Regulation of the appearance of cytoplasmic RNAs from region 1 of the adenovirus 2 genome

Heat Shock Protein 90 Chaperones E1A Early Protein of Adenovirus 5 and Is Essential for Replication of the Virus

Adenovirus infections tend to be mild, but they may pose a serious threat for young and immunocompromised individuals. The treatment is complicated because there are no approved safe and specific drugs for adenovirus infections. Here, we present evidence that 17-(Allylamino)-17-demethoxygeldanamycin (17-AAG), an inhibitor of Hsp90 chaperone, decreases the rate of human adenovirus 5 (HAdV-5) replication in cell cultures by 95%. 17-AAG inhibited the transcription of early and late genes of HAdV-5, replication of viral DNA, and expression of viral proteins. 6 h after infection, Hsp90 inhibition results in a 6.3-fold reduction of the newly synthesized E1A protein level without a decrease in the E1A mRNA level. However, the Hsp90 inhibition does not increase the decay rate of the E1A protein that was constitutively expressed in the cell before exposure to the inhibitor. The co-immunoprecipitation proved that E1A protein interacted with Hsp90. Altogether, the presented results show, for the first time. that Hsp90 chaperones newly synthesized, but not mature, E1A protein. Because E1A serves as a transcriptional co-activator of adenovirus early genes, the anti-adenoviral activity of the Hsp90 inhibitor might be explained by the decreased E1A level.

Cell-to-cell and genome-to-genome variability of adenovirus transcription tuned by the cell cycle

In clonal cultures, not all cells are equally susceptible to virus infection, and the mechanisms underlying this are poorly understood. Here, we developed image-based single-cell measurements to scrutinize the heterogeneity of adenovirus (AdV) infection. AdV delivers, transcribes and replicates a linear double-stranded DNA genome in the nucleus. We measured the abundance of viral transcripts using single-molecule RNA fluorescence in situ hybridization (FISH) and the incoming 5-ethynyl-2'-deoxycytidine (EdC)-tagged viral genomes using a copper(I)-catalyzed azide-alkyne cycloaddition (click) reaction. Surprisingly, expression of the immediate early gene E1A only moderately correlated with the number of viral genomes in the cell nucleus. Intranuclear genome-to-genome heterogeneity was found at the level of viral transcription and, in accordance, individual genomes exhibited heterogeneous replication activity. By analyzing the cell cycle state, we found that G1 cells exhibited the highest E1A gene expression and displayed increased correlation between E1A gene expression and viral genome copy numbers. The combined image-based single-molecule procedures described here are ideally suited to explore the cell-to-cell variability in viral gene expression in a range of different settings, including the innate immune response.

Characterization of the 55-residue protein encoded by the 9S E1A mRNA of species C adenovirus

Early region 1A (E1A) of human adenovirus (HAdV) has been the focus of over 30 years of investigation and is required for the oncogenic capacity of HAdV in rodents. Alternative splicing of the E1A transcript generates mRNAs encoding multiple E1A proteins. The 55-residue (55R) E1A protein, which is encoded by the 9S mRNA, is particularly interesting due to the unique properties it displays relative to all other E1A isoforms. 55R E1A does not contain any of the conserved regions (CRs) present in the other E1A isoforms. The C-terminal region of the 55R E1A protein contains a unique sequence compared to all other E1A isoforms, which results from a frameshift generated by alternative splicing. The 55R E1A protein is thought to be produced preferentially at the late stages of infection. Here we report the first study to directly investigate the function of the species C HAdV 55R E1A protein during infection. Polyclonal rabbit antibodies (Abs) have been generated that are capable of immunoprecipitating HAdV-2 55R E1A. These Abs can also detect HAdV-2 55R E1A by immunoblotting and indirect immunofluorescence assay. These studies indicate that 55R E1A is expressed late and is localized to the cytoplasm and to the nucleus. 55R E1A was able to activate the expression of viral genes during infection and could also promote productive replication of species C HAdV. 55R E1A was also found to interact with the S8 component of the proteasome, and knockdown of S8 was detrimental to viral replication dependent on 55R E1A.

Adenovirus type 5 early region 1B 156R protein promotes cell transformation independently of repression of p53-stimulated transcription

Early region 1B (E1B) of adenovirus type 5 (Ad5) encodes at least five different polypeptides generated by alternative splicing of a common mRNA precursor. Two of these gene products, E1B-19K and E1B-55K, are individually capable of cooperating with the Ad5 E1A proteins to completely transform rodent cells in culture. Substantial evidence suggests that these two E1B proteins contribute to cell transformation by antagonizing growth arrest and apoptosis. Here, we performed genetic and biochemical analyses to assess the attributes of the remaining E1B proteins (E1B-156R, E1B-93R, and E1B-84R). Our results show that E1B-156R, which comprises the 79 amino-terminal and 77 carboxy-terminal amino acids of E1B-55K, also enhances focal transformation of primary rat cells in cooperation with E1A. Since E1B-156R seemed unable to relocalize p53 and inhibit its transactivating function, it must be assumed that it contributes to transformation independently of repression of p53-stimulated transcription. Furthermore, we discovered that E1B-156R contains a functional transcriptional repression domain and binds Ad5 E4orf6 and the cellular apoptosis regulator Daxx. While the ability to bind E4orf6 could indicate further biological functions of E1B-156R in viral infection, the interaction with Daxx might also be linked to its transforming potential. Taken together, these analyses introduce E1B-156R as a novel transformation-promoting E1B protein that acts without repressing p53 transactivation. Moreover, identification of the interaction partners E4orf6 and Daxx provides a first glance of E1B-156R's potential functions.

Remodelling of the host cell RNA splicing machinery during an adenovirus infection

Adenovirus makes extensive use of RNA splicing to produce a complex set of spliced mRNAs during virus replication. All transcription units, except pIX and IVa2, encode multiple alternatively spliced mRNAs. The accumulation of viral mRNAs is subjected to a temporal regulation, a mechanism that ensures that proteins that are needed at certain stages of the viral life cycle are produced. The complex interaction between host cell RNA splicing factors and viral regulatory elements has been studied intensely during the last decade. Such studies have begun to produce a picture of how adenovirus remodels the host cell RNA splicing machinery to orchestrate the shift from the early to the late profile of viral mRNA accumulation. Recent progress has to a large extent focused on the mechanisms regulating E1A and L1 alternative splicing. Here we will review the current knowledge of cis-acting sequence element, trans-acting factors and mechanisms controlling E1A and L1 alternative splicing.

The A1 and A1B proteins of heterogeneous nuclear ribonucleoparticles modulate 5' splice site selection in vivo

Recent in vitro results suggest that the heterogeneous nuclear ribonucleoparticle (hnRNP) A1 protein modulates alternative splicing by favoring distal 5' splice site (5'SS) selection and exon skipping. We used a mouse erythroleukemia (MEL) cell line (CB3C7) deficient in the expression of hnRNP A1 to test whether variations in hnRNP A1 and AlB protein levels affected alternative splicing in vivo. In contrast to A1-expressing MEL cell lines, CB3C7 cells preferentially selected the proximal 13S and 12S 5'SS on the adenovirus E1A pre-mRNA. Transiently expressing the A1 or A1B cDNA in CB3C7 cells shifted 5'SS selection toward the more distal 9S donor site. A1 protein synthesis was required for this effect since the expression of a mutated A1 cDNA did not affect 5'SS selection. These results demonstrate that in vivo variations in hnRNP A1 protein levels can influence 5'SS selection.

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.

The E1B transcription map of the enteric adenovirus type 41

Enteric adenovirus type 41 (Ad41) is defective for growth in conventional established cell lines. Ad41 is dependent on the Ad5 early regions E1A/E1B since it cannot grow in HEK cells but only in 293 HEK cells transformed by Ad5 E1 region. However, Hep-2 cells have also been shown to support the growth of Ad41 to some extent. The nucleotide sequence of the E1B region of the Ad41 strain D389 has been determined. When compared to the corresponding region of the Ad41 prototype strain (Tak) the degree of homology in the DNA sequences was close to 100%. The mRNAs from the E1B region of the Ad41 strain D389 have been studied by Northern blot, primer extension, and polymerase chain reaction-cDNA analysis. E1B transcripts corresponding to Ad2 14 S, 22 S, and 9 S mRNAs were identified but no 13 S mRNA equivalent was detected, a pattern similar to that seen in the Ad40 and Ad12 transcription maps. However, the Ad41 E1B 14S mRNA equivalent has one additional small exon of 23 nucleotides, created by a donor and an acceptor splice site located at positions not seen in other E1B transcripts of human adenoviruses analyzed so far. The coding potential for E1B 19K, 55K, and 15K proteins and for pIX is retained in the Ad41 transcripts. In contrast to other adenoviruses, except for the closely related Ad40, the ORF of pIX starts in the intron of the 22 S mRNA.

Regulation of alternative pre-mRNA splicing by hnRNP A1 and splicing factor SF2

When messenger RNA precursors (pre-mRNAs) containing alternative 5' splice sites are spliced in vitro, the relative concentrations of the heterogeneous ribonucleoprotein (hnRNP) A1 and the essential splicing factor SF2 precisely determine which 5' splice site is selected. In general, an excess of hnRNP A1 favors distal 5' splice sites, whereas an excess of SF2 results in utilization of proximal 5' splice sites. The regulation of these antagonistic activities may play an important role in the tissue-specific and developmental control of gene expression by alternative splicing.

A novel protein factor is required for use of distal alternative 5' splice sites in vitro

Adenovirus E1A pre-mRNA was used as a model to examine alternative 5' splice site selection during in vitro splicing reactions. Strong preference for the downstream 13S 5' splice site over the upstream 12S or 9S 5' splice sites was observed. However, the 12S 5' splice site was used efficiently when a mutant pre-mRNA lacking the 13S 5' splice site was processed, and 12S splicing from this substrate was not reduced by 13S splicing from a separate pre-mRNA, demonstrating that 13S splicing reduced 12S 5' splice site selection through a bona fide cis-competition. DEAE-cellulose chromatography of nuclear extract yielded two fractions with different splicing activities. The bound fraction contained all components required for efficient splicing of simple substrates but was unable to utilize alternative 5' splice sites. In contrast, the flow-through fraction, which by itself was inactive, contained an activity required for alternative splicing and was shown to stimulate 12S and 9S splicing, while reducing 13S splicing, when added to reactions carried out by the bound fraction. Furthermore, the activity, which we have called distal splicing factor (DSF), enhanced utilization of an upstream 5' splice site on a simian virus 40 early pre-mRNA, suggesting that the factor acts in a position-dependent, substrate-independent fashion. Several lines of evidence are presented suggesting that DSF is a non-small nuclear ribonucleoprotein protein. Finally, we describe a functional interaction between DSF and ASF, a protein that enhances use of downstream 5' splice sites.

A novel protein factor is required for use of distal alternative 5' splice sites in vitro

Adenovirus E1A pre-mRNA was used as a model to examine alternative 5' splice site selection during in vitro splicing reactions. Strong preference for the downstream 13S 5' splice site over the upstream 12S or 9S 5' splice sites was observed. However, the 12S 5' splice site was used efficiently when a mutant pre-mRNA lacking the 13S 5' splice site was processed, and 12S splicing from this substrate was not reduced by 13S splicing from a separate pre-mRNA, demonstrating that 13S splicing reduced 12S 5' splice site selection through a bona fide cis-competition. DEAE-cellulose chromatography of nuclear extract yielded two fractions with different splicing activities. The bound fraction contained all components required for efficient splicing of simple substrates but was unable to utilize alternative 5' splice sites. In contrast, the flow-through fraction, which by itself was inactive, contained an activity required for alternative splicing and was shown to stimulate 12S and 9S splicing, while reducing 13S splicing, when added to reactions carried out by the bound fraction. Furthermore, the activity, which we have called distal splicing factor (DSF), enhanced utilization of an upstream 5' splice site on a simian virus 40 early pre-mRNA, suggesting that the factor acts in a position-dependent, substrate-independent fashion. Several lines of evidence are presented suggesting that DSF is a non-small nuclear ribonucleoprotein protein. Finally, we describe a functional interaction between DSF and ASF, a protein that enhances use of downstream 5' splice sites.

