The stability of early adenovirus mRNA is controlled by the viral 72 kd DNA-binding protein

The adenovirus DNA-binding protein DBP

Adenoviruses are a group of double-stranded DNA viruses that can mainly cause respiratory, gastrointestinal, and eye infections in humans. In addition, adenoviruses are employed as vector vaccines for combatting viral infections, including SARS-CoV-2, and serve as excellent gene therapy vectors. These viruses have the ability to modulate the host cell machinery to their advantage and trigger significant restructuring of the nuclei of infected cells through the activity of viral proteins. One of those, the adenovirus DNA-binding protein (DBP), is a multifunctional non-structural protein that is integral to the reorganization processes. DBP is encoded in the E2A transcriptional unit and is highly abundant in infected cells. Its activity is unequivocally linked to the formation, structure, and integrity of virus-induced replication compartments, molecular hubs for the regulation of viral processes, and control of the infected cell. DBP also plays key roles in viral DNA replication, transcription, viral gene expression, and even host range specificity. Notably, post-translational modifications of DBP, such as SUMOylation and extensive phosphorylation, regulate its biological functions. DBP was first investigated in the 1970s, pioneering research on viral DNA-binding proteins. In this literature review, we provide an overview of DBP and specifically summarize key findings related to its complex structure, diverse functions, and significant role in the context of viral replication. Finally, we address novel insights and perspectives for future research.

Global Transcriptome Analyses of Cellular and Viral mRNAs during HAdV-C5 Infection Highlight New Aspects of Viral mRNA Biogenesis and Cytoplasmic Viral mRNA Accumulations

It is well established that human adenoviruses such as species C, types 2 and 5 (HAdV-C2 and HAdV-C5), induce a nearly complete shutoff of host-cell protein synthesis in the infected cell, simultaneously directing very efficient production of viral proteins. Such preferential expression of viral over cellular genes is thought to be controlled by selective nucleocytoplasmic export and translation of viral mRNA. While detailed knowledge of the regulatory mechanisms responsible for the translation of viral mRNA is available, the viral or cellular mechanisms of mRNA biogenesis are not completely understood. To identify parameters that control the differential export of viral and cellular mRNAs, we performed global transcriptome analyses (RNAseq) and monitored temporal nucleocytoplasmic partitioning of viral and cellular mRNAs during HAdV-C5 infection of A549 cells. Our analyses confirmed previously reported features of the viral mRNA expression program, as a clear shift in viral early to late mRNA accumulation was observed upon transition from the early to the late phase of viral replication. The progression into the late phase of infection, however, did not result in abrogation of cellular mRNA export; rather, viral late mRNAs outnumbered viral early and most cellular mRNAs by several orders of magnitude during the late phase, revealing that viral late mRNAs are not selectively exported but outcompete cellular mRNA biogenesis.

Adipocyte commitment of 3T3-L1 cells is required to support human adenovirus 36 productive replication concurrent with altered lipid and glucose metabolism

Human adenovirus 36 (HAdV-D36) can cause obesity in animal models, induces an adipogenic effect and increased adipocyte differentiation in cell culture. HAdV-D36 infection alters gene expression and the metabolism of the infected cells resulting in increased glucose internalization and triglyceride accumulation. Although HAdV-D36 prevalence correlates with obesity in humans, whether human preadipocytes may be targeted in vivo has not been determined and metabolic reprogramming of preadipocytes has not been explored in the context of the viral replication cycle. HAdV-D36 infection of the mouse fibroblasts, 3T3-L1 cells, which can differentiate into adipocytes, promotes proliferation and differentiation, but replication of the virus in these cells is abortive as indicated by short-lived transient expression of viral mRNA and a progressive loss of viral DNA. Therefore, we have evaluated whether a productive viral replication cycle can be established in the 3T3-L1 preadipocyte model under conditions that drive the cell differentiation process. For this purpose, viral mRNA levels and viral DNA replication were measured by RT-qPCR and qPCR, respectively, and viral progeny production was determined by plaque assay. The lipogenic effect of infection was evaluated with Oil Red O (ORO) staining, and expression of genes that control lipid and glucose metabolism was measured by RT-qPCR. In the context of a viral productive cycle, HAdV-D36 modulated the expression of the adipogenic genes, C/EBPα, C/EBPβ and PPARγ, as well as intracellular lipid accumulation, and the infection was accompanied by altered expression of glucolytic genes. The results show that only adipocyte-committed 3T3-L1 cells are permissive for the expression of early and late viral mRNAs, as well as viral DNA replication and progeny production, supporting productive HAdV-D36 viral replication, indicating that a greater effect on adipogenesis occurs in adipocytes that support productive viral replication.

PML Alternative Splice Products Differentially Regulate HAdV Productive Infection

Promyelocytic leukemia nuclear bodies (PML-NBs) were considered to maintain antiviral capacity, as these spherical complexes are antagonized by viruses. Actual work provides evidence, that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Several early HAdV proteins target PML-NBs, such as E4orf3 that promotes redistribution into track-like structures. PML-associated dependency factors that enhance viral gene expression, such as Sp100A remain in the nuclear tracks while restrictive factors, such as Daxx, are inhibited by either proteasomal degradation or relocalization to repress antiviral functions. Here, we did a comprehensive analysis of nuclear PML isoforms during HAdV infection. Our results show cell line specific differences as PML isoforms differentially regulate productive HAdV replication and progeny production. Here, we identified PML-II as a dependency factor that supports viral progeny production, while PML-III and PML-IV suppress viral replication. In contrast, we identified PML-I as a positive regulator and PML-V as a restrictive factor during HAdV infection. Solely PML-VI was shown to repress adenoviral progeny production in both model systems. We showed for the first time, that HAdV can reorganize PML-NBs that contain PML isoforms other then PML-II. Intriguingly, HAdV was not able to fully disrupt PML-NBs composed out of the PML isoforms that inhibit viral replication, while PML-NBs composed out of PML isoforms with beneficial influence on the virus formed tracks in all examined cells. In sum, our findings clearly illustrate the crucial role of PML-track formation in efficient viral replication. IMPORTANCE Actual work provides evidence that PML-NB-associated factors might also be beneficial for distinct viral processes indicating why genomes and replication centers of nuclear replicating viruses are often found juxtaposed to PML-NBs. Alternatively spliced PML isoforms I-VII are expressed from one single pml gene containing nine exons and their transcription is tightly controlled and stimulated by interferons and p53. Several early HAdV proteins target PML-NBs, such as E4orf3, promoting redistribution into track-like structures. Our comprehensive studies indicate a diverging role of PML isoforms throughout the course of productive HAdV infection in either stably transformed human lung (H1299) or liver (HepG2) cells, in which we observed a multivalent regulation of HAdV by all six PML isoforms. PML-I and PML-II support HAdV-mediated track formation and efficient formation of viral replication centers, thus promoting HAdV productive infection. Simultaneously, PML-III, -IV,-V, and -VI antagonize viral gene expression and particle production.

E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral gene expression in HAdV-C5 infection

The multifunctional adenoviral E1B-55K phosphoprotein is a major regulator of viral replication and plays key roles in virus-mediated cell transformation. While much is known about its function in oncogenic cell transformation, underlying features and exact mechanisms that implicate E1B-55K in regulation of viral gene expression are less well understood. Therefore, this work aimed at unravelling basic intranuclear principles of E1B-55K-regulated viral mRNA biogenesis using wild type HAdV-C5 E1B-55K, a virus mutant with abrogated E1B-55K expression and a mutant that expresses a phosphomimetic E1B-55K. By subnuclear fractionation, mRNA, DNA and protein analyses as well as luciferase reporter assays, we show that (i) E1B-55K promotes efficient release of viral late mRNAs from their site of synthesis in viral replication compartments (RCs) to the surrounding nucleoplasm, that (ii) E1B-55K modulates the rate of viral gene transcription and splicing in RCs, that (iii) E1B-55K participates in the temporal regulation of viral gene expression, that (iv) E1B-55K can enhance or repress the expression of viral early and late promoters and that (v) the phosphorylation of E1B-55K regulates the temporal effect of the protein on each of these activities. Together, these data demonstrate that E1B-55K is a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes during HAdV-C5 infection. Importance Human adenoviruses are useful models to study basic aspects of gene expression and splicing. Moreover, they are one of the most commonly used viral vectors for clinical applications. However, key aspects of the activities of essential viral proteins that are commonly modified in adenoviral vectors have not been fully described. A prominent example is the multifunctional adenoviral oncoprotein E1B-55K that is known to promote efficient viral genome replication and expression while simultaneously repressing host gene expression and antiviral host responses. Our study combined different quantitative methods to study how E1B-55K promotes viral mRNA biogenesis. The data presented here propose a novel role for E1B-55K as a phosphorylation-dependent transcriptional and post-transcriptional regulator of viral genes.

Double-edged role of PML nuclear bodies during human adenovirus infection

PML nuclear bodies are matrix-bound nuclear structures with a variety of functions in human cells. These nuclear domains are interferon regulated and play an essential role during virus infections involving accumulation of SUMO-dependent host and viral factors. PML-NBs are targeted and subsequently manipulated by adenoviral regulatory proteins, illustrating their crucial role during productive infection and virus-mediated oncogenic transformation. PML-NBs have a longstanding antiviral reputation; however, the genomes of Human Adenoviruses and initial sites of viral transcription/replication are found juxtaposed to these domains, resulting in a double-edged capacity of these nuclear multiprotein/multifunctional complexes. This enigma provides evidence that Human Adenoviruses selectively counteract antiviral responses, and simultaneously benefit from or even depend on proviral PML-NB associated components by active recruitment to PML track-like structures, that are induced during infection. Thereby, a positive microenvironment for adenoviral transcription and replication is created at these nuclear subdomains. Based on the available data, this review aims to provide a detailed overview of the current knowledge of Human Adenovirus crosstalk with nuclear PML body compartments as sites of SUMOylation processes in the host cells, evaluating the currently known principles and molecular mechanisms.

Viral DNA Binding Protein SUMOylation Promotes PML Nuclear Body Localization Next to Viral Replication Centers

Human adenoviruses (HAdVs) have developed mechanisms to manipulate cellular antiviral measures to ensure proper DNA replication, with detailed processes far from being understood. Host cells repress incoming viral genomes through a network of transcriptional regulators that normally control cellular homeostasis. The nuclear domains involved are promyelocytic leukemia protein nuclear bodies (PML-NBs), interferon-inducible, dot-like nuclear structures and hot spots of SUMO posttranslational modification (PTM). In HAdV-infected cells, such SUMO factories are found in close proximity to newly established viral replication centers (RCs) marked by the adenoviral DNA binding protein (DBP) E2A. Here, we show that E2A is a novel target of host SUMOylation, leading to PTMs supporting E2A function in promoting productive infection. Our data show that SUMOylated E2A interacts with PML. Decreasing SUMO-E2A protein levels by generating HAdV variants mutated in the three main SUMO conjugation motifs (SCMs) led to lower numbers of viral RCs and PML-NBs, and these two structures were no longer next to each other. Our data further indicate that SUMOylated E2A binds the host transcription factor Sp100A, promoting HAdV gene expression, and represents the molecular bridge between PML tracks and adjacent viral RCs. Consequently, E2A SCM mutations repressed late viral gene expression and progeny production. These data highlight a novel mechanism used by the virus to benefit from host antiviral responses by exploiting the cellular SUMO conjugation machinery.IMPORTANCE PML nuclear bodies (PML-NBs) are implicated in general antiviral defense based on recruiting host restriction factors; however, it is not understood so far why viruses would establish viral replication centers (RCs) juxtaposed to such "antiviral" compartments. To understand this enigma, we investigate the cross talk between PML-NB components and viral RCs to find the missing link connecting both compartments to promote efficient viral replication and gene expression. Taken together, the current concept is more intricate than originally believed, since viruses apparently take advantage of several specific PML-NB-associated proteins to promote productive infection. Simultaneously, they efficiently inhibit antiviral measures to maintain the viral infectious program. Our data provide evidence that SUMOylation of the viral RC marker protein E2A represents the basis of this virus-host interface and regulates various downstream events to support HAdV productive infection. These results are the basis of our current attempts to generate and screen for specific E2A SUMOylation inhibitors to constitute novel therapeutic approaches to limit and prevent HAdV-mediated diseases and mortality of immunosuppressed patients.

Viral and cellular interactions during adenovirus DNA replication

Adenoviruses represent ubiquitous and clinically significant human pathogens, gene-delivery vectors, and oncolytic agents. The study of adenovirus-infected cells has long been used as an excellent model to investigate fundamental aspects of both DNA virus infection and cellular biology. While many key details supporting a well-established model of adenovirus replication have been elucidated over a period spanning several decades, more recent findings suggest that we have only started to appreciate the complex interplay between viral genome replication and cellular processes. Here, we present a concise overview of adenovirus DNA replication, including the biochemical process of replication, the spatial organization of replication within the host cell nucleus, and insights into the complex plethora of virus-host interactions that influence viral genome replication. Finally, we identify emerging areas of research relating to the replication of adenovirus genomes.

In Vivo Labelling of Adenovirus DNA Identifies Chromatin Anchoring and Biphasic Genome Replication

Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Adenoviruses are DNA viruses with a lytic infection cycle. Following the fate of incoming as well as recently replicated genomes during infections is a challenge. In this study, we used the ANCHOR3 technology based on a bacterial partitioning system to establish a versatile in vivo imaging system for adenoviral genomes. The system allows the visualization of both individual incoming and newly replicated genomes in real time in living cells. We demonstrate that incoming adenoviral genomes are attached to condensed cellular chromatin during mitosis, facilitating the equal distribution of viral genomes in daughter cells after cell division. We show that the formation of replication centers occurs in conjunction with in vivo genome replication and determine replication rates. Visualization of adenoviral DNA revealed that adenoviruses exhibit two kinetically distinct phases of genome replication. Low-level replication occurred during early replication, while high-level replication was associated with late replication phases. The transition between these phases occurred concomitantly with morphological changes of viral replication compartments and with the appearance of virus-induced postreplication (ViPR) bodies, identified by the nucleolar protein Mybbp1A. Taken together, our real-time genome imaging system revealed hitherto uncharacterized features of adenoviral genomes in vivo. The system is able to identify novel spatiotemporal aspects of the adenovirus life cycle and is potentially transferable to other viral systems with a double-stranded DNA phase.

IMPORTANCE Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Granzyme H destroys the function of critical adenoviral proteins required for viral DNA replication and granzyme B inhibition

Granzymes are key components of the immune response that play important roles in eliminating host cells infected by intracellular pathogens. Several granzymes are potent inducers of cell death. However, whether granzymes use additional mechanisms to exert their antipathogen activity remains elusive. Here, we show that in adenovirus-infected cells in which granzyme B (gzmB) and downstream apoptosis pathways are inhibited, granzyme H (gzmH), an orphan granzyme without known function, directly cleaves the adenovirus DNA-binding protein (DBP), a viral component absolutely required for viral DNA replication. We directly addressed the functional consequences of the cleavage of the DBP by gzmH through the generation of a virus that encodes a gzmH-resistant DBP. This virus demonstrated that gzmH directly induces an important decay in viral DNA replication. Interestingly, gzmH also cleaves the adenovirus 100K assembly protein, a major inhibitor of gzmB, and relieves gzmB inhibition. These results provide the first evidence that granzymes can mediate antiviral activity through direct cleavage of viral substrates, and further suggest that different granzymes have synergistic functions to outflank viral defenses that block host antiviral activities.

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.

A comparative analysis of the phosphorylation and biochemical properties of wild type and host range variant DNA binding proteins of human adenovirus 5

Specific mutation of the DNA binding protein (DBP) of human adenovirus types 2 and 5 can extend the host range of these viruses to simian cells. These mutations replace histidine at position 130 in the highly phosphorylated N-terminal domain of DBP with a potentially phophorylatable tyrosine residue. To investigate the possibility that alternative phosphorylation might contribute to the functional differences between wild type (wt) and host range (hr) DBP molecules, radiolabeled proteins were compared by partial proteolysis and tyrosine phosphorylation was analyzed. These studies confirmed the previous tentative assignment of a chymotrypsin-sensitive site at position 121 of DBP. No host range-specific tyrosine phosphorylation was detected, and no gross difference in the extent of phosphorylation between wt and hr DBP was observed. However, the cleaved N-terminal domains of wt and hr DBP exhibited different sensitivities to further chymotryptic digestion in vitro and different fragmentation patterns, suggesting that they might have different conformations. Such a difference could underlie the differing ability of these proteins to support Ad replication in simian cells.

Control of adenovirus early gene expression during the late phase of infection

The adenovirus gene regulatory program occurs in two distinct phases, as defined by the onset of DNA replication. During the early phase, the E1A, E1B, E2, E3, and E4 genes are maximally expressed, while the major late promoter (MLP) is minimally expressed and transcription is attenuated. After the onset of DNA replication, the IVa2 and pIX genes are expressed at high levels, transcription from the MLP is unattenuated and fully activated, and early gene expression is repressed. Although the cis elements and trans-acting factors responsible for the late-phase activation of the MLP have been identified and characterized and the role of DNA replication in activation has been established, the mechanism(s) underlying the commensurate decrease in early gene expression has yet to be elucidated. The results of this study demonstrate that this decrease depends on a fully functional MLP. Specifically, virus mutants with severely deficient transcription from the MLP exhibit a marked increase in expression of the E1A, E1B, and E2 early genes. These increases were observed at the level of transcription initiation, mRNA accumulation, and protein production. In addition, expression from the late gene pIX, which is not contained within the major late transcription unit (MLTU), is also markedly increased. To begin the analysis of the mechanisms underlying these late-phase effects, mixed-infection experiments with mutant and wild-type viruses were performed. The results show that the effects on early gene expression, as measured both at the protein and RNA levels, are mediated in trans and not in cis. These observations are consistent either with a model in which one or more late protein products encoded by the MLTU acts as a repressor of early gene expression or with one in which the wild-type MLP competes with early promoters for limiting transcription factors.

E1A 12S and 13S of the transformation-defective adenovirus type 12 strain CS-1 inactivate proteins of the RB family, permitting transactivation of the E2F-dependent promoter

The transformation-defective Vero cell host range mutant CS-1 of the highly oncogenic adenovirus type 12 (Ad12) (Ad12-CS-1) has a 69-bp deletion in the early region 1A (E1A) gene that removes the carboxy-terminal half of conserved region 2 and the amino-terminal half of the Ad12-specific so-called spacer that seems to play a pivotal role in the oncogenicity of the virus. Despite its deficiency in immortalizing and transforming primary rodent cells, we found that the E1A 13S protein of Ad12-CS-1 retains the ability to bind p105-RB, p107, and p130 in nuclear extract binding assays with glutathione S-transferase-E1A fusion proteins and Western blot analysis. Like wild-type E1A, the mutant protein was able to dissociate E2F from retinoblastoma-related protein-containing complexes, as judged from gel shift experiments with purified 12S and 13S proteins from transfection experiments with an E1A expression vector or from infection with the respective virus. Moreover, in transient expression assays, the 12S and 13S products of wild-type Ad12 and Ad12-CS-1 were shown to transactivate the Ad12 E1A promoter containing E2F-1 and E2F-5-motifs, respectively, in a comparable manner. The same results were obtained from transfection assays with the E2F motif-dependent E2 promoter of adenovirus type 5 or the human dihydrofolate reductase promoter. These data suggest that efficient infection by Ad12 and the correlated virus-induced reprogramming of the infected cells, including the induction of cell cycle-relevant mechanisms (e.g. E2F activation), can be uncoupled from the transformation properties of the virus.

Construction, characterization, and utilization of cell lines which inducibly express the adenovirus DNA-binding protein

To further our understanding of structure-function relationships within the multifunctional adenovirus DNA binding protein (DBP) a more diverse collection of mutants is necessary. DBP-expressing cell lines (gmDBP) were previously constructed that complemented DBP-negative mutants for viral growth. However, they did not allow severely defective viruses to form plaques. Since efficient mutant construction is reliant on plaque isolation of the desired mutant virus as a final step, additional gmDBP cell lines were constructed which allow all DBP-negative mutants to form plaques. Here we describe the construction and characterization of 12 new gmDBP cell lines. The utility of these lines was demonstrated by the efficient construction of a new defective mutant, H5in804, using a combination of DBP-expressing lines. The H5in804 mutation adds 22 amino acids at the carboxyl end of an otherwise wild type protein. Characterization of H5in804 revealed that it was altered in its ability to replicate viral DNA. The depression of DNA synthesis most probably results from a reduced ability of H5in804 DBP to bind ssDNA.

Structural alterations of double-stranded DNA in complex with the adenovirus DNA-binding protein. Implications for its function in DNA replication

The Adenovirus DNA-binding protein (DBP) binds to single-stranded (ss) DNA as well as to double-stranded (ds) DNA and forms multimeric protein-DNA complexes with both. Gel retardation assays indicate rapid complex formation for both DNAs. DBP rapidly dissociates from dsDNA, indicating a dynamic equilibrium, whereas the ssDNA-DBP complex is much more stable. We investigated the complex between DBP and dsDNA in more detail. Electron microscopical analysis shows thick filament-like and beaded structures in which the length of the DNA is not significantly altered. Cryo-electron micrographs suggest the presence of interwound protein fibres around the DNA. Ligase-mediated cyclization, but not linear multimerization, of DBP-saturated DNA fragments exceeding the persistence length was severely inhibited. This suggests that DNA may be organized by DBP into a rigid structure. Under those conditions, DBP induces distinct changes in the circular dichroism spectrum of the DNA, indicative of structural DNA changes. No bending or twisting of the complex was observed. Hydroxyl radical footprinting showed that the breakdown pattern of DNA at saturating DBP concentrations is much more regular than the protein-free DNA. This suggests the removal of tertiary structures, which may be related to the effects of DBP on enhanced NFI binding and chain elongation during Adenovirus DNA replication. Using purified proteins in an in vitro replication system, we correlate the structural changes with the effects of DBP on enhancement of NFI-binding as well as on DNA replication.

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.

A zinc-binding motif located between amino acids 273 and 286 in the adenovirus DNA-binding protein is necessary for ssDNA binding

The human adenovirus single-stranded (ss) DNA-binding protein (DBP) possesses a highly conserved carboxyl domain which contains a putative zinc-binding motif between amino acids (aa) 273 and 286. Using a zinc blotting technique DBP was shown to bind 65Zn at levels similar to other documented zinc metalloproteins. In competition experiments, DBP bound specifically to the zinc ion even in the presence of other divalent ions such as Ca+2, Mg+2, Cd+2, Co+2, and Mn+2. The zinc-binding ability of DBP was also confirmed by zinc affinity chromatography. Site-directed mutagenesis was utilized to construct a mutant which deleted the entire zinc region (pKZNdl 273-286) and a mutant which contained a Cys to Ser substitution at aa residue 284 (pKZNpt 284). The deletion mutant was unable to bind zinc, and the point mutant showed limited binding suggesting that aa 273-286 are responsible for the interaction of DBP with zinc. The DBP zinc mutants were also examined for their ability to bind to ssDNA. The deletion mutant was unable to bind ssDNA cellulose while the point mutant exhibited decreased affinity. Thus, the region between aa 273 and 286 which mediates zinc binding also appears fundamental for the ssDNA-binding function of DBP.

Functional changes in temperature-sensitive mutants of the adenovirus single-stranded DNA-binding protein are accompanied by structural alterations

Adenovirus requires the virus-encoded single-stranded DNA-binding protein (DBP) to replicate its DNA. We have previously shown (M. Tsuji, P. C. van der Vliet, and G. R. Kitchingman, J. Biol. Chem. 266:16178-16187, 1991) that the inability of three temperature-sensitive (ts) mutant DBPs (Ad2+ ND1ts23, Ad2ts111A, and Ad5ts125) to support DNA replication at the nonpermissive temperature was associated with impaired ability to bind to DNA. In this study, we examined these mutant proteins for structural alterations that might be linked to the functional changes. All three ts mutants, but not the wild-type protein, showed different proteolytic cleavage patterns before and after heating at 40 degrees C (the nonpermissive temperature), suggesting a possible conformational change during heating. The Ad2+ND1ts23 and Ad2ts111A DBPs have single amino acid changes located in a putative zinc finger subdomain (positions 282 and 280). In the presence of zinc ions, these ts mutants showed significantly increased resistance to inactivation at 40 degrees C. Surprisingly, however, the stabilizing effect of zinc was also observed with the Ad5ts125DBP, which contains a mutation located more than 100 amino acids from the zinc finger. Other related metal ions, such as cobalt, cadmium, and mercury, did not protect the ts DBPs from inactivation at 40 degrees C. These results indicate that functional changes of the ts DBPs in DNA replication and DNA binding are accompanied by structural alterations in the protein and that zinc and the metal-binding subdomain may play an important role in the structure and/or function of the DBP.

Regulation of N-myc gene expression: use of an adenovirus vector to demonstrate posttranscriptional control

We present evidence that differences in the levels of N-myc mRNA among different cell types are the result of posttranscriptional control. First, we noted that while steady-state mouse N-myc mRNA could be detected only in fetal mouse brain, it was transcribed at an equivalent rate in adult brain, liver, spleen, and placenta and in fetal brain. Similarly, the human N-myc gene was transcribed at an equivalent rate in HeLa cells, which do not accumulate this RNA in the cytoplasm, and cell lines G401 (a Wilms tumor-derived cell line) and SKNMc (established from a primitive neuroepithelioma), which do express N-myc RNA. As expected, the N-myc promoter functioned at equivalent rates, as demonstrated by the level of a reporter gene, when introduced into these cell types by using a recombinant adenovirus vector. The suggestion that posttranscriptional mechanisms control the level of this RNA was supported by the observation that sequences in the N-myc third exon specifically decreased the level of E1A mRNA when these sequences were placed downstream of the E1A promoter in a recombinant adenovirus. Finally, we further localized these sequences to a 600-bp fragment of the third exon by introducing various subclones of this sequence downstream of the E1A promoter in both viral and plasmid vectors.

Regulation of N-myc gene expression: use of an adenovirus vector to demonstrate posttranscriptional control

We present evidence that differences in the levels of N-myc mRNA among different cell types are the result of posttranscriptional control. First, we noted that while steady-state mouse N-myc mRNA could be detected only in fetal mouse brain, it was transcribed at an equivalent rate in adult brain, liver, spleen, and placenta and in fetal brain. Similarly, the human N-myc gene was transcribed at an equivalent rate in HeLa cells, which do not accumulate this RNA in the cytoplasm, and cell lines G401 (a Wilms tumor-derived cell line) and SKNMc (established from a primitive neuroepithelioma), which do express N-myc RNA. As expected, the N-myc promoter functioned at equivalent rates, as demonstrated by the level of a reporter gene, when introduced into these cell types by using a recombinant adenovirus vector. The suggestion that posttranscriptional mechanisms control the level of this RNA was supported by the observation that sequences in the N-myc third exon specifically decreased the level of E1A mRNA when these sequences were placed downstream of the E1A promoter in a recombinant adenovirus. Finally, we further localized these sequences to a 600-bp fragment of the third exon by introducing various subclones of this sequence downstream of the E1A promoter in both viral and plasmid vectors.

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

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

The adenovirus EII early promoter has multiple EIA-sensitive elements, two of which function cooperatively in basal and virus-induced transcription

The mechanism by which the adenovirus-encoded nuclear oncogene EIA activates transcription of several viral and host promoters is an important issue in the regulation of eucaryotic gene expression and virus-host cell interactions. Identification of cis-acting elements of the promoters and the cognate host transcription factors that are targets for EIA action is crucial for our understanding of the EIA-mediated control of coordinately regulated genes. The adenovirus EII early promoter has a complex architecture and contains two overlapping promoters with start sites at +1 (major promoter) and -26 (minor promoter). The major promoter responds strongly to virus-encoded trans activators EIA and EIV and contains four elements: a TAGA motif analogous to the TATA box, two EIIF sites present in an inverted orientation, and an ATF/CREB site. To determine precisely the roles played by these cis-acting elements in both basal and virus-induced transcription when the promoter is situated in its natural context, we investigated the phenotype of a series of linker scan promoter substitution mutants inserted into the viral chromosome. Promoter constructs harboring linker scan mutations in each element were rebuilt into a novel EIA- adenovirus vector, and transcriptional activity was monitored in virus-infected cells. In the absence of virus-encoded trans activators, basal activity in vivo was dependent on all four cis-acting elements. Surprisingly, a promoter mutant with only one of the two EIIF sites intact could not promote transcription in vivo, suggesting that the two EIIF sites function cooperatively even in basal transcription. Promoters harboring mutations in either of these two EIIF sites also failed to bind to an infection-specific form of EIIF in gel shift assays and competed only very weakly for EIIF binding with the wild-type promoter fragment. The dramatic cooperativity shown by the two inverted EIIF sites of the EII promoter both in vivo and in vitro could reflect simultaneous contact of both sites by the transcription factor EIIF. Furthermore, promoter mutants with mutations in the TAGA motif, the two EIIF sites, and the single ATF site all failed to respond to virus-encoded trans activators. Whereas recent results demonstrate that EIIF activity can be modulated independently by EIV, leading to transactivation of this promoter, our results and those published previously strongly indicate that the three different transcription factors that bind to TAGA, EIIF, and ATF motifs of the EII early promoter are all targets for EIA regulation in vivo. Thus, strong transactivation of the EII early promoter through these multiple EIA-sensitive elements and independently by the recently discovered EIV pathway suggests that the EII early promoter is stringently regulated in virus-infected cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The adenovirus DNA-binding protein stimulates the rate of transcription directed by adenovirus and adeno-associated virus promoters

The DNA-binding protein (DBP) encoded by the E2A region of adenovirus type 5 was found to enhance the expression of a reporter gene controlled by several different promoters within transfected cells. The rate of synthesis of correctly initiated transcripts was increased by the DBP. The adeno-associated virus P5 promoter and the adenovirus E1A and E2A early and major late promoters responded to the DBP by increases in expression ranging from 6- to 27-fold, while the adenovirus E4 promoter was slightly inhibited by DBP. The adenovirus major late promoter showed a greater response to DBP than to the E1A transactivator protein, suggesting that the DBP plays a central role in activation of the late promoter.

Sequences upstream of AAUAAA influence poly(A) site selection in a complex transcription unit

The adenovirus major late transcription unit (MLTU) encodes five colinear mRNA families, L1 through L5, each distinguished by a unique poly(A) site. Site selection is regulated during the course of infection, predominating early at the L1 site and late at the L2 through L5 sites. Two general mechanisms can be invoked to explain predominant usage of the L1 site early in infection. MLTU site selection may proceed in a first-come, first-serve manner whereby the L1 site is used most frequently because it is closest to the promoter. Alternatively, specific sequences flanking the L1 site may control predominant L1 site usage in a position-independent manner. To distinguish between these mechanisms, we constructed deletions in the L1 flanking sequences and inserted the mutated sites into either simple transcription units or mini-MLTUs encoding two poly(A) sites. The pattern of site selection for each construct was then quantitated by S1 nuclease analysis after transfection into 293 cells. The results indicated that L1 sequences upstream of AAUAAA define a novel selector element that can cause predominant L1 site usage at either position of a tandem transcription unit. The element did not significantly affect the stability or nucleocytoplasmic transport of L1 transcripts and was not required for efficient 3'-end processing in simple transcription units. Predominant L1 site usage required physical linkage of the processing signals and was independent of the major late promoter.

Mutations that affect phosphorylation of the adenovirus DNA-binding protein alter its ability to enhance its own synthesis

The multifunctional adenovirus single-strand DNA-binding protein (DBP) is highly phosphorylated. Its phosphorylation sites are located in the amino-terminal domain of the protein, and its DNA- and RNA-binding activity resides in the carboxy-terminal half of the polypeptide. We have substituted cysteine or alanine for up to 10 of these potential phosphorylation sites by using oligonucleotide-directed mutagenesis. Alteration of one or a few of these sites had little effect on the viability of virus containing the mutated DBP. However, when eight or more sites were altered, viral growth decreased significantly. This suggests that the overall phosphorylation state of the protein was more important than whether any particular site was modified. The reduction in growth correlated with both depressed DNA replication and expression of late genes. This reduction was probably the result of lower DBP accumulation in mutant-infected cells. Interestingly, although the stability of the mutated DBP was not affected, DBP synthesis and the level of its mRNA were depressed 5- to 10-fold for the underphosphorylated protein. These results suggest that DBP enhances its own expression and imply that phosphorylation of the DBP may be important for this function. Similarities to several eucaryotic transcriptional activators, which are composed of negatively charged activating domains and separate binding domains, are discussed.

Sequences upstream of AAUAAA influence poly(A) site selection in a complex transcription unit

The adenovirus major late transcription unit (MLTU) encodes five colinear mRNA families, L1 through L5, each distinguished by a unique poly(A) site. Site selection is regulated during the course of infection, predominating early at the L1 site and late at the L2 through L5 sites. Two general mechanisms can be invoked to explain predominant usage of the L1 site early in infection. MLTU site selection may proceed in a first-come, first-serve manner whereby the L1 site is used most frequently because it is closest to the promoter. Alternatively, specific sequences flanking the L1 site may control predominant L1 site usage in a position-independent manner. To distinguish between these mechanisms, we constructed deletions in the L1 flanking sequences and inserted the mutated sites into either simple transcription units or mini-MLTUs encoding two poly(A) sites. The pattern of site selection for each construct was then quantitated by S1 nuclease analysis after transfection into 293 cells. The results indicated that L1 sequences upstream of AAUAAA define a novel selector element that can cause predominant L1 site usage at either position of a tandem transcription unit. The element did not significantly affect the stability or nucleocytoplasmic transport of L1 transcripts and was not required for efficient 3'-end processing in simple transcription units. Predominant L1 site usage required physical linkage of the processing signals and was independent of the major late promoter.

Characterization of adenovirus type 5 insertion and deletion mutants encoding altered DNA binding proteins

We have introduced insertion and deletion mutations in the cloned DNA binding protein (DBP) gene of adenovirus type 5. The mutated DBP genes were subsequently introduced in the viral genome by a combination of in vitro and in vivo methods. The resulting mutant viruses were tested for their viability in human 293 cells and an initial characterization of these viruses was performed. Viable mutants with insertions in the carboxyl-terminal portion of the gene could not be obtained. In contrast, a number of viable mutants were constructed that contained insertions or deletions in the amino-terminal half of DBP. Several of these, which covered the region between amino acid (aa) residues 39 and 81, were phenotypically wild type, implying that this segment is completely dispensable for DBP function. However, mutations altering the region encompassed by aa 2-38 were, at least, partially defective suggesting that this region is important for full activity of the protein.

Isolation and characterization of a viable adenovirus mutant defective in nuclear transport of the DNA-binding protein

The isolation and characterization of an adenovirus mutant, Ad5dl802r1, containing two independent deletions in the 72-kilodalton (kDa) DNA-binding protein (DBP) gene is described. The two deletions remove amino acids 23 through 105 of DBP, resulting in the production of a 50-kDa product. Expression of this truncated DBP was delayed 12 to 24 h compared with that of the 72-kDa protein produced by wild-type adenovirus type 5. The DBP was located primarily in the cytoplasm of infected cells, whereas the wild-type product was predominantly nuclear. Therefore, DBP appears to contain a nuclear localization signal within the deleted region. Ad5dl802r1 DNA synthesis, viral late gene expression, and virus production were all delayed 12 to 24 h and were approximately 10-fold lower than with wild-type adenovirus type 5. These phenotypic properties can be accounted for by the delay in synthesis and the inefficient accumulation of the 50-kDa DBP within the nucleus of infected cells. The truncated DBP also lacks the majority of amino acids which are phosphorylated in the normal protein. The loss of these phosphorylation sites does not appear to seriously impair the ability of the protein to carry out its functions.

