Regulation of adenovirus mRNA formation

Understanding Post Entry Sorting of Adenovirus Capsids; A Chance to Change Vaccine Vector Properties

Adenovirus vector-based genetic vaccines have emerged as a powerful strategy against the SARS-CoV-2 health crisis. This success is not unexpected because adenoviruses combine many desirable features of a genetic vaccine. They are highly immunogenic and have a low and well characterized pathogenic profile paired with technological approachability. Ongoing efforts to improve adenovirus-vaccine vectors include the use of rare serotypes and non-human adenoviruses. In this review, we focus on the viral capsid and how the choice of genotypes influences the uptake and subsequent subcellular sorting. We describe how understanding capsid properties, such as stability during the entry process, can change the fate of the entering particles and how this translates into differences in immunity outcomes. We discuss in detail how mutating the membrane lytic capsid protein VI affects species C viruses' post-entry sorting and briefly discuss if such approaches could have a wider implication in vaccine and/or vector development.

Imaging the adenovirus infection cycle

Incoming adenoviruses seize control of cytosolic transport mechanisms to relocate their genome from the cell periphery to specialized sites in the nucleoplasm. The nucleus is the site for viral gene expression, genome replication, and the production of progeny for the next round of infection. By taking control of the cell, adenoviruses also suppress cell-autonomous immunity responses. To succeed in their production cycle, adenoviruses rely on well-coordinated steps, facilitated by interactions between viral proteins and cellular factors. Interactions between virus and host can impose remarkable morphological changes in the infected cell. Imaging adenoviruses has tremendously influenced how we delineate individual steps in the viral life cycle, because it allowed the development of specific optical markers to label these morphological changes in space and time. As technology advances, innovative imaging techniques and novel tools for specimen labeling keep uncovering previously unseen facets of adenovirus biology emphasizing why imaging adenoviruses is as attractive today as it was in the past. This review will summarize past achievements and present developments in adenovirus imaging centered on fluorescence microscopy approaches.

Inhibition of adenovirus multiplication by short interfering RNAs directly or indirectly targeting the viral DNA replication machinery

Human adenoviruses are a common threat to immunocompromised patients, e.g., HIV-positive individuals or solid-organ and, in particular, allogeneic stem cell transplant recipients. Antiviral drugs have a limited effect on adenoviruses, and existing treatment modalities often fail to prevent fatal outcome. Silencing of viral genes by short interfering RNAs (siRNAs) holds a great promise in the treatment of viral infections. The aim of the present study was to identify adenoviral candidate targets for RNA interference-mediated inhibition of adenoviral replication. We investigated the impact of silencing of a set of early, middle, and late viral genes on the replication of adenovirus 5 in vitro. Adenovirus replication was inhibited by siRNAs directed against the adenoviral E1A, DNA polymerase, preterminal protein (pTP), IVa2, hexon, and protease genes. Silencing of early and middle genes was more effective in inhibiting adenovirus multiplication than was silencing of late genes. A siRNA directed against the viral DNA polymerase mRNA decreased viral genome copy numbers and infectious virus progeny by several orders of magnitude. Since silencing of any of the early genes directly or indirectly affected viral DNA synthesis, our data suggest that reducing viral genome copy numbers is a more promising strategy for the treatment of adenoviral infections than is reducing the numbers of proteins necessary for capsid generation. Thus, adenoviral DNA replication was identified as a key target for RNAi-mediated inhibition of adenovirus multiplication. In addition, the E1A transcripts emerged as a second important target, because its knockdown markedly improved the viability of cells at late stages of infection.

Transcription releases protein VII from adenovirus chromatin

Adenovirus protein VII is the major protein component of the viral nucleoprotein core. It is a nonspecific DNA-binding protein that condenses viral DNA inside the capsid. Protein VII remains associated with viral chromatin throughout early phase, indicating its continuing role during infection. Here we characterize the release of protein VII from infectious genomes during a time period that corresponds to the late phase of infection. Interestingly, the early viral transactivator E1A, but not other early gene products, is responsible for releasing protein VII by a mechanism that requires ongoing transcription but not viral DNA replication. Moreover transcription per se, in the absence of E1A, is also sufficient to trigger release. Accordingly, a recombinant genome containing only non-coding "stuffer" DNA is unable to support release of protein VII. Our data support a model in which early gene transcription results in a change in the structure of the viral chromatin.

The adenovirus L4 33-kilodalton protein binds to intragenic sequences of the major late promoter required for late phase-specific stimulation of transcription

The adenovirus late IVa2 protein is required for maximally efficient transcription from the viral major late (ML) promoter, and hence, the synthesis of the majority of viral late proteins. This protein is a sequence-specific DNA-binding protein that also promotes the assembly of progeny virus particles. Previous studies have established that a IVa2 protein dimer (DEF-B) binds specifically to an intragenic ML promoter sequence necessary for late phase-specific stimulation of ML transcription. However, activation of transcription from the ML promoter correlates with binding of at least one additional infected-cell-specific protein, termed DEF-A, to the promoter. Using an assay for the DNA-binding activity of DEF-A, we identified the unknown protein by using conventional purification methods, purification of FLAG-tagged IVa2-protein-containing complexes, and transient synthesis of viral late proteins. The results of these experiments established that the viral L4 33-kDa protein is the only component of DEF-A: the IVa2 and L4 33-kDa proteins are necessary and sufficient for formation of all previously described complexes in the intragenic control region of the ML promoter. Furthermore, the L4 33-kDa protein binds to the promoter with the specificity characteristic of DEF-A and stimulates transcription from the ML promoter in transient-expression assays.

Studies on the activity of a bidirectional promoter of Mungbean yellow mosaic India virus by agroinfiltration

The AV promoter expressing AV1 and AV2 genes and AC1 promoter expressing AC1 gene are present in opposite orientation in the intergenic region of Mungbean yellow mosaic India virus (MYMIV). Transient Agrobacterium-mediated delivery of putative promoter constructs into Nicotiana benthamiana and different legumes, followed by reporter gene (beta-d-glucuronidase, GUS) assay, identified the promoter region of both AC1 and AV genes that is necessary for transcriptional initiation. Transcription activator protein-independent activity of AV promoter and differential regulation of AC1 promoter are unique to MYMIV. The AV promoter is a composite core promoter having both TATA box and Initiator elements (TATA(+)Inr(+)). Many transcription factor binding sites were identified in the upstream promoter sequences of both virion and complementary sense genes, which might be used in the transcription regulation studies of the host plant as well as the virus.

Viral DNA synthesis-dependent titration of a cellular repressor activates transcription of the human adenovirus type 2 IVa2 gene

Synthesis of progeny DNA genomes in cells infected by human subgroup C adenoviruses leads to several changes in viral gene expression. These changes include transcription from previously silent, late promoters, such as the IV(a2) promoter, and a large increase in the efficiency of major-late (ML) transcription. Some of these changes appear to take place sequentially, because the product of the IV(a2) gene has been implicated in stimulation of ML transcription. Our previous biochemical studies suggested that IV(a2) transcription is regulated by viral DNA synthesis-dependent relief of transcriptional repression by a cellular protein that we termed IV(a2)-RF. To test the relevance of such a repressor-titration mechanism during the viral infectious cycle, we introduced into the endogenous IV(a2) promoter two mutations that impair in vitro-binding of IV(a2)-RF, but introduce no change (Rep7) or one conservative amino acid substitution (Rep6) into the overlapping coding sequence for the viral DNA polymerase. The results of run-on transcription assays indicated that both mutations induced earlier-than-normal and more efficient IV(a2) transcription. Both mutations were also observed to result in modest increases in the efficiency of viral DNA synthesis. However, measurement of the concentration of IV(a2) transcripts as a function of IV(a2) template concentration demonstrated that the Rep mutations increased by up to 60-fold the efficiency with which IV(a2) templates were used during the initial period of the late phase of infection, as predicted by the repressor titration hypothesis. These mutations also increased the efficiency of ML transcription in infected cells.

In vivo recruitment of exon junction complex proteins to transcription sites in mammalian cell nuclei

Studies over the past years indicate that there is extensive coupling between nuclear export of mRNA and pre-mRNA processing. Here, we visualized the distribution of exon junction complex (EJC) proteins and RNA export factors relative to sites of abundant pre-mRNA synthesis in the nucleus. We analyzed both HeLa cells infected with adenovirus and murine erythroleukemia (MEL) cells stably transfected with the human beta-globin gene. Using in situ hybridization and confocal microscopy, we observe accumulation of EJC proteins (REF/Aly, Y14, SRm160, UAP56, RNPS1, and Magoh) and core spliceosome components (U snRNPs) at sites of transcription. This suggests that EJC proteins bind stably to pre-mRNA cotranscriptionally. No concentration of the export factors NXF1/TAP, p15, and Dbp5 was detected on nascent transcripts, arguing that in mammalian cells these proteins bind the mRNA shortly before or after release from the sites of transcription. These results also suggest that binding of EJC proteins to the mRNA is not sufficient to recruit TAP-p15, consistent with recent findings showing that the EJC does not play a crucial role in mRNA export. Contrasting to the results obtained in MEL cells expressing normal human beta-globin transcripts, mutant pre-mRNAs defective in splicing and 3'end processing do not colocalize with SRm160, REF, UAP56, or Sm proteins. This shows that the accumulation of EJC proteins at transcription sites requires efficient processing of the nascent pre-mRNAs, arguing that transcription per se is not sufficient for the stable assembly of the EJC.

DNA synthesis-dependent relief of repression of transcription from the adenovirus type 2 IVa(2) promoter by a cellular protein

The promoter of the human adenovirus type 2 IVa(2) gene, which becomes active only during the late phase of infection, is built largely from sequences spanning, and downstream of, the sites of initiation of transcription. These sequences comprise an initiator, an intragenic sequence necessary for efficient transcription from the promoter by RNA polymerase II, and an intragenic binding site for a cellular repressor of IVa(2) transcription. The properties of the latter protein, which is termed IVa(2)-RF, suggested that it might account for the viral DNA synthesis-dependent activation of IVa(2) transcription during the adenoviral productive cycle. Here we report the results of experiments to assess the contributions of DNA template concentration and IVa(2)-RF binding to the activity of the IVa(2) promoter using a transient expression system. When a IVa(2)-EGFP reporter gene was introduced into HeLa cells, in which IVa(2)-RF was identified, no EFGP synthesis could be detected. In contrast, in IVa(2)-RF-containing cells in which the plasmid carrying the chimeric gene replicated, synthesis of both the EGFP protein and the IVa(2)-EGFP mRNA was readily detected. A vector mutation that blocked plasmid replication reduced IVa(2) promoter activity to undetectable levels. In contrast, a IVa(2) promoter substitution that impaired binding of IVa(2)-RF increased IVa(2) promoter activity under all conditions examined. Furthermore, introduction of DNA containing the IV-RF binding site with the chimeric reporter genes resulted in increased transcription from the IVa(2) promoter in the absence of plasmid replication. These properties are consistent with the hypothesis that the relative concentration of the IVa(2) promoter and of the cellular repressor that binds to it governs transcription from this adenoviral promoter.

Adenovirus early region 4 promotes the localization of splicing factors and viral RNA in late-phase interchromatin granule clusters

Adenovirus early region 4 (E4) mutants are defective for late gene expression and show reduced levels of late RNA in both the cytoplasm and the nucleus. These reductions reflect a posttranscriptional defect in the production of viral late RNA. We find that E4 mutants form replication centers during the initial stages of infection and are able to redistribute splicing factors to transcription sites that surround viral replication centers. However, E4 mutant infected cultures have reduced numbers of cells with splicing factors localized in enlarged interchromatin granule clusters during the late phase. Although the late-phase interchromatin granule clusters that formed in wild-type and E4 mutant infected cells had similar levels of poly(A) RNA, they contained reduced levels of viral RNA. These results suggest that E4 mutants do not efficiently accumulate viral late RNA in late-phase interchromatin granule clusters following the onset of late RNA transcription.

Unusual properties of adenovirus E2E transcription by RNA polymerase III

In adenovirus type 5-infected cells, RNA polymerase III transcription of a gene superimposed on the 5' end of the E2E RNA polymerase II transcription unit produces two small (<100-nucleotide) RNAs that accumulate to low steady-state concentrations (W. Huang, R. Pruzan, and S. J. Flint, Proc. Natl. Acad. Sci. USA 91:1265-1269, 1984). To gain a better understanding of the function of this RNA polymerase III transcription, we have examined the properties of the small E2E RNAs and E2E RNA polymerase III transcription in more detail. The accumulation of cytoplasmic E2E RNAs and the rates of E2E transcription by the two RNA polymerases during the infectious cycle were analyzed by using RNase T(1) protection and run-on transcription assays, respectively. Although the RNA polymerase III transcripts were present at significantly lower concentrations than E2E mRNA throughout the period examined, E2E transcription by RNA polymerase III was found to be at least as efficient as that by RNA polymerase II. The short half-lifes of the small E2E RNAs estimated by using the actinomycin D chase method appear to account for their limited accumulation. The transcription of E2E sequences by RNA polymerase II and III in cells infected by recombinant adenoviruses carrying ectopic E2E-CAT (chloramphenicol transferase) reporter genes with mutations in E2E promoter sequences was also examined. The results of these experiments indicate that recognition of the E2E promoter by the RNA polymerase II transcriptional machinery in infected cells limits transcription by RNA polymerase III, and vice versa. Such transcriptional competition and the properties of E2E RNAs made by RNA polymerase III suggest that the function of this viral RNA polymerase III transcription unit is unusual.

Effects of mutations in the adenoviral E1B 55-kilodalton protein coding sequence on viral late mRNA metabolism

The human subgroup C adenoviral E1B 55-kDa protein cooperates with the viral E4 Orf6 protein to induce selective export of viral, late mRNAs from the nucleus to the cytoplasm. Previous studies have suggested that such preferential transport of viral mRNA and the concomitant inhibition of export of cellular mRNAs are the result of viral colonization of specialized microenvironments within the nucleus. However, neither the molecular basis of this phenomenon nor the mechanism by which the E1B 55-kDa protein acts has been elucidated. We therefore examined viral late mRNA metabolism in HeLa cells infected with a series of mutant viruses that carry insertions at various positions in the E1B protein coding sequence (P. R. Yew, C. C. Kao, and A. J. Berk, Virology 179:795-805, 1990). All the mutations examined impaired cytoplasmic accumulation of viral L2 mRNAs and reduced L2 mRNA export efficiency. However, in most cases these defects could be ascribed to reduced E1B 55-kDa protein concentration or the unexpected failure of the altered E1B proteins to enter the nucleus efficiently. The latter property, the pleiotropic defects associated with all the mutations that impaired nuclear entry of the E1B protein, and consideration of its primary sequence suggest that these insertions result in misfolding of the protein. Insertion of four amino acids at residue 143 also inhibited viral mRNA export but resulted in increased rather than decreased accumulation of the E1B 55-kDa protein in the nucleus. This mutation specifically impaired the previously described association of the E1B protein with intranuclear structures that correspond to sites of adenoviral DNA replication and transcription (D. Ornelles and T. Shenk, J. Virol. 65:424-439, 1991) and the colocalization of the E1B and E4 Orf6 proteins. As this insertion has been shown to inhibit the interaction of the E1B with the E4 Orf6 protein in infected cell extracts (S. Rubenwolf, H. Schütt, M. Nevels, H. Wolf, and T. Dobner, J. Virol. 71:1115-1123, 1997), these phenotypes provide direct support for the hypothesis that selective viral mRNA export is determined by the functional organization of the infected cell nucleus.

Truncation of the human adenovirus type 5 L4 33-kDa protein: evidence for an essential role of the carboxy-terminus in the viral infectious cycle

The subgroup C human adenovirus L4 33-kDa protein is a nuclear phosphoprotein that plays a direct, but dispensable, role in virion assembly. The r-strand open reading frame (ORF) for this protein lies opposite to the 5' end of the l-strand E2 early (E2E) transcription units. To facilitate studies of regulation of E2E transcription, we wished to construct a mutant virus in which the 33-kDa ORF was truncated to serve as a background into which specific E2E mutations could be introduced without also altering the 33-kDa protein. We constructed viral DNA (vDNA) containing within the 33-kDa ORF two tandem, premature stop codons that should prevent translation of the C-terminal 47 amino acids of the protein (Delta47). We report here the unanticipated lethality of such truncation of the L4 33-kDa protein. Viral DNA harboring the Delta47 mutations did not produce infectious virus when transfected into cultured cells. In contrast, infectious virus was recovered upon transfection of revertant vDNA, indicating that the Delta47 mutations were responsible for the observed phenotype. The Delta47 mutations did not affect E2E transcription or production of the E2 DNA-binding protein. Transfected Delta47 vDNA was replicated and directed the production of early and late viral proteins, including hexon protein in the trimer conformation. However, no virus particles of any kind were produced. We propose that truncation of the adenovirus 33-kDa protein results in a lethal, late block in the infectious cycle during the assembly of progeny virions and discuss the implications of this phenotype for the mechanism of virion assembly.

Superimposed promoter sequences of the adenoviral E2 early RNA polymerase III and RNA polymerase II transcription units

The human adenovirus type 2 E2 early (E2E) transcriptional control region contains an efficient RNA polymerase III promoter, in addition to the well characterized promoter for RNA polymerase II. To determine whether this promoter includes intragenic sequences, we examined the effects of precise substitutions introduced between positions +2 and +62 on E2E transcription in an RNA polymerase III-specific, in vitro system. Two noncontiguous sequences within this region were necessary for efficient or accurate transcription by this enzyme. The sequence and properties of the functional element proximal to the sites of initiation identified it as an A box. Although a B box sequence could not be unambiguously located, substitutions between positions +42 and +62 that severely impaired transcription also inhibited binding of the human general initiation protein TFIIIC. Thus, this region of the RNA polymerase III E2E promoter contains a B box sequence. We also identified previously unrecognized intragenic sequences of the E2E RNA polymerase II promoter. In conjunction with our previous observations, these data establish that RNA polymerase II and RNA polymerase III promoter sequences are superimposed from approximately positions -30 to +20 of the complex E2E transcriptional control region. The alterations in transcription induced by certain mutations suggest that components of the RNA polymerase II and RNA polymerase III transcriptional machines compete for access to overlapping binding sites in the E2E template.

Identification of a cellular repressor of transcription of the adenoviral late IVa(2) gene that is unaltered in activity in infected cells

The gene encoding the adenovirus type 2 IVa(2) protein, a sequence-specific activator of transcription from the viral major late promoter, is itself transcribed only during the late phase of infection. We previously identified a cellular protein (IVa(2)-RF) that binds specifically to an intragenic sequence of the IVa(2) transcription unit. We now report that precise substitutions within the IVa(2)-RF-binding site that decreased binding affinity increased the efficiency of IVa(2) transcription in in vitro reactions containing IVa(2)-RF. Consistent with the conclusion that this cellular protein represses IVa(2) transcription, mutations that led to more efficient transcription in the presence of IVa(2)-RF were without effect in reactions lacking this cellular protein. No change in the concentration or activity of IVa(2)-RF could be detected in adenovirus-infected cells during the period in which the IVa(2) gene is transcribed. We therefore propose that restriction of IVa(2) transcription to the late phase is the result of titration of this cellular repressor as the number of copies of the IVa(2) promoter increases upon replication of the viral genome.

Adenovirus late gene expression does not require a Rev-like nuclear RNA export pathway

Adenovirus late mRNA export is facilitated by viral early proteins of 55 and 34 kDa. The 34-kDa protein contains a leucine-rich nuclear export signal (NES) similar to that of the human immunodeficiency virus Rev protein. It was proposed that the 34-kDa protein might facilitate the export of adenovirus late mRNA through a Rev-like NES-mediated export pathway. We have tested the role of NES-mediated RNA export during adenovirus infection, and we find that it is not essential for the expression of adenovirus late genes.

The tripartite leader sequence of subgroup C adenovirus major late mRNAs can increase the efficiency of mRNA export

The subgroup C human adenoviruses induce selective export of newly synthesized viral mRNA from the nucleus to the cytoplasm, with concomitant inhibition of export of the majority of cellular mRNA species. Such posttranscriptional regulation of viral and cellular gene expression in infected cells requires viral E1B and E4 proteins. To facilitate the investigation of parameters that govern selective export in adenovirus-infected cells, we constructed a marked human beta-actin minigene under the control of the glucocorticoid-inducible enhancer-promoter of mouse mammary tumor virus and introduced it into the left end of the adenovirus type 5 (Ad5) genome. Transcription of this reporter gene (designated MA) as well as of a sibling, which differed only in the inclusion of a cDNA copy of the Ad2 major late tripartite leader sequence upstream of beta-actin sequences (termed MtplA), in recombinant virus-infected cells was strictly dependent on the addition of dexamethasone to the medium. When transcription of the MA gene was induced during the late phase of infection, newly synthesized MA RNA entered the cytoplasm. These transcripts, which contain no viral sequences, therefore reproduce the behavior of exceptional cellular mRNA species observed when transcription of their genes is activated during the late phase of infection (U.-C. Yang, W. Huang, and S. J. Flint, J. Virol. 70:4071-4080, 1996). Unexpectedly, however, higher concentrations of newly synthesized RNA accumulated in the cytoplasm when the tripartite leader sequence was present in the reporter RNA, despite equal rates of transcription of the two reporter genes. Examination of the partitioning of both newly synthesized and steady-state populations of MA and MtplA RNAs between nuclear and cytoplasmic compartments indicated that the tripartite leader sequence did not increase RNA stability in the cytoplasm. Comparison of nuclear and cytoplasmic reporter RNA species by Northern blotting, primer extension, and reverse transcription-PCR provided no evidence for altered processing induced by the tripartite leader sequence. We therefore conclude that the tripartite leader sequence, long known to facilitate the translation of mRNAs during the late phase of adenovirus infection, can also modulate mRNA export from the nucleus.

Characterization of a replication-incompetent adenovirus type 5 mutant deleted for the preterminal protein gene

An adenovirus type 5 mutant deleted for the preterminal protein (pTP) gene was constructed using cell lines that express pTP. The pTP deletion mutant virus is incapable of replicating in the absence of complementation and does not express detectable levels of viral mRNAs that are expressed only after the onset of replication. Accumulation of early-region mRNAs, including that for E1A, exhibits a lag relative to that observed from the wild-type virus. However, E1A mRNA accumulation attains a steady-state level similar to the level of expression during the early phase of infection with the wild-type virus. In 293-pTP cells (human embryonic kidney cells that express pTP in addition to high levels of adenovirus E1A and E1B proteins), the pTP deletion mutant virus replicates efficiently and yields infectious titers within 5-fold of that of the wild-type virus. The deletion of 1.2 kb of pTP-encoding sequence increases the size of foreign DNA that can be introduced into the virus and, with an absolute block to replication, makes this virus an important tool for gene therapy.

Spliced exons of adenovirus late RNAs colocalize with snRNP in a specific nuclear domain

Posttranscriptional steps in the production of mRNA include well characterized polyadenylation and splicing reactions, but it is also necessary to understand how RNA is transported within the nucleus from the site of its transcription to the nuclear pore, where it is translocated to the cytoplasmic compartment. Determining the localization of RNA within the nucleus is an important aspect of understanding RNA production and may provide clues for investigating the trafficking of RNA within the nucleus and the mechanism for its export to the cytoplasm. We have previously shown that late phase adenovirus-infected cells contain large clusters of snRNP and non-snRNP splicing factors; the presence of these structures is correlated with high levels of viral late gene transcription. The snRNP clusters correspond to enlarged interchromatin granules present in late phase infected cells. Here we show that polyadenylated RNA and spliced tripartite leader exons from the viral major late transcription unit are present in these same late phase snRNP-containing structures. We find that the majority of the steady state viral RNA present in the nucleus is spliced at the tripartite leader exons. Tripartite leader exons are efficiently exported from the nucleus at a time when we detect their accumulation in interchromatin granule clusters. Since the enlarged interchromatin granules contain spliced and polyadenylated RNA, we suggest that viral RNA may accumulate in this late phase structure during an intranuclear step in RNA transport.

mRNA export correlates with activation of transcription in human subgroup C adenovirus-infected cells

To investigate the mechanisms by which viral mRNA species are distinguished from their cellular counterparts for export to the cytoplasm during the late phase of subgroup C adenovirus infection, we have examined the metabolism of several cellular and viral mRNAs in human cells productively infected by adenovirus type 5 (Ad5). Several cellular mRNAs that were refractory to, or could escape from, adenovirus-induced inhibition of export of mRNA from the nucleus have been identified. This group includes Hsp70 mRNAs synthesized upon heat shock of Ad5-infected 293 or HeLa cells during the late phase of infection. However, successful export in Ad5-infected cells is not a specific response to heat shock, for beta-tubulin and interferon-inducible mRNAs were also refractory to virus-induced export inhibition. The export of these cellular mRNAs, like that of viral late mRNAs, required the E1B 55-kDa protein. Export to the cytoplasm during the late phase of Ad5 infection of several cellular mRNAs, including members of the Hsp70 family whose export was inhibited under some, but not other, conditions, indicates that viral mRNA species cannot be selectively exported by virtue of specific sequence or structural features. Cellular and viral late mRNAs that can be exported from the nucleus to the cytoplasm were expressed from genes whose transcription was induced or activated during the late phase of Ad5 infection. Consistent with the possibility that successful export is governed by transcriptional activation in the late phase of adenovirus infection, newly synthesized viral early E1A mRNA was subject to export inhibition during the late phase of infection.

Adenoviral E1B-55kDa protein inhibits yeast mRNA export and perturbs nuclear structure

The mechanisms of export of RNA from the nucleus are poorly understood; however, several viral proteins modulate nucleocytoplasmic transport of mRNA. Among these are the adenoviral proteins E1B-55kDa and E4-34kDa. Late in infection, these proteins inhibit export of host transcripts and promote export of viral mRNA. To investigate the mechanism by which these proteins act, we have expressed them in Saccharomyces cerevisiae. Overexpression of either or both proteins has no obvious effect on cell growth. By contrast, overexpression of E1B-55kDa bearing a nuclear localization signal (NLS) dramatically inhibits cell growth. In this situation, the NLS-E1B-55kDa protein is localized to the nuclear periphery, fibrous material is seen in the nucleoplasm, and poly(A)+ RNA accumulates in the nucleus. Simultaneous overexpression of E4-34kDa bearing or lacking an NLS does not modify these effects. We discuss the mechanisms of selective mRNA transport.

The metabolism of small cellular RNA species during productive subgroup C adenovirus infection

During the late phase of subgroup C adenovirus infection, export of cellular mRNA from the nucleus to the cytoplasm is inhibited. In one approach to investigate the mechanism whereby viral late mRNAs are selected for export, we have examined the metabolism of small cellular RNA species transcribed by all three RNA polymerases during the late phase of Ad5 infection. No changes in the quantities of [3H]uridine-labeled 5S rRNA or tRNAs entering the cytoplasm were observed in infected cells. Adenovirus type 5 infection reduced the nuclear and cytoplasmic populations of the newly synthesized, snRNP-associated snRNAs U1, U2, U4, U5, and U6. Transcription of a representative snRNA, U1 RNA, was not inhibited, indicating that the post-transcriptional metabolism of snRNAs was perturbed during the late phase of infection. The increased cytoplasmic concentration of newly synthesized U1 RNA in Ad5- compared to mock-infected cells, and the greater reduction of the snRNP-associated compared to the total U1 RNA population, indicated that snRNP assembly in the cytoplasm was impaired. As adenovirus infection does not perturb export from the nucleus of small cellular mRNAs transcribed by RNA polymerases II and III, viral mRNA must be distinguished for selective export at a nuclear step upstream of translocation to the cytoplasm via nuclear pore complexes.

Dynamic organization of splicing factors in adenovirus-infected cells

Adenovirus infection affects the nuclear distribution of host splicing factors. Late phase-infected cells contain discrete clusters of small nuclear ribonucleoproteins (snRNPs) that are separate from centers containing the viral 72-kilodalton DNA-binding protein (72K protein). In the present study, we demonstrate that these snRNP clusters also contain splicing factors from the SR protein family. We show that a previously described monoclonal antibody, 3C5, detects SR proteins. Furthermore, we demonstrate that late region 3 transcription occurs at a maximal rate in infected cultures in which greater than 90% of the cells contain the snRNP clusters, indicating that such cells are actively transcribing their late genes. During the onset of the late phase, the intranuclear distribution of splicing factors is very different from that seen after the late phase is established. When late viral transcription commences, cells with snRNP clusters are less prevalent than in cultures that are maintaining maximum levels of late transcription. Instead, a cell type which shows snRNPs, concentrated in foci that also contain the viral 72K DNA-binding protein is detected. This cell type disappears from cultures by 18 to 20 h after a high-multiplicity infection. These results suggest a dynamic organization of splicing factors in infected cells that can be correlated to the status of viral gene expression. Our work also provides an explanation for the differing results that have been published concerning the organization of splicing factors in the adenovirus-infected cell nucleus (L. F. Jiménez-García and D. L. Spector, Cell 73:47-59, 1993). During the present study we observed that a monoclonal antibody against the SC-35 protein, which was used by Jiménez-García and Spector to study the localization of the SC-35 splicing factor in adenovirus-infected cells, cross-reacts with the adenovirus 72K DNA-binding protein and is thus unsuitable for this type of study.

Intragenic activating and repressing elements control transcription from the adenovirus IVa2 initiator

The downstream stimulatory segment of the adenovirus type 2 IVa promoter includes a TA-rich sequence that binds recombinant TATA-binding proteins (TBP) in vitro. We now demonstrate that when placed upstream of the IVa2, initiator, this TA-rich sequence operated as a TATA element but exhibited significantly lower transcriptional and TBP-binding activities than did the TATA box of the adenovirus major late (ML) promoter. In sharp contrast, changing the IVa2 TA-rich sequence in its natural, intragenic context to the ML TATA sequence increased the activity of the IVa2 promoter only slightly. In view of this discrepancy, we examined the effects of single, double, and clustered point mutations in the downstream sequence on the activity of a minimal IVa2 promoter. Mutations between positions +21 and +29 inhibited IVa2 transcription, in some cases to the very low level directed by the IVa2 initiator alone. By contrast, substitutions within the TA-rich sequence increased the efficiency of IVa2 transcription. These results indicated that the downstream, TA-rich sequence does not function as an intragenic TFIID-binding site but rather is included within a negative regulatory element. Electrophoretic mobility shift and methylation interference assays using wild-type and mutated, intragenic promoter sequences identified a HeLa cell component whose binding to the sequence +11 to +27 correlated with repression of IVa2 transcription, suggesting that a negative regulatory element is superimposed upon the intragenic sequence required for efficient transcription from the IVa2 initiator.

Intragenic activating and repressing elements control transcription from the adenovirus IVa2 initiator

The downstream stimulatory segment of the adenovirus type 2 IVa promoter includes a TA-rich sequence that binds recombinant TATA-binding proteins (TBP) in vitro. We now demonstrate that when placed upstream of the IVa2, initiator, this TA-rich sequence operated as a TATA element but exhibited significantly lower transcriptional and TBP-binding activities than did the TATA box of the adenovirus major late (ML) promoter. In sharp contrast, changing the IVa2 TA-rich sequence in its natural, intragenic context to the ML TATA sequence increased the activity of the IVa2 promoter only slightly. In view of this discrepancy, we examined the effects of single, double, and clustered point mutations in the downstream sequence on the activity of a minimal IVa2 promoter. Mutations between positions +21 and +29 inhibited IVa2 transcription, in some cases to the very low level directed by the IVa2 initiator alone. By contrast, substitutions within the TA-rich sequence increased the efficiency of IVa2 transcription. These results indicated that the downstream, TA-rich sequence does not function as an intragenic TFIID-binding site but rather is included within a negative regulatory element. Electrophoretic mobility shift and methylation interference assays using wild-type and mutated, intragenic promoter sequences identified a HeLa cell component whose binding to the sequence +11 to +27 correlated with repression of IVa2 transcription, suggesting that a negative regulatory element is superimposed upon the intragenic sequence required for efficient transcription from the IVa2 initiator.

Organization of the double-stranded RNA-activated protein kinase DAI and virus-associated VA RNAI in adenovirus-2-infected HeLa cells

We have examined the cellular distribution of the double-stranded RNA-activated protein kinase DAI in adenovirus 2 (Ad2)-infected and uninfected HeLa cells. In uninfected cells DAI was found to be concentrated in the cytoplasm. In addition, DAI was localized in the nucleoli and diffusely distributed throughout the nucleoplasm. Cells treated with alpha-interferon displayed a similar pattern of distribution for DAI. When RNA polymerase I activity was inhibited by the drug actinomycin D, nucleoli segregated and DAI was found to colocalize with the dense fibrillar region of the nucleoli. During mitosis, the distribution of DAI paralleled that of rRNA. In adenovirus-infected cells the localization of DAI was similar to that in uninfected interphase cells. VA RNAI was detected in Ad2-infected cells by 10–14 hours post-infection as fine dots in the nucleoplasm. By 18–24 hours post-infection, VA RNAI appeared in bigger and more abundant dots in the nucleoplasm and the cytoplasm was intensively labeled. Transient expression of the VA RNAI gene in uninfected cells resulted in a similar localization of the RNA. Our results are consistent with a role for DAI and VA RNAI in protein synthesis and suggest that DAI may play an early role in ribosome biogenesis in the nucleolus in addition to its cytoplasmic role in translation.

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.

Anatomy of an unusual RNA polymerase II promoter containing a downstream TATA element

The adenovirus type 2 IVa2 promoter lacks a conventional TATA element yet directs transcription from two closely spaced initiation sites. To define elements required for in vitro transcription of this promoter, IVa2 templates carrying 5' deletions or linker-scanning mutations were transcribed in HeLa whole-cell extracts and the transcripts were analyzed by primer extension. Mutation of the sequence centered on position -47, which is specifically recognized by a cellular factor, reduced the efficiency of IVa2 transcription two- to threefold, whereas mutation of the sequence centered on position -30 selectively impaired utilization of the minor in vivo initiation site. Utilization of the major in vivo site was decreased no more than fivefold by deletion of all sequences upstream of position -15. By contrast, mutation of the region from +13 to +19 or of the initiation region severely impaired IVa2 transcription. The sequence spanning the initiation sites was sufficient to direct accurate initiation by RNA polymerase II from the major in vivo site. Thus, the two initiation sites of the IVa2 promoter are specified by independent elements, and a downstream element is the primary determinant of efficient transcription from both of these sites. The downstream element identified by mutational analysis altered the TATA element-like sequence TATAGAAA lying at positions +21 to +14 in the coding strand. Transcription from the wild-type IVa2 promoter was severely inhibited when endogenous TFIID was inactivated by mild heat treatment. Exogenous human TATA-binding protein (TBP) synthesized in Escherichia coli restored specific IVa2 transcription from both initiation sites when added to such heat-treated extracts. Although efficient IVa2 transcription requires both the downstream TATA sequence and active TFIID, bacterially synthesized TBP also stimulated the low level of IVa2 transcription observed when the TATA sequence was mutated to a sequence that failed to bind TBP.

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.

Anatomy of an unusual RNA polymerase II promoter containing a downstream TATA element

The adenovirus type 2 IVa2 promoter lacks a conventional TATA element yet directs transcription from two closely spaced initiation sites. To define elements required for in vitro transcription of this promoter, IVa2 templates carrying 5' deletions or linker-scanning mutations were transcribed in HeLa whole-cell extracts and the transcripts were analyzed by primer extension. Mutation of the sequence centered on position -47, which is specifically recognized by a cellular factor, reduced the efficiency of IVa2 transcription two- to threefold, whereas mutation of the sequence centered on position -30 selectively impaired utilization of the minor in vivo initiation site. Utilization of the major in vivo site was decreased no more than fivefold by deletion of all sequences upstream of position -15. By contrast, mutation of the region from +13 to +19 or of the initiation region severely impaired IVa2 transcription. The sequence spanning the initiation sites was sufficient to direct accurate initiation by RNA polymerase II from the major in vivo site. Thus, the two initiation sites of the IVa2 promoter are specified by independent elements, and a downstream element is the primary determinant of efficient transcription from both of these sites. The downstream element identified by mutational analysis altered the TATA element-like sequence TATAGAAA lying at positions +21 to +14 in the coding strand. Transcription from the wild-type IVa2 promoter was severely inhibited when endogenous TFIID was inactivated by mild heat treatment. Exogenous human TATA-binding protein (TBP) synthesized in Escherichia coli restored specific IVa2 transcription from both initiation sites when added to such heat-treated extracts. Although efficient IVa2 transcription requires both the downstream TATA sequence and active TFIID, bacterially synthesized TBP also stimulated the low level of IVa2 transcription observed when the TATA sequence was mutated to a sequence that failed to bind TBP.

Adenovirus E1A protein activates transcription of the E1A gene subsequent to transcription complex formation

The mechanism of transcriptional activation of the adenovirus E1A and E3 genes by E1A protein during infection was examined by using transcription-competition assays. Infection of HeLa cells with one virus led to inhibition of mRNA accumulation from a superinfecting virus. Synthesis of the E1A 289R protein by the first virus to infect reduced inhibition of transcription of the superinfecting virus, indicating that the E1A 289R protein was limiting for E1A-activated transcription. Infection with an E1A- virus, followed 6 h later by superinfection with a wild-type virus, led to preferential transcriptional activation of the E1A gene of the first virus, suggesting that a host transcription component(s) stably associated with the E1A promoter in the absence of E1A protein and that this complex was the substrate for transcriptional activation by E1A protein. The limiting host transcription component(s) bound to the E1A promoter to form a complex with a half-life greater than 24 h in the absence of E1A 289R protein, as demonstrated in a challenge assay with a large excess of superinfecting virus. In the presence of the E1A 289R protein, the E1A gene of the superinfecting virus was gradually activated with a reduction in E1A mRNA accumulation from the first virus. The kinetics of the activation suggest that this was due to an indirect effect rather than to destabilization of stable transcription complexes by the 289R protein.

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.

Adenovirus terminal protein mediates both nuclear matrix association and efficient transcription of adenovirus DNA

Adenovirus DNA is tightly bound to the nuclear matrix throughout the course of infection. Analysis of adenovirus DNA from infected HeLa cell nuclei after extraction with lithium diiodosalicylate and digestion with restriction enzymes demonstrated that the sites of tightest attachment occur in the terminal fragments of the linear viral chromosome. Analysis of viruses mutated in the precursor terminal protein coding sequence demonstrated that the terminal protein, which is covalently attached to the 5' end of each DNA strand, mediates the tight binding. Virions containing chromosomes with mutant terminal proteins were unpackaged and viral DNA accumulated in the nucleus at a normal rate and competed for the limiting component during transcription complex formation, but their early genes were transcribed at reduced efficiency by both RNA polymerases II and III. The transcriptional defects were not complemented by coinfection with a wild-type virus. We propose that the adenovirus chromosome may exist as a single chromatin domain during infection and that binding of DNA to the nuclear matrix may play a critical role in adenovirus transcription.

The adenovirus L4 100-kilodalton protein is necessary for efficient translation of viral late mRNA species

When screening a number of adenovirus type 5 (Ad5) temperature-sensitive mutants for defects in viral gene expression, we observed that H5ts1-infected 293 cells accumulated reduced levels of newly synthesized viral late proteins. Pulse-labeling and pulse-chase experiments were used to establish that the late proteins synthesized in H5ts1-infected cells under nonpermissive conditions were as stable as those made in Ad5-infected cells. H5ts1-infected cells contained normal levels of viral late mRNAs. Because these observations implied that translation of viral mRNA species was defective in mutant virus-infected cells, the association of viral late mRNAs with polyribosomes was examined during the late phase of infection at a nonpermissive temperature. In Ad5-infected cells, the majority of the viral L2, L3, L4, pIX, and IVa2 late mRNA species were polyribosome bound. By contrast, these same mRNA species were recovered from H5ts1-infected cells in fractions nearer the top of polyribosome gradients, suggesting that initiation of translation was impaired. During the late phase of infection, neither the polyribosome association nor the translation of most viral early mRNA species was affected by the H5ts1 mutation. This lesion, mapped by marker rescue to the L4 100-kilodalton (kDa) nonstructural protein, has been identified as a single base pair substitution that replaces Ser-466 of the Ad5 100-kDa protein with Pro. A set of temperature-independent revertants of H5ts1 was isolated and characterized. Either true reversion of the H5ts1 mutation or second-site mutation of Pro-466 of the H5ts1 100-kDa protein to Thre, Leu, or His restored both temperature-independent growth and the efficient synthesis of viral late proteins. We therefore conclude that the Ad5 L4 100-kDa protein is necessary for efficient initiation of translation of viral late mRNA species during the late phase of infection.

Phosphorylation at serine 89 induces a shift in gel mobility but has little effect on the function of adenovirus type 5 E1A proteins

Phosphorylation at serine 89 was shown to be the major cause of the shift in gel migration of the 289R and 243R early region 1A (E1A) proteins of human adenovirus type 5. However, conversion of Ser-89 to alanine by site-directed mutagenesis did not abolish E1A transactivating or transforming activities, suggesting that phosphorylation at this site is not necessary for these E1A functions.

Effects of adriamycin and etoposide on the replication of adenovirus 5 in sensitive and resistant human tumour cells

Adenovirus is a potential probe for identifying and understanding drug sensitivity in primary, nonproliferating cultures of human normal and tumour cells but the scope and limitations of such an approach first need to be evaluated in established cell lines. For this purpose we have identified an ovarian tumour cell line (CI-80-13S) with natural resistance to adriamycin, etoposide and crosslinking agents compared with other human tumour lines. Resistance to adriamycin correlated poorly with resistance to etoposide in these cell lines (r = 0.05). Adenovirus replication in drug-treated cells (viral capacity) was found to be differentially inhibited in sensitive cells when the drug was administered to cells simultaneously with infection (adriamycin) or 20 hr after infection (etoposide). Viral capacity could not be inhibited by more than 90% in sensitive cells. In contrast, no such plateau was exhibited in the dose-responses of cell survival or inhibition of cellular DNA synthesis, both of which distinguished sensitive from resistant cells. Adenovirus was not inactivated by preincubation with high doses of adriamycin or etoposide, thus confirming that no functionally-relevant damage is directly induced by these agents in DNA. Uptake of adriamycin and etoposide was similar in sensitive and resistant cells and both agents blocked cells in the G2 phase of the cell cycle. Protein-linked DNA was induced in sensitive cells. The results indicate that (a) these drugs have two dose-dependent effects in cells, one of which does not inhibit replication of adenovirus; and (b) inhibition of adenovirus replication could in principle be used to predict sensitivity to adriamycin and etoposide.

A rapid and sensitive solution hybridisation assay for the quantitative determination of specific viral RNA sequences

A solution hybridisation assay has been developed which allows the quantitation of specific viral (RNA) sequences in infected cells. The assay makes use of single-stranded (ss) RNA probes of known polarity synthesised at high specific activity in vitro from cDNA clones of the relevant viral gene by the SP6 or T7 RNA polymerase. These probes are used together with samples containing the RNA to be detected at a known concentration to construct a calibration curve to relate RNase resistant radioactivity following solution hybridisation to amount of RNA. The amount of RNA in experimental samples is then determined using the calibration curve that is produced each time the assay is performed. The UKtc strain of Rotavirus growing in BSC-1 cells was used to develop this method but with the substitution of suitable cDNA clones it could be applied to any viral system.

The role of the two E1a mRNA products of subgroup B adenoviruses in the regulation of early promoters of subgroup C adenoviruses

HeLa cells were co-transfected with recombinant plasmids carrying adenovirus (Ad)2 or Ad3 E1a promoters fused to the chloramphenicol acetyl transferase gene (cat), and a plasmid encoding the Ad3 E1a promoter. Whereas no stimulating effect was observed on the Ad3 E1a promoter, the Ad2 promoter was inhibited. To determine which of the E1a gene products of Ad3 was responsible for the repressive effect, plasmids were constructed in which only the 13S or 12S mRNA product of Ad3 was expressed. Both the 12S and 13S mRNA products of Ad3 E1a were found to depress the transcription from the Ad2 E1a promoter. Each Ad3 E1a gene product was able to stimulate transcription from the Ad5 E2a early promoter in a manner similar to that of the Ad2 E1a gene products. In the case of the Ad5 E3 promoter, neither of the Ad3 E1a gene products stimulated transcription, but an inhibition was observed. These results suggest that both mRNA products of the Ad3 E1a region inhibit transcription at the TATA box transcription complex.

Activation of adenovirus promoters by the adenovirus E1A protein in cell-free extracts

The primary product of the adenovirus E1A gene is a protein that is sufficient for controlling host-cell proliferation and immortalizing primary rodent cells. The mechanism by which the protein induces these cellular effects is poorly understood, but might be linked to its ability to regulate RNA transcription from a number of viral and cellular genes. The mechanism of E1A's transcriptional-activation (trans-activation) was studied here by monitoring the protein's effect on specific adenovirus promoters in two types of transcriptional systems in vitro. One of these systems consisted of extracts from transformed cells constitutively expressing E1A, and the other consisted of extracts of HeLa cells supplemented with a plasmid-encoded E1A protein purified from Escherichia coli. The results show that the E1A protein specifically stimulates transcription from adenovirus promoters; thus, the induction of cellular transcription factors is not necessary to explain the stimulation of transcription by E1A.

Recombinant adenovirus induces antibody response to hepatitis B virus surface antigen in hamsters

Recombinant adenoviruses carrying the hepatitis B virus surface antigen coding sequence in the adenovirus E3 region were constructed using DNA from either adenovirus type 5 or an adenovirus type 5 E3-region deletion mutant. Both of these recombinant adenoviruses replicated as efficiently as wild-type adenovirus in all human cells tested, including the human diploid cell strain WI-38. This indicates that insertion of the hepatitis B virus surface antigen gene into the E3 region does not significantly affect viral replication. Human cells infected with these recombinant adenoviruses secreted immunoreactive hepatitis B virus surface antigen. Since a practical small animal model for human adenoviruses was lacking, a hamster model was developed to evaluate the immunogenic potential of these recombinant adenoviruses. Upon intranasal inoculation, both wild-type adenovirus and the adenovirus E3-region deletion mutant replicated in the lungs of these animals and induced an antibody response against adenovirus. Hamsters similarly immunized with the live recombinant adenoviruses produced antibody against both adenovirus and hepatitis B virus surface antigen.

Partition of E1A proteins between soluble and structural fractions of adenovirus-infected and -transformed cells

The partition of E1A proteins between soluble and structural framework fractions of human cells infected or transformed by subgroup C adenoviruses was investigated by using gentle cell fractionation conditions. A polyclonal antibody raised against a trpE-E1A fusion protein (K.R. Spindler, D.S.E. Rosser, and A. J. Berk, J. Virol. 132-141, 1984) synthesized in Escherichia coli was used to measure the steady-state levels of E1A proteins recovered in the various fractions by immunoblotting. The relative concentration of E1A proteins recovered in the soluble fraction of adenovirus type 2-infected cells was at least fivefold greater than the relative concentration in the corresponding fraction of transformed 293 cells. The observed distribution of E1A proteins was not altered by the sulfhydryl-blocking reagent N-ethylmaleimide. E1A proteins were recovered in nuclear matrix, chromatin, and cytoskeleton fractions after further fractionation of the structural framework fraction. However, the E1A protein species that could be identified by one-dimensional gel electrophoresis were not uniformly distributed among the subcellular fractions examined. The results obtained when fractionation was performed in the presence of the oxidation catalysts Cu2+ or (ortho-phenanthroline)2 Cu2+ indicate that E1A proteins can be efficiently cross-linked, via disulfide bonds, to the structural framework of both adenovirus-infected and adenovirus-transformed cells.