Modulation of alternative splicing of adenoviral E1A transcripts: factors involved in the early-to-late transition

The E1A pre-mRNA of adenovirus is spliced into three mRNA species (13S, 12S, and 9S mRNAs) by the use of three alternative 5'-splice sites. The 13S and 9S mRNAs predominate during the early and late periods of infection, respectively. With HeLa nuclear extracts isolated in early and late periods of infection, we were able to reproduce a 13S-9S modulation that resembles that occurring in infected cells. An in vitro analysis of the cis-acting parameters involved in the 13S-9S switch indicates that the 13S mRNA splicing inhibition is one of the first events of the late period and leads to the subsequent stimulation of the 9S mRNA reaction. The new abilities of the late nuclear extract for the 9S mRNA reaction were also confirmed by analyzing splicing of a major late transcript containing leaders 1 and 2 separated by the wild-type intervening sequence (IVS) of 1021 nucleotides. Complementation experiments show that the trans-acting factor(s) are micrococcal nuclease sensitive. They were partially characterized by induction experiments, and we show that the primary factors responsible for the 13S-9S modulation in vitro are viral RNAs of high molecular weight that accumulate late in infection. We postulate that the splicing modulation of E1A pre-mRNA results from an indirect mode of action for these viral RNAs, based on a sequestration of common splicing factors that are not present in vast excess in HeLa cells.

Expression and interactions of human adenovirus oncoproteins

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Isolation and characterization of novel adenovirus type 12 E1A mRNAs by cDNA PCR technique

The technique of cDNA polymerase chain reaction was used to study the heterogeneity of adenovirus type 12 early region 1A mRNAs in infected and transformed cells. In addition to 13 S and 12 S mRNAs, three transcripts of 10 S, 9.5 S, and 9 S could be isolated from infected HeLa cells. A fourth transcript of 11 S was found in transformed cells. The 9 S mRNA is generated by removing one intron (nucleotide (nt) 589 to nt 1143) from the RNA precursor. For the 11 S and 10 S mRNAs the primary transcript is spliced twice. The first splicing event removes a common intron from nt 589 to nt 715; the second splicing event removes introns deleted also during the processing of either the 13 S mRNA (leading to the 11 S mRNA) or the 12 S mRNA (leading to the 10 S mRNA). The 9.5 S mRNA is also generated by removing two introns (first splice junction: nt 588/852; the second splice junction is identical to that of the 12 S mRNA). In comparison to the 13 S and 12 S transcripts, the four smaller mRNAs alter the translational reading frames with the beginning of their second exons. In vitro translation of these mRNAs resulted in protein products with molecular weights of 10,100 (11 S and 10 S mRNAs), 10,500 (9.5 S mRNA), and 6200 (9 S mRNA) kDa.

Two-step transformation of rat 3Y1 cells by the adenovirus E1A and E1B genes

The transformation of rodent cells by the adenovirus E1A and E1B genes was very efficient when these genes were physically linked. When they were cleaved, the transformation became very inefficient. To clarify this difference, the chimeric E1B genes in which either the adenovirus enhancer or the human beta-actin promoter was linked to the 5' side of the E1B gene were introduced into rat 3Y1 cells. The saturation density of these cell lines (eB or APrB) was similar to that of parental 3Y1 cells. When eB or APrB cell lines were supertransfected with the E1A gene, discrete dense foci were developed after 5-6 weeks, while the supertransfection of 3Y1 derivative cell lines, in which the enhancer-unlinked E1B gene was introduced, did not develop any dense foci. Analysis of the E1A and E1B transcripts in these cell lines indicated that the E1B gene is efficiently expressed in the presence of the E1A gene products if the enhancer is linked to the E1B gene and that an increased level of E1B proteins is required for an efficient expression of the E1A gene. These results indicated that E1A and E1B genes in separate pieces of DNA are capable of cooperatively transforming 3Y1 cells if appropriate cis-acting elements are attached and high-level expressions are achieved.

Intranuclear location of the adenovirus type 5 E1B 55-kilodalton protein

The intracellular location of the adenovirus type 5 E1B 55-kilodalton (kDa) protein, particularly the question of whether it is associated with nuclear pore complexes, was examined. Fractionation of adenovirus type 5-infected HeLa cell nuclei by an established procedure (N. Dwyer and G. Blobel, J. Cell. Biol. 70:581-591, 1976) yielded one population of E1B 55-kDa protein molecules released by digestion of nuclei with RNase A and a second population recovered in the pore complex-lamina fraction. Free and E1B 55-kDa protein-bound forms of the E4 34-kDa protein (P. Sarnow, C. A. Sullivan, and A. J. Levine, Virology 120:387-394, 1982) were largely recovered in the pore complex-lamina fraction. Nevertheless, the association of E1B 55-kDa protein molecules with this nuclear envelope fraction did not depend on interaction of the E1B 55-kDa protein with the E4 34-kDa protein. Comparison of the immunofluorescence patterns observed with antibodies recognizing the E1B 55-kDa protein or cellular pore complex proteins and of the behavior of these viral and cellular proteins during in situ fractionation suggests that the E1B 55-kDa protein does not become intimately or stably associated with pore complexes in adenovirus-infected cells.

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.

cis-dominant defect in activation of adenovirus type 5 E1b early RNA synthesis

A cis-dominant mutation in the adjacent E1a gene disrupted the accumulation of adenovirus type 5 E1b mRNA during the early phase of infection. Steady-state levels of cytoplasmic and nuclear E1b RNAs in cells infected with dl312, a strain that lacks the E1a TATA box, cap site, and much of the coding sequence, were reduced 5- to 10-fold even when the E1a activator was provided in trans. The strain was defective for early E1b RNA synthesis but not for E1b RNA made late or during prolonged incubation in the presence of an inhibitor of DNA replication. The defect in E1b RNA synthesis could not be attributed to the E1a promoter sequences missing in dl312 DNA. If the E1a protein-coding region contains cis-acting regulatory sequences, they are not part of the previously mapped E1b transcriptional control region and may represent additional regulatory elements that ensure prompt and efficient E1b expression during the early phase of infection.

The adenovirus E1B 19-kilodalton protein stimulates gene expression by increasing DNA levels

In transient expression assays, the adenovirus E1B 19-kilodalton (19K) tumor antigen increases expression from viral promoters and the promoter for the cellular 70-kilodalton heat shock protein (hsp70). To study the mechanism of this effect, we constructed HeLa cell lines that contain stably integrated copies of the 19K gene. Compared with a 19K- control cell line, 19K+ cells produced a significantly higher level of expression from every promoter introduced into the cells by transfection. The 19K protein also increased expression of an RNA polymerase III-transcribed gene but did not affect the level of expression of the endogenous hsp70 gene. The rate of transcription from transfected promoters, as measured by a nuclear run-on assay, was higher in the 19K+ cells than in the 19K- control cells. Furthermore, the level of plasmid DNA remained higher in the 19K+ cell line, suggesting that the 19K protein stabilizes transfected plasmid DNA. The elevated DNA levels seemed to account in full for the increased transcription. The role of the 19K protein in increasing gene expression during viral infection was found to be due to a replication-dependent increase in viral DNA levels. Thus, the 19K protein activates transcription indirectly by producing a higher level of viral or plasmid DNA. The DNA stabilization function of the 19K protein is probably related to the protective role of the 19K protein during viral infection and represents the first example of a viral oncogene product that modulates gene expression by regulating viral and plasmid DNA levels.

The adenovirus E1B 19-kilodalton protein stimulates gene expression by increasing DNA levels

In transient expression assays, the adenovirus E1B 19-kilodalton (19K) tumor antigen increases expression from viral promoters and the promoter for the cellular 70-kilodalton heat shock protein (hsp70). To study the mechanism of this effect, we constructed HeLa cell lines that contain stably integrated copies of the 19K gene. Compared with a 19K- control cell line, 19K+ cells produced a significantly higher level of expression from every promoter introduced into the cells by transfection. The 19K protein also increased expression of an RNA polymerase III-transcribed gene but did not affect the level of expression of the endogenous hsp70 gene. The rate of transcription from transfected promoters, as measured by a nuclear run-on assay, was higher in the 19K+ cells than in the 19K- control cells. Furthermore, the level of plasmid DNA remained higher in the 19K+ cell line, suggesting that the 19K protein stabilizes transfected plasmid DNA. The elevated DNA levels seemed to account in full for the increased transcription. The role of the 19K protein in increasing gene expression during viral infection was found to be due to a replication-dependent increase in viral DNA levels. Thus, the 19K protein activates transcription indirectly by producing a higher level of viral or plasmid DNA. The DNA stabilization function of the 19K protein is probably related to the protective role of the 19K protein during viral infection and represents the first example of a viral oncogene product that modulates gene expression by regulating viral and plasmid DNA levels.

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.

Organization of the transcriptional control region of the E1b gene of adenovirus type 5

Genetic analysis of the transcriptional control sequences of the E1b gene of adenovirus type 5 identified two regions that stimulated specific transcription by whole cell extracts from uninfected cells. The first region, located within 50 nucleotides (position -50) 5' to the transcription initiation (cap) site, contains a G+C-rich consensus-binding site (GC box) for the transcription factor Sp1 and a TATA box. Unambiguous stimulatory activity of the second region, between positions -358 and -127, was observed only in the absence of the GC box. DNase I protection experiments (footprinting) with crude nuclear extracts from uninfected cells revealed multiple DNA-protein interactions at the control region. Proximal to the initiation site, both the GC box and the cap site were protected; however, protection of the TATA box was not observed. In the distal region, four protein-binding sites, designated I through IV, were located between positions -250 and -120. Three of the four mapped in protein-coding sequences of the adjacent E1a gene. Sites I and II were 5' to position -218 whereas sites III and IV were 3' to position -218. This finding was consistent with results of the transcriptional analysis indicating that subsets of the distal region were sufficient for stimulation of transcription in vitro in the absence of the GC box. Within the boundaries of site I, a 10-base-pair protected sequence was similar to one located 5' to the adenovirus E1a, E2a, E3, E4, E2 late, and polypeptide IX transcription initiation sites. Sequences within the boundaries of the other three sites were similar to those within other viral and cellular enhancers.

Detailed kinetics of adenovirus type-5 steady-state transcripts during early infection

The appearance and steady-state accumulation of specific viral RNAs during the early phase of adenovirus type 5 (Ad5) infection was examined. HeLa cells were synchronously infected and harvested at 30 min intervals throughout the first 12 h of infection. Total cytoplasmic RNA was extracted from infected cells and analyzed by hybridization-selection and translation to identify the viral mRNAs from each early region on the basis of the protein products they encode. The same RNA samples were used for S-1 nuclease and Northern blot analyses to quantitatively compare the levels of individual viral RNAs that accumulate within each early transcription region (E1A, E1B, L1, E2A, E3 and E4). The salient features of this analysis show that RNA accumulation occurs first from E1A followed by E2A, E3 and E4, E1B and lastly, L1. Although the profile of RNA accumulation was unique for each early region, overlapping RNAs within E1A, E3, and E4, respectively, remained generally parallel to one another throughout early infection, in contrast to RNAs from E1B and L1, respectively. Since both the appearance and quantitative accumulation of specific early viral mRNAs were examined at many time points, a number of subtleties associated with the complex dynamics of early Ad5 gene expression were revealed. In particular, the L1 region was shown to transcribe from the major late promoter two early RNAs of 3.81 Kb and 3.5 Kb, either or both of which encode the 52,55 kDa proteins; the auxiliary i leader sequence was found on the 3.81 Kb RNA but not on the 3.5 Kb RNA.

Selective induction of human heat shock gene transcription by the adenovirus E1A gene products, including the 12S E1A product

We have previously shown that the human 70-kilodalton heat shock protein gene (hsp70) is induced by the adenovirus E1A gene product and during the S-G2 phase of the cell cycle. In this study, we investigated the effect of E1A on the expression of other human hsp genes. A gene encoding one form of the hsp89 protein (hsp89 alpha) was activated during an adenovirus infection with kinetics similar to those of activation of hsp70. The induction required a functional E1A gene. However, the hsp89 transcript was not cell cycle regulated. Genes encoding another form of hsp89 and the hsp27 protein were not induced by E1A or during the cell cycle. Further examination of hsp70 expression revealed a greater complexity than previously seen. S1 nuclease analysis using an hsp70 cDNA as well as a distinct hsp70 genomic clone demonstrated three related hsp70 transcripts; two were induced by E1A, and one was not. Both of the E1A-inducible genes were regulated during the cell cycle. All three were induced by heat shock. These results suggest common aspects of control among certain members of this family of cellular genes distinct from heat shock control. Finally, using viruses that express the individual E1A proteins, we found that the hsp70 gene is induced by the 12S and the 13S E1A products. The efficiency of induction by the 12S product was somewhat less than that by the 13S product but only by a factor of less than 2. This is in contrast to the induction of early viral genes, for which the 13S product is considerably more efficient than the 12S product.

Participation of two human cytomegalovirus immediate early gene regions in transcriptional activation of adenovirus promoters

The participation of human cytomegalovirus (HCMV) immediate early genes in the activation of the expression of adenovirus genes in trans (trans-activation) was examined. The initial strategy used was to determine the ability of HCMV genes to complement mutants of adenovirus E1a, an immediate early gene which encodes a trans-activator. The HCMV immediate early gene regions IE1 and IE2 complemented E1a-deficient mutants in three separate assays. IE1 and IE2 substituted for E1a in the synthesis of infectious adenovirus, late adenovirus RNA, and adenovirus DNA. Complementation by the IE2 gene region alone, but not by IE1 alone, was observed using the most discriminating assay, that for late adenovirus RNA synthesis. A role for both HCMV gene regions in positive transcriptional control was indicated by their ability to increase expression of chloramphenicol acetyltransferase (CAT) mediated by the adenovirus E2a promoter. The IE2 region alone activated CAT synthesis but IE1 alone had no detectable activity. Moreover, the activity of both gene regions was about 10-fold higher than that of IE2 alone. These data indicate that efficient complementation of E1a-deficient mutants and trans-activation of adenovirus early promoters involved the participation of both HCMV immediate early gene regions.

Selective induction of human heat shock gene transcription by the adenovirus E1A gene products, including the 12S E1A product

We have previously shown that the human 70-kilodalton heat shock protein gene (hsp70) is induced by the adenovirus E1A gene product and during the S-G2 phase of the cell cycle. In this study, we investigated the effect of E1A on the expression of other human hsp genes. A gene encoding one form of the hsp89 protein (hsp89 alpha) was activated during an adenovirus infection with kinetics similar to those of activation of hsp70. The induction required a functional E1A gene. However, the hsp89 transcript was not cell cycle regulated. Genes encoding another form of hsp89 and the hsp27 protein were not induced by E1A or during the cell cycle. Further examination of hsp70 expression revealed a greater complexity than previously seen. S1 nuclease analysis using an hsp70 cDNA as well as a distinct hsp70 genomic clone demonstrated three related hsp70 transcripts; two were induced by E1A, and one was not. Both of the E1A-inducible genes were regulated during the cell cycle. All three were induced by heat shock. These results suggest common aspects of control among certain members of this family of cellular genes distinct from heat shock control. Finally, using viruses that express the individual E1A proteins, we found that the hsp70 gene is induced by the 12S and the 13S E1A products. The efficiency of induction by the 12S product was somewhat less than that by the 13S product but only by a factor of less than 2. This is in contrast to the induction of early viral genes, for which the 13S product is considerably more efficient than the 12S product.

Transcription of adenovirus 2 major late and peptide IX genes under conditions of in vitro nucleosome assembly

Plasmid DNA containing adenovirus 2 major late and peptide IX genes was assembled into nucleosomes in vitro, and the assembled nucleosomes were used as a template to study the regulatory mechanism of transcription initiation under these conditions. Neither the major late nor peptide IX genes was transcribed on the already-assembled nucleosomes. However, the major late gene, but not the peptide IX gene, was transcribed efficiently when the DNA was incubated with HeLa cell extracts prior to assembly into nucleosomes. These results indicate that prebinding of some component in the cell extracts to DNA is essential to activate transcription of the major late gene on nucleosomes assembled under the conditions used here. Since gene IX on the nucleosomes was not transcriptionally active regardless of preincubation of DNA with the extracts, some other component or another, different template structure which is not able to be identified in an in vitro system with deproteinized DNA template might be required for activation of peptide IX gene transcription. To know the function of the upstream sequences of the major late gene, effects of the deletion on transcription of nucleosomes were compared with that of deproteinized DNA. The result showed that depression of transcription by deleting the upstream sequences had more effect on nucleosomes than on deproteinized DNA.

An adenovirus type 5 E1A protein with a single amino acid substitution blocks wild-type E1A transactivation

The E1A gene of adenovirus type 5 encodes a 289-amino-acid (289R) protein that transactivates early adenovirus promoters. We showed that the 289R protein of the E1A missense mutant gene hr5 is novel in that it inhibits the wild-type (wt) E1A protein from stimulating transcription from each of the early viral promoters E2, E3, and E4. Since both the hr5 and wt genes produced similar levels of E1A proteins, the ability of hr5 E1A to block transactivation was attributed to the replacement of serine by asparagine as position 185. We confirmed that this single amino acid substitution was responsible for blocking transactivation by showing equal inhibition with an hr5-wt hybrid E1A gene containing this missense mutation as the only alteration. The smaller 243R E1A protein of hr5 was not necessary for inhibition. Transcriptional activity from each early promoter was inhibited by at least 50% when the hr5 and wt E1A genes were present in equimolar amounts; complete inhibition occurred with a fivefold molar excess of the hr5 gene. Two other E1A missense mutant genes (hr3 and hr4) with amino acid substitutions in close proximity to that of hr5 failed to block wt E1A-induced transcription when similarly tested. Also, the hr5 E1A gene failed to impede the pseudorabies immediate early gene from transactivating the adenovirus E3 promoter, demonstrating that hr5 E1A inhibits wt E1A activation at the transcriptional, rather than the posttranscriptional, level. Although several possibilities were considered to account for this inhibition, the most likely is that the nonfunctional hr5 E1A protein competes with the wt 289R protein for a cellular transcription factor required for transactivation.

Selective inhibition of adenovirus type 2 early region II and III transcription by an anisomycin block of protein synthesis

The transcription of adenovirus type 2 genes proceeds through a broad three-phase program. From 1 to 4 h postinfection six early transcription units (EIa, EIb, EII, EIII, EIV, and the promoter-proximal segment of the late transcription unit) are activated. From 4 to 6 h postinfection transcription of the early genes is depressed. After the onset of viral DNA replication at approximately 6 h postinfection, the transcript from the late promoter is antiterminated, and this transcript dominates viral RNA synthesis. The early activation period also proceeds through a series of stages; early regions EIa and EIV are activated first, followed by early region EII. We show that in the presence of anisomycin, a stringent inhibitor of protein synthesis, nuclear RNA and cytoplasmic polyadenylated RNA from regions EIa and EIV accumulate at normal rates, whereas RNAs from regions EII and EIII do not accumulate. We also show that failure to accumulate RNAs from regions EII and EIII is due to reduction of the rate of transcription by greater than 90%. We conclude that the regulation of the promoters for early regions EII and EIII is distinct from the mechanism that operates on the other early transcription units. The promoters for early regions EII and EIII diverge and lie approximately 500 nucleotides apart. We examined the structure of viral chromatin in this region early during infection by mild DNase I digestion in isolated nuclei and indirect end labeling with a DNA probe near these promoters. In control, drug-free cells where EII and EIII are transcribed and in anisomycin-treated cells where EII and EIII are not transcribed we detect the same regular DNase I pattern. We suggest that regulation of EII and EIII is not mediated through a change in gross chromatin structure, but rather by a viral effector, possibly a product of region EIa.

Transcription of adenovirus 2 major late and peptide IX genes under conditions of in vitro nucleosome assembly

Plasmid DNA containing adenovirus 2 major late and peptide IX genes was assembled into nucleosomes in vitro, and the assembled nucleosomes were used as a template to study the regulatory mechanism of transcription initiation under these conditions. Neither the major late nor peptide IX genes was transcribed on the already-assembled nucleosomes. However, the major late gene, but not the peptide IX gene, was transcribed efficiently when the DNA was incubated with HeLa cell extracts prior to assembly into nucleosomes. These results indicate that prebinding of some component in the cell extracts to DNA is essential to activate transcription of the major late gene on nucleosomes assembled under the conditions used here. Since gene IX on the nucleosomes was not transcriptionally active regardless of preincubation of DNA with the extracts, some other component or another, different template structure which is not able to be identified in an in vitro system with deproteinized DNA template might be required for activation of peptide IX gene transcription. To know the function of the upstream sequences of the major late gene, effects of the deletion on transcription of nucleosomes were compared with that of deproteinized DNA. The result showed that depression of transcription by deleting the upstream sequences had more effect on nucleosomes than on deproteinized DNA.

An adenovirus type 5 E1A protein with a single amino acid substitution blocks wild-type E1A transactivation

The E1A gene of adenovirus type 5 encodes a 289-amino-acid (289R) protein that transactivates early adenovirus promoters. We showed that the 289R protein of the E1A missense mutant gene hr5 is novel in that it inhibits the wild-type (wt) E1A protein from stimulating transcription from each of the early viral promoters E2, E3, and E4. Since both the hr5 and wt genes produced similar levels of E1A proteins, the ability of hr5 E1A to block transactivation was attributed to the replacement of serine by asparagine as position 185. We confirmed that this single amino acid substitution was responsible for blocking transactivation by showing equal inhibition with an hr5-wt hybrid E1A gene containing this missense mutation as the only alteration. The smaller 243R E1A protein of hr5 was not necessary for inhibition. Transcriptional activity from each early promoter was inhibited by at least 50% when the hr5 and wt E1A genes were present in equimolar amounts; complete inhibition occurred with a fivefold molar excess of the hr5 gene. Two other E1A missense mutant genes (hr3 and hr4) with amino acid substitutions in close proximity to that of hr5 failed to block wt E1A-induced transcription when similarly tested. Also, the hr5 E1A gene failed to impede the pseudorabies immediate early gene from transactivating the adenovirus E3 promoter, demonstrating that hr5 E1A inhibits wt E1A activation at the transcriptional, rather than the posttranscriptional, level. Although several possibilities were considered to account for this inhibition, the most likely is that the nonfunctional hr5 E1A protein competes with the wt 289R protein for a cellular transcription factor required for transactivation.

Cellular promoters incorporated into the adenovirus genome. Effect of viral DNA replication on endogenous and exogenous gene transcription

Cell-specific utilization of the albumin and immunoglobulin promoters within non-replicating adenovirus genomes has been established. With such viruses plus helper viruses we now consider the impact of infection by replicating viruses on liver-specific gene expression in hepatoma cells and of the effect of replication on the previously established limits of cell-specific expression. We find that replication of any adenovirus, whether or not it contains the albumin promoter, decreases albumin and apolipoprotein A transcription in hepatoma cells but does not affect transcription of at least four other genes expressed mainly (or only) in the liver. Thus, it may be that some hepatoma-specific genes and late adenoviral genes require a factor(s) in common, and these factors become limiting during replication. In hepatoma cells, the increased copy number of the exogenous promoters resulting from viral DNA replication did not influence the cell-specific expression noted previously; only albumin and not globin or immunoglobulin promoters in the virus were active. In contrast, replication by the same viruses in HeLa cells resulted in high levels of expression from all the exogenous promoters.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.