Estrogen-dependent activation of the avian very low density apolipoprotein II and vitellogenin genes. Transient alterations in mRNA polyadenylation and stability early during induction

Administration of estrogen to egg-laying vertebrates activates unscheduled, hepatic expression of major, egg-yolk protein genes in immature animals and mature males. Two avian yolk protein genes, encoding very low density apolipoprotein II (apoVLDLII) and vitellogenin II, are dormant prior to stimulation with estrogen, but within three days their cognate mRNAs accumulate to become two of the most abundant species in the liver. Accumulation of these mRNAs has been attributed to both induction of transcription and selective, estrogen-dependent mRNA stabilization. We have detected alterations in the size of apoVLDLII mRNA that occur during the first 24 hours that are attributable to a shift in the extent of polyadenylation as steady-state is approached. In vitro transcription assays indicate that primary activation of both genes takes place relatively slowly and that maximal rates of mRNA accumulation occur when the apoVLDLII and vitellogenin II genes are expressed at only 30% and 10% of their fully induced levels, respectively. Transcription data combined with the structural alteration of apoVLDLII mRNA suggest that stability of the two mRNAs may change as steady-state is approached. We have assessed the compatibility of this suggestion with earlier estimates of the kinetics of accumulation of both mRNAs by developing a generally useful algorithm that predicts approach to steady-state kinetics under conditions where both the rate of synthesis and mRNA stability change throughout the accumulation phase of the response. The results predict that the stability of both mRNAs decreases by at least two- to threefold during the approach to steady-state and that, although an additional destabilization of apoVLDLII mRNA may occur following withdrawal of estrogen, the steady-state stability of vitellogenin mRNA is not significantly decreased upon removal of hormone.

A subclass of the adenovirus 72K DNA binding protein specifically associating with the cytoskeletal framework of the plasma membrane

We have analyzed by immunofluorescence microscopy and by biochemical cell fractionation the subcellular distribution of the adenovirus type 2 72K DNA binding protein (DBP) during the course of infection in HeLa cells. Early in infection, the 72K DBP was strictly localized in the cell nucleus. However, as infection progressed, the 72K DBP was additionally found in other subcellular fractions, notably in association with the cytoskeletal framework of the plasma membrane, the plasma membrane lamina. Pulse-chase experiments demonstrated that this association was specific. Control experiments excluded the possibility of an artificial redistribution of the 72K DBP during cell fractionation. Our data, therefore, demonstrate that a significant portion of the 72K DBP during late times of infection associates specifically with the cytoskeletal framework of plasma membranes of infected cells.

Role of the adenovirus 72-kDa DNA binding protein in the rapid decay of early viral mRNA

Previous experiments have shown that the early adenovirus E1A and E1B mRNAs decay with a half-life of 20 min in a lytic infection dependent on the action of the viral 72-kDa DNA binding protein. In contrast, the same E1A and E1B mRNAs are stable when synthesized in 293 cells, an adenovirus-transformed cell line that is devoid of the 72-kDa protein. If 293 cells are infected with the E1A deletion mutant dl312, the endogenous E1A RNA disappears after 4 hr of infection, a time coincident with the appearance of the 72-kDa protein. The induction of decay is specific since there is no decrease in the level of actin or certain other cellular mRNAs. Thus, the stability of the early RNAs is variable and correlates with the presence of the 72-kDa protein. An interaction of the 72-kDa DNA binding protein with RNA inside the cell has been demonstrated by in vivo crosslinking of protein to RNA. However, the protein is found in association with actin mRNA as well as E1A mRNA. Thus, although the 72-kDa protein appears to be required for the rapid decay of viral mRNA it apparently does not impart specificity to the process.

Delineation of DNA sequences that are important for in vitro transcription from the adenovirus EIIa late promoter

Late in infection, transcription of the EIIa gene is initiated primarily at map unit 72 of the adenovirus genome. A cell-free nuclear extract system was used to determine sequence elements important for the function of this late promoter. In such a system, the transcriptional activity of a circular template was found to be much higher (5- to 10-fold) than that of a linear template. The effect of template topology appeared to be dependent on two distal upstream elements with 5' boundaries located near -265 to -223 and -147 to -133 (in relation to the initiation site), since deletions of these regions reduced transcription of the circular template, in a stepwise fashion, to a level similar to that observed with the linear template. Further deletions revealed an element in the -116 region that appeared to be more important for transcription of the circular template (10-fold reduction) than for transcription of the linear template (3-fold reduction). Lastly, deletion of the TACAAA sequence in the -29 region resulted in further reduction in transcription, indicating that this element functions as a promoter in vitro.

Physical organization of the enteric adenovirus type 41 early region 1A

Enteric adenovirus types 40 and 41 (Ad40 and Ad41), representing subgenus F, differ from all other human adenoviruses by being so fastidious that productive replication does not occur in conventional established cell lines. They are dependent of the Ad5 early regions E1A and E1B since they can not grow in HEK cells, only in 293 HEK cells transformed by Ad5 E1. The overall genetic organization of Ad41 E1A is similar to the E1A region of other characterized human adenoviruses but it is slightly shorter, comprising 1350 bp. The inverted terminal repeat (ITR) at the 5' end of both Ad40 and AD41 consists of 163 nucleotides, being similar to the ITR of Ad12 (subgenus A) and longer than the ITRs of adenoviruses of subgenera B, C, and E. The early mRNA products (12 and 13 S) can be translated into a 222-amino acid (aa) and a 251-aa tentative protein, respectively. In a comparison of the Ad41 251-aa protein with corresponding peptides of Ad12, Ad7, Ad5, and Ad4, three conserved amino acid sequences CS1-CS3 can be found. In the second conserved domain CS2, which is particularly acidic, the homology is very high within all five serotypes compared. Only one among eight conserved amino acids differs in the Ad41 251-aa protein. Within CS1 and CS3 which exhibit a hydrophilic and a hydrophobic character, respectively, the amino acid composition of the Ad41 protein is less conserved than the corresponding regions in all other analyzed adenovirus types. Ten of 16 conserved amino acids in CS1 are shared by Ad41 and 18 of 23 conserved amino acids in CS3 are shared by Ad41.

Induced heat shock mRNAs escape the nucleocytoplasmic transport block in adenovirus-infected HeLa cells

Under conditions in which cytoplasmic accumulation of HeLa cell mRNAs has been blocked by adenovirus infection, hsp70 family mRNAs are transported from the nucleus to the cytoplasm at near normal efficiency subsequent to heat shock. Heat shock does not reverse the general virus-induced block to host cell mRNA transport. The heat shock mRNAs are translated within the cytoplasm of the infected cell but at substantially reduced efficiency compared with that of uninfected cells. Thus, the hsp70 family of mRNAs can escape the transport block but not the translational block instituted late after adenovirus infection. The beta-tubulin gene family is induced by the viral E1A gene after infection, and its mRNAs also accumulate in the cytoplasmic compartment. Given these two examples, it seems likely that the process of transcriptional induction allows the resulting mRNA to escape the viral block of transport.

The adenovirus tripartite leader sequence can alter nuclear and cytoplasmic metabolism of a non-adenovirus mRNA within infected cells

All mRNAs encoded by the adenovirus major late transcription unit share a common 5' noncoding region, 200 nucleotides in length, termed the tripartite leader sequence. To assess function of the tripartite leader, recombinant viruses were prepared which carried either a bona fide herpes simplex virus thymidine kinase gene or a modified thymidine kinase gene whose normal 5' noncoding domain was replaced with the adenovirus leader sequence. The tripartite leader simultaneously decreased the nuclear half-life and increased the cytoplasmic half-life of the thymidine kinase-specific mRNA. The tripartite leader stabilized the non-adenovirus mRNA only within the environment of an adenovirus-infected cell during the late phase of the infectious cycle.

The genes encoding the DNA binding protein and the 23K protease of adenovirus types 40 and 41

The adenovirus (Ad) single-stranded DNA binding protein (DBP) is a multifunctional protein. It is thought to consist of two domains, the amino-terminal domain involved in host-range determination and the carboxyl-terminal domain functioning in DNA replication and DNA binding. We have determined the nucleotide sequences of the DBP genes of Ad40 and Ad41, two human adenoviral serotypes that differ significantly from other adenoviruses. Regions of structural and functional importance in the corresponding proteins could be identified by comparison of the amino acid sequences with those of other known DBPs. In addition, the nucleotide sequences of the DBP early promoters, of the 23K protease genes, and of parts of the hexon and 100K protein genes have been determined. It can be deduced from the nucleotide sequences, that the Ad40 and Ad41 DBPs are relatively small (473 and 474 amino acids (a.a.), respectively, versus 529 a.a. for the Ad5 DBP). This is caused by the presence of very small amino-terminal domains of 119 a.a. (Ad40) and 120 a.a. (Ad41), as compared to 173 a.a. for the corresponding Ad5 domain. Only a few amino acids in this domain have been conserved in all known DBPs. The carboxyl-terminal domains show a higher degree of sequence conservation. In this domain, four strongly conserved regions can be identified, one of which might form a metal-binding site. The 23K proteases of both Ad40 and Ad41 show a strong homology to the Ad2 and Ad5 proteins, with the exception of the carboxyl-terminal end of the proteins. The 23K protease gene of Ad41 has an open reading frame that extends beyond the polyadenylation signal, in contrast to the Ad40 gene that ends well in front of the signal.

Induced heat shock mRNAs escape the nucleocytoplasmic transport block in adenovirus-infected HeLa cells

Under conditions in which cytoplasmic accumulation of HeLa cell mRNAs has been blocked by adenovirus infection, hsp70 family mRNAs are transported from the nucleus to the cytoplasm at near normal efficiency subsequent to heat shock. Heat shock does not reverse the general virus-induced block to host cell mRNA transport. The heat shock mRNAs are translated within the cytoplasm of the infected cell but at substantially reduced efficiency compared with that of uninfected cells. Thus, the hsp70 family of mRNAs can escape the transport block but not the translational block instituted late after adenovirus infection. The beta-tubulin gene family is induced by the viral E1A gene after infection, and its mRNAs also accumulate in the cytoplasmic compartment. Given these two examples, it seems likely that the process of transcriptional induction allows the resulting mRNA to escape the viral block of transport.

Adenovirus proteins associated with mRNA and hnRNA in infected HeLa cells

The proteins that interact with cytoplasmic and nuclear polyadenylated RNA in adenovirus type 5 (Ad5) infection of HeLa cells were examined by UV-induced RNA-protein cross-linking in intact cells. The Ad5 100-kilodalton late nonvirion protein (100K protein) was cross-linked to both host and viral polyadenylated cytoplasmic RNA (mRNA). The cross-linking of the 100K protein to mRNA appears to correlate with productive infection, because the protein is not cross-linked to mRNA in abortive infection of wild-type Ad5 in monkey cells (CV-1) even though normal amounts of it are produced. However, when CV-1 cells are infected with Ad5 hr404, and Ad5 mutant which overcomes the host restriction to wild-type Ad5 infection in these cells, the 100K protein is cross-linked to mRNA. To identify and obtain antibodies to RNA-contacting proteins, a mouse was immunized with oligo(dT)-selected cross-linked RNA-protein complexes from Ad5-infected cells and the serum was used for immunoblotting experiments. It was found that in addition to the 100K protein, the Ad5 72K DNA-binding protein is also associated with RNA in the infected cells. The 72K DNA-binding protein is cross-linked to polyadenylated nuclear RNA sequences. These findings indicate that adenovirus proteins interact with RNAs in the infected cell and suggest possible mechanisms for the effects of the virus on mRNA metabolism.

Herpes simplex virus virion stimulatory protein mRNA leader contains sequence elements which increase both virus-induced transcription and mRNA stability

To investigate the role of 5' noncoding leader sequence of herpes simplex virus type 1 (HSV-1) mRNA in infected cells, the promoter for the 65,000-dalton virion stimulatory protein (VSP), a beta-gamma polypeptide, was introduced into plasmids bearing the chloramphenicol acetyltransferase (cat) gene together with various lengths of adjacent viral leader sequences. Plasmids containing longer lengths of leader sequence gave rise to significantly higher levels of CAT enzyme in transfected cells superinfected with HSV-1. RNase T2 protection assays of CAT mRNA showed that transcription was initiated from an authentic viral cap site in all VSP-CAT constructs and that CAT mRNA levels corresponded to CAT enzyme levels. Use of cis-linked simian virus 40 enhancer sequences demonstrated that the effect was virus specific. Constructs containing 12 and 48 base pairs of the VSP mRNA leader gave HSV infection-induced CAT activities intermediate between those of the leaderless construct and the VSP-(+77)-CAT construct. Actinomycin D chase experiments demonstrated that the longest leader sequences increased hybrid CAT mRNA stability at least twofold in infected cells. Cotransfection experiments with a cosmid bearing four virus-specified transcription factors (ICP4, ICP0, ICP27, and VSP-65K) showed that sequences from -3 to +77, with respect to the viral mRNA cap site, also contained signals responsive to transcriptional activation.