclear localization within infected cells. Finally, analysis of a mutant carrying deletions in both the E4-34K and E1B-55K polypeptides indicated that the complex formed between these two proteins was a functional unit in lytic infection.

Functional analysis of the adenovirus type 5 DNA-binding protein: site-directed mutants which are defective for adeno-associated virus helper activity

We generated four point mutations in the DNA-binding protein (DBP) gene of adenovirus type 5 by oligonucleotide-directed site-specific mutagenesis. The sites mutated were in the three conserved regions (CR; amino acids 178-186 [CR1], 322-330 [CR2], and 464-475 [CR3]) identified previously by comparative sequence analysis (G. R. Kitchingman, Virology 146:90-101, 1985). The mutations resulted in changes in amino acids 181 (Trp to Leu), 323 (Arg to Leu), 324 (Trp to Leu), and 469 (Phe to Ile). The mutated DBP genes were put under the control of the simian virus 40 early promoter and analyzed by transfection for their ability to help adeno-associated virus replicate its DNA in COS-1 monkey cells. Mutations in the aromatic amino acids 324 and 469 reduced the amount of AAV DNA replication approximately 10-fold, while the mutation in Arg 323 produced a reduction of approximately fourfold. The Trp-to-Leu mutation in amino acid 181 had no effect on AAV DNA replication. The decreased helper activity of the 323, 324, and 469 mutations was not caused by any effect of the mutation on the stability of the DBP. These results suggest that CR2 and CR3 are involved in AAV helper activity, specifically in AAV DNA replication. The relevance of these findings to the identification of residues important for the functions of DBP in adenovirus infection is discussed.

Biosynthesis and properties of the adenovirus 2 L1-encoded 52,000- and 55,000-Mr proteins

The adenovirus type 2 L1 region, which is located at 30.7 to 39.2 map units on the viral genome, is transcribed from the major late promoter during both early and late stages of virus replication, and a 52,000-Mr (52K) protein-55K protein doublet has been translated in vitro on L1-specific RNA. To investigate the biosynthesis and properties of the L1 52K and 55K proteins, we prepared antibody against a synthetic peptide encoded near the predicted N terminus. As determined by immunoprecipitation and immunoblot analysis, the antipeptide antibody recognized major 52K and 55K proteins synthesized in adenovirus type 2-infected cells that appeared to be identical to the 52K-55K doublet translated in vitro. The immunoprecipitated 52K and 55K proteins were very closely related, as shown by a peptide map analysis. Both L1 proteins were phosphorylated, and they were phosphorylated at similar sites. No precursor-product relationship was detected between the 52K and 55K proteins by a pulse-chase analysis. Biosynthesis of the L1 52K and 55K proteins began about 6 to 7 h postinfection, after biosynthesis of the early region 1A and early region 1B 19K (175R) T antigens, and reached a maximum rate at about 15 h; the maximum rate was maintained until at least 25 h postinfection. At all times, the 55K protein appeared to be synthesized at a severalfold-higher level than the 52K protein. Both proteins were quite stable and accumulated until late times after infection. Viral DNA replication was not essential for formation of the L1 proteins. Thus, the L1 52K-55K gene appears to be regulated in a manner different from the classical early and late viral genes but similar to the protein encoded by the i-leader (Symington et al., J. Virol. 57:849-856, 1986). The L1 proteins were detected in the cell nucleus by immunofluorescence microscopy with antipeptide antibody and were found to be primarily associated with the nuclear membrane by an immunoblot analysis of subcellular fractions.

Adenoviral early region 4 is required for efficient viral DNA replication and for late gene expression

H2dl808 is a defective deletion mutant of human adenovirus 2 lacking most of transcriptional early region 4. Although the mutant can be grown in the complementing cell line W162, it is defective in human cell lines normally used to propagate adenovirus. In such nonpermissive cells, H2dl808 exhibits a severe defect in late gene expression, accumulating very small amounts of viral late messages and producing correspondingly small amounts of viral late proteins. H2dl808 also exhibits a defect in viral DNA synthesis: 24 h after infection, H2dl808-infected nonpermissive cells contain five- to sevenfold less viral DNA than those cells infected with wild-type adenovirus. H2dl808-infected nonpermissive cells eventually accumulate a significant amount of viral DNA. However, the rate of synthesis of viral proteins late in mutant infection remains much lower than that observed in wild-type infection at a time when DNA accumulation is comparable. Thus, the mutant's late protein synthesis defect is probably not due solely to its reduced accumulation of viral DNA. Finally, H2dl808 is much less efficient than wild-type virus in the inhibition of host cell protein synthesis in infections of nonpermissive cells. These observations imply roles for early region 4 products in several aspects of the viral growth cycle, including DNA replication, late gene expression, and host cell shutoff.

Adenovirus early region 4 encodes functions required for efficient DNA replication, late gene expression, and host cell shutoff

To delineate the function of adenovirus early region 4 (E4) gene products, we constructed a set of mutant viruses which carry defined lesions within this coding region. Deletion and insertion mutations within six of seven known E4 coding regions had no measurable effect on virus growth in cultured cells. A variant carrying a deletion within the last coding region (encoding a 34,000-molecular-weight polypeptide) was modestly defective, and a mutant lacking the majority of the E4 region was severely defective for growth. The phenotypes of the two defective mutants are similar and complex. Both display perturbations in DNA replication, translation of the E2A mRNA, accumulation of late viral mRNAs, and host cell shutoff.

Organization and expression of the E4 region of adenovirus 2

The E4 region of Adenovirus 2 is a leftward transcribed part of the viral genome. Its nucleotide sequence has already been analysed. From this sequence several open reading frames were defined, which could be used in the coding of the E4 proteins. Using S1 digestion of mRNA-DNA hybrids a precise mapping of donor and acceptor sites was done. Their use in various combinations allows the synthesis of mRNAs, able to direct the synthesis of at least 7 polypeptides, ranging in size from 9K to 34K. Comparison of the sequences of the different acceptor sites indicates that they all conform to the consensus sequence. Analysis of the ATG surrounding sequence shows that initiator ATG may be positively selected according to Kozak's rule.

mRNAs from human adenovirus 2 early region 4

The molecular structure of the mRNAs from early region 4 of human adenovirus 2 has been studied by Northern blot analysis, S1 nuclease analysis, and sequence analysis of cDNA clones. The results make it possible to identify four different splice donor sites and six different splice acceptor sites. The structure of 12 different mRNAs can be deduced from the analysis. The mRNAs have identical 5' and 3' ends and are thus likely to be processed from a common mRNA precursor by differential splicing. The different mRNA species are formed by the removal of one to three introns, and they all carry a short 5' leader segment. The introns appear to serve two functions; they either place a 5' leader segment in juxtaposition with an open reading frame or fuse two open translational reading frames. The early region 4 mRNAs can encode at least seven unique polypeptides.