Cellular promoters incorporated into the adenovirus genome: cell specificity of albumin and immunoglobulin expression

Recombinant adenoviruses were constructed in which the viral E1A gene was deleted and the E1B promoter was replaced by the rat albumin, mouse beta-major globin, or mouse immunoglobulin heavy-chain promoter. After infection of human or rat hepatoma cells, E1B-containing mRNAs could be detected only from the virus containing the albumin promoter. Conversely, only the immunoglobulin promoter was active in virus-infected myeloma cells. However, in hepatoma cells transcription from the albumin promoter in the virus was much less than that of the endogenous cellular albumin gene or of other viral genes. In primary mouse hepatocytes endogenous albumin gene transcription was high immediately after plating but declined within 24 h. Expression of the albumin promoter in the virus paralleled that of the cellular albumin gene. From these results it appears that cell-specific expression of albumin depends on the presence of tissue-specific trans-acting factors, but the presence of such factors does not suffice for a maximal rate of transcription, a conclusion that requires direct comparison within a differentiated cell of a newly introduced and preexisting active cell gene.

Adenovirus 2 peptide IX gene is expressed only on replicated DNA molecules

Transcription of the adenovirus type 2 peptide IX (pIX) gene was examined in transient expression assays. When a nonreplicating plasmid DNA containing the pIX gene was introduced into HeLa cells by the DEAE-dextran method, no pIX gene transcript was detected. In contrast, efficient transcription was observed in the cells transfected with a replicating plasmid containing the pIX gene. Adenovirus early genes did not affect the level of transcription of the pIX gene on either a nonreplicating or a replicating plasmid. Inhibition of plasmid replication with cytosine arabinoside prevented transcription of the pIX gene. By quantitative analysis of the amount of the pIX gene and its transcript in transfected cells, it was concluded that active transcription of the pIX gene occurred only on DNA molecules replicated in the cell.

Expression of adenovirus E1B mutant phenotypes is dependent on the host cell and on synthesis of E1A proteins

Adenovirus mutants containing genetic alterations in the gene encoding the E1B 19,000-molecular-weight (19K) tumor antigen induce the degradation of host cell chromosomal DNA (deg phenotype) and enhanced cytopathic effect (cyt phenotype) after infection of HeLa and KB cells. The deg and cyt phenotypes are a consequence of viral early gene expression in the absence of the E1B 19K protein. The role of the E1A proteins in induction of the cyt and deg phenotypes was investigated by constructing E1A-E1B double mutant viruses. Viruses were constructed to express the individual E1A 13S, 12S, or 9S cDNA genes in the presence of a mutation in the gene encoding the E1B 19K tumor antigen. Expression of either the 13S or 12S E1A proteins in the absence of functional E1B 19K protein produced the deg and cyt phenotypes. In contrast, a virus which expressed exclusively the 9S E1A gene product in the absence of the E1B 19K gene product did not induce the deg and cyt phenotypes, even at high multiplicities of infection. Therefore, both the 13S and 12S E1A gene products could directly or indirectly cause the deg and cyt phenotypes during infection of HeLa cells with an E1B 19K gene mutant virus. Furthermore, the deg phenotype was found to be host cell type specific, occurring in HeLa and KB cells but not in growth-arrested human WI38 cells. These results indicate that expression of the E1A trans-activating and transforming proteins is necessary for the induction of the cyt and deg phenotypes and that host cell factors also play a role.

Association of adenovirus early-region 1A proteins with cellular polypeptides

Extracts from adenovirus-transformed human 293 cells were immunoprecipitated with monoclonal antibodies specific for the early-region 1A (E1A) proteins. In addition to the E1A polypeptides, these antibodies precipitated a series of proteins with relative molecular weights of 28,000, 40,000, 50,000, 60,000, 80,000, 90,000, 110,000, 130,000, and 300,000. The two most abundant of these polypeptides are the 110,000-molecular-weight protein (110K protein) and 300K protein. Three experimental approaches have suggested that the 110K and 300K polypeptides are precipitated because they form stable complexes with the E1A proteins. The 110K and 300K polypeptides do not share epitopes with the E1A proteins, they copurify with a subset of the E1A proteins, and they bind to the E1A proteins following mixing in vitro. The 110K and 300K polypeptides are not adenoviral proteins, but are encoded by cellular DNA. Both the 12S and the 13S E1A proteins bind to the 110K and 300K species, and these complexes are found in adenovirus-transformed and -infected cells.

The E1A 13S product of adenovirus 5 activates transcription of the cellular human HSP70 gene

Expression of the human gene encoding the major heat shock protein, HSP70, was induced during cell growth by serum stimulation and after infection with adenovirus 5. In this study we showed that HSP70 gene expression could be induced by adenovirus 5 infection, even in the absence of exogenous serum factors. Whereas serum stimulation induced the expression of the endogenous HSP70 gene, it had no effect on early adenovirus promoters. However, expression of both the cellular HSP70 gene and the adenovirus E3 promoter were activated during adenovirus infection. By using a collection of reconstructed mutant viruses, we identified the 13S product of the E1A region as the specific transcriptional trans-activator of the HSP70 gene.

Adenovirus 2 peptide IX gene is expressed only on replicated DNA molecules

Transcription of the adenovirus type 2 peptide IX (pIX) gene was examined in transient expression assays. When a nonreplicating plasmid DNA containing the pIX gene was introduced into HeLa cells by the DEAE-dextran method, no pIX gene transcript was detected. In contrast, efficient transcription was observed in the cells transfected with a replicating plasmid containing the pIX gene. Adenovirus early genes did not affect the level of transcription of the pIX gene on either a nonreplicating or a replicating plasmid. Inhibition of plasmid replication with cytosine arabinoside prevented transcription of the pIX gene. By quantitative analysis of the amount of the pIX gene and its transcript in transfected cells, it was concluded that active transcription of the pIX gene occurred only on DNA molecules replicated in the cell.

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.

Regulation of adenovirus gene expression in human WI38 cells by an E1B-encoded tumor antigen

Adenovirus mutants carrying alterations in the gene encoding the E1B 19-kilodalton tumor antigen (19K protein) cause enhanced cytopathic effect (cyt phenotype) and the degradation of host-cell chromosomal DNA (deg phenotype) upon infection of human HeLa or KB cells. Furthermore, E1B 19K gene mutant viruses are defective for cellular transformation. We report that these mutant viruses possess a host-range phenotype for growth in human cells. In human HeLa cells the mutant viruses grew to the same levels as the wild-type virus, but they were severely defective for growth in KB cells. In human WI38 cells, the E1B 19K gene mutant viruses had a substantial growth advantage over the wild-type virus, yielding 500-fold-higher titers. Viral DNA synthesis was reduced 10- to 20-fold in WI38 cells infected with the wild-type virus relative to that synthesized by the E1B mutant viruses. Viral early and late protein synthesis was similarly reduced in wild type- relative to mutant-infected cells. These reduced levels of early gene expression in wild-type virus-infected cells were paralleled by comparably reduced levels of early cytoplasmic mRNA. The primary cause of this host-range phenotype appeared at the level of early gene transcription, since transcription of viral early genes in the mutant-infected cells was substantially greater than levels found in cells infected with the wild-type virus. These results implicate the E1B 19K tumor antigen in the regulation of adenovirus early gene expression. Specifically, the E1B 19K protein directly or indirectly exerts a negative effect on early gene transcription accounting for efficient gene expression from the E1B mutant viruses in WI38 cells. Based on these findings it is probable that the cyt and deg phenotypes observed in mutant-infected HeLa and KB cells are the result of the pleiotropic effect of this altered gene regulation.

Cellular promoters incorporated into the adenovirus genome: cell specificity of albumin and immunoglobulin expression

Recombinant adenoviruses were constructed in which the viral E1A gene was deleted and the E1B promoter was replaced by the rat albumin, mouse beta-major globin, or mouse immunoglobulin heavy-chain promoter. After infection of human or rat hepatoma cells, E1B-containing mRNAs could be detected only from the virus containing the albumin promoter. Conversely, only the immunoglobulin promoter was active in virus-infected myeloma cells. However, in hepatoma cells transcription from the albumin promoter in the virus was much less than that of the endogenous cellular albumin gene or of other viral genes. In primary mouse hepatocytes endogenous albumin gene transcription was high immediately after plating but declined within 24 h. Expression of the albumin promoter in the virus paralleled that of the cellular albumin gene. From these results it appears that cell-specific expression of albumin depends on the presence of tissue-specific trans-acting factors, but the presence of such factors does not suffice for a maximal rate of transcription, a conclusion that requires direct comparison within a differentiated cell of a newly introduced and preexisting active cell gene.

Genetic analysis of mRNA synthesis in adenovirus region E3 at different stages of productive infection by RNA-processing mutants

Region E3 of adenovirus encodes about nine overlapping mRNAs (a to i) with different spliced structures and with two major RNA 3' end sites termed E3A and E3B. Synthesis of E3 mRNAs was examined by the nuclease-gel procedure at early and late stages of infection by wild-type virus (rec700) and by several E3 deletion mutants. Our results, together with those obtained by electron microscopy (L. T. Chow, T. R. Broker, and J. B. Lewis, J. Mol. Biol. 134:265-303, 1979), suggest that E3 may be differentially regulated at early and late stages at both the promoter and RNA-processing levels. Early after infection, the E3 promoter is used to make mainly mRNAs a and h. Late after infection, the E3 promoter appears to be shut off and the major late promoter is used to make mainly mRNAs d and e. The late L4 mRNA 3' end site is not used early even though early E3 pre-mRNAs transcribe through the L4 RNA 3' end site. The nucleotide 768-951 exon, which is the y leader on many L5 mRNAs, is very abundant late. (Nucleotide +1 is the major E3 transcription initiation site.) Early after infection, the 951 5' splice site is enhanced 5- to 10-fold in the dl712 (delta 1691 to 2122) group of mutants; late after infection, these mutants resemble the wild type. We speculate that the activity of the 951 5' splice site is regulated at early and late stages; it is suppressed early to permit synthesis of mRNA a, and it is activated late to permit synthesis of mRNAs d, e, and L5. With dl719 (delta 2173 to 2237), the 951----2157 splice is enhanced both early and late; we suggest that this deletion enhances the 2157 3' splice site.

Vector expression of adenovirus type 5 E1a proteins: evidence for E1a autoregulation

We transiently expressed adenovirus type C E1a proteins in wild-type or mutant form from plasmid vectors which have different combinations of E1a and simian virus 40 enhancer elements and which contain the DNA replication origin of SV40 and can replicate in COS 7 cells. We measured the levels of E1a mRNA encoded by the vectors and the transition regulation properties of the protein products. Three vectors encoded equivalent levels of E1a mRNA in COS 7 cells: (i) a plasmid encoding the wt 289-amino acid E1a protein (this complemented the E1a deletion mutant dl312 for early region E2a expression under both replicative and nonreplicative conditions); (ii) a vector for the wt 243-amino acid E1a protein (this complemented dl312 weakly and only under conditions of high multiplicities of dl312); (iii) a mutant, pSVXL105, in which amino acid residues-38 through 44 of the 289-amino acid E1a protein (which includes two highly conserved residues) are replaced by 3 novel amino acids (this also complemented dl312 efficiently). A fourth vector, mutant pSVXL3 with which linker substitution shifts the reading frame to encode a truncated 70-amino acid fragment from the amino terminus of the 289-amino acid protein, was unable to complement dl312. Surprisingly, pSVXL3 overexpressed E1a mRNA approximately 30-fold in COS 7 cells in comparison with the other vectors. The pSVXL3 overexpression could be reversed by cotransfection with a wt E1a vector. We suggest that wt E1a proteins regulate the levels of their own mRNAs through the recently described transcription repression functions of the 289- and 243-amino acid E1a protein products and that pSVXL3 fails to autoregulate negatively.