Cis and trans activation of adenovirus IVa2 gene transcription

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.

Identification of a previously unrecognized promoter that drives expression of the UXP transcription unit in the human adenovirus type 5 genome

We previously identified an adenovirus (Ad) protein named U exon protein (UXP) encoded by a leftward-strand (l-strand) transcription unit. Here we identify and characterize the UXP promoter. Primer extension and RNase protection assays mapped the transcription initiation site at 32 nucleotides upstream of the UXP gene initiation codon. A series of viral mutants with mutations at two putative inverted CCAAT (I-CCAAT) boxes and two E2F sites were generated. With mutants lacking the proximal I-CCAAT box, the UXP mRNA level decreased significantly to 30% of the Ad type 5 (Ad5) mRNA level as measured by quantitative reverse transcription-PCR. Decreased UXP was also observed by immunoblotting and immunofluorescence. UXP mRNA and protein levels were similar to those of Ad5 for mutants lacking the distal I-CCAAT box or both putative E2F sites. Ad DNA levels were similar in mutant- and wild-type Ad5-infected cells during the late stage of infection, strongly suggesting that the decreased UXP mRNA and protein from mutants lacking the proximal I-CCAAT box was due to decreased promoter activity. Electrophoretic mobility shift assays (EMSA) indicated that a cellular factor binds specifically to the proximal I-CCAAT box of the UXP promoter. An in vitro luciferase reporter assay demonstrated that basal promoter activity lies between bp -158 and +30 of the transcription initiation site. No E1A-mediated promoter transactivation was observed in 293 cells compared with A549 cells. Thus, we propose that there is a previously unidentified Ad5 promoter that drives expression of the UXP transcription unit. This promoter is embedded within the gene for fiber, and it contains a proximal I-CCAAT box critical for UXP mRNA transcription.

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

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

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

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

Differential effects of cisplatin on the expression of chimeric marker genes in CV-1 cells

The effect of the antitumor drug cisplatin on marker gene expression in CV-1 monkey cells was measured. When non-replicating test genes were introduced by transient transfection, there was strong differential inhibition caused by the drug. Expression of certain genes was relatively insensitive, but expression of others was inhibited as strongly as was DNA replication. Stronger promoters led to stronger inhibition. This selective inhibition was not observed with the pharmacologically inactive isomer transplatin. The results raise the possibility that inhibition of strongly expressed genes by cisplatin may contribute to the antitumor activity of the drug.

Cisplatin-induced imbalances in the pattern of chimeric marker gene expression in HeLa cells

The effects of the antitumor drug cisplatin on gene expression have been measured during transient transfections in HeLa cells. The results indicate a surprising diversity of response. Expression from two promoters is strongly induced, both in non-replicating plasmids and cellular integrants, whereas expression from two other promoters is strongly inhibited. The results suggest that a drug-induced imbalance in gene expression may contribute to the antitumor properties of cisplatin.

Synthesis of adeno-associated virus structural proteins requires both alternative mRNA splicing and alternative initiations from a single transcript

The three adeno-associated virus type 2 (AAV2) structural proteins (A, B, and C) are specified by transcripts generated from the most-rightward promoter (p40). Protein C (60 kilodaltons [kDa]), the most abundantly produced, is entirely contained within B (72 kDa) which, in turn, is contained within A (90 kDa). Although neither of the known structures of p40 transcripts, an unspliced 2.6-kilobase (kb) RNA and a spliced 2.3-kb RNA, possesses an AUG-initiated open reading frame that accounts for the synthesis of proteins A and B, recent evidence indicates that B is initiated by a unique eucaryotic initiation codon (ACG) (S.P. Becerra, J.A. Rose, M. Hardy, B. Baroudy, and C.W. Anderson, Proc. Natl. Acad. Sci. USA 82:7919-7923, 1985). In the present study, we analyzed the in vitro translation of AAV capsid proteins from synthetic transcripts and the in vivo expression of AAV mRNA and capsid proteins in 293 cells transfected with AAV DNA constructs. The results demonstrated that AAV transcripts contain only one functional 5' splice donor site, that synthesis of capsid proteins from the unspliced 2.6-kb transcript is very inefficient, that transcripts without the intervening sequence (IVS) (i.e., the 2.3-kb RNA) do not produce protein A but effectively synthesize proteins B and C, and that protein A is actively synthesized from transcripts which contain the last 34 bases of the IVS. Protein A initiates within this 34-base segment in reading frame 1, apparently with the AUG codon at nucleotide 2203, and then elongates into the B and C open reading frame. Because A is inefficiently synthesized from the 2.6-kb transcript, we conclude that an effective A transcript is generated by alternative splicing and that the alternative 3' acceptor site may lie at nucleotide 2200 within a context of...CAG]GTA. The levels of B and C produced by a synthetic transcript devoid of the IVS suggest that the known 2.3-kb RNA is the main source of these proteins and indicate that this single RNA species expresses both proteins by alternative use of their respective initiation codons.

Mutations in the adenovirus major late promoter: effects on viability and transcription during infection

We developed an experimental system to examine the effects of mutations in the adenovirus major late promoter in its correct genomic location during a productive infection. A virus was constructed whose genome could be digested to give a rightward terminal DNA fragment extending from the XhoI site at 22.9 map units, which can be ligated or recombined with plasmid DNA containing adenovirus sequences extending from 0 to 22.9 or 26.5 map units, respectively. Mutations were made by bisulfite mutagenesis in the region between base pairs -52 and -12 with respect to the cap site at +1 and transferred to the appropriate plasmids for viral reconstruction. Of 19 mutant plasmid sequences containing single or multiple G-to-A transitions, 14 could be placed in the viral genome with no apparent change in phenotype. These mutant sequences included those which contained four transitions in the string of G residues immediately downstream of the TATA box. There were no alterations in rates of transcription from the major late promoter, sites of transcription initiation, or steady-state levels of late mRNAs. All of the five mutant sequences which could not be placed in virus contained multiple transitions both up- and downstream of the TATA box. Two of these apparently lethal mutant sequences were used in promoter fusion experiments to test their ability to promote transcription of rabbit beta-globin sequences placed in the dispensable E1 region of the virus. Both sequences showed diminished ability compared with wild-type sequences to promote transcription in this context. Comparisons between these two sequences and the viable mutant sequences suggest a role for the string of G residues located between -38 and -33 in promoting transcription from the major late promoter. The data as a whole also demonstrate that the specific nucleotide sequence of this region of the major late promoter, which overlaps transcription elements of the divergent IVa2 transcription unit and coding sequences of the adenovirus DNA polymerase, is not rigidly constrained but can mutate extensively without loss of these several functions.

Cotransfection with adenovirus DNA enhances transcription from linear DNA containing eucaryotic promoters

Linear DNAs, containing a copy of the adenovirus serotype 2 (Ad2) inverted terminal repeat sequence at each end, replicate in 293 cells when cotransfected with Ad2 DNA (Hay et al., J. Mol. Biol. 175:493-510, 1984). We have linked either the Ad2 IVa2 promoter (IVa2) or major late promoter (MLP) to the chloramphenicol acetyltransferase gene and inserted this DNA into such a plasmid (pARKR) between its two inverted terminal repeats. These recombinant plasmids were linearized and then used to transfect 293 cells in the presence or absence of Ad2 helper DNA. Synthesis of IVa2 and MLP RNAs, and production of chloramphenicol acetyltransferase was increased dramatically when the Ad2 DNA was included. However, unlike the patterns of temporal regulation which are seen during a cycle of virus replication when these genes are contained within the virion, there was no obvious difference in the timing of RNA synthesis from plasmid IVa2 or MLP after cotransfection. When linearized plasmids containing IVa2 and MLP sequences but lacking inverted terminal repeats at their ends (replication deficient plasmids) were used for transfection, an increase in RNA synthesis from IVa2 or MLP was also observed and similarly required cotransfection with Ad2 DNA. When HeLa cells, which do not constitutively express the adenovirus E1a gene, were cotransfected with linearized plasmids and adenovirus DNA that lacks the E1a region (H5dl312), a stimulation of transcription was also observed, although it was less than the level observed with wild-type DNA. The results of the present study demonstrate that an early gene product(s) besides E1a functions in trans to regulate transcription.

Mutations in the adenovirus major late promoter: effects on viability and transcription during infection

We developed an experimental system to examine the effects of mutations in the adenovirus major late promoter in its correct genomic location during a productive infection. A virus was constructed whose genome could be digested to give a rightward terminal DNA fragment extending from the XhoI site at 22.9 map units, which can be ligated or recombined with plasmid DNA containing adenovirus sequences extending from 0 to 22.9 or 26.5 map units, respectively. Mutations were made by bisulfite mutagenesis in the region between base pairs -52 and -12 with respect to the cap site at +1 and transferred to the appropriate plasmids for viral reconstruction. Of 19 mutant plasmid sequences containing single or multiple G-to-A transitions, 14 could be placed in the viral genome with no apparent change in phenotype. These mutant sequences included those which contained four transitions in the string of G residues immediately downstream of the TATA box. There were no alterations in rates of transcription from the major late promoter, sites of transcription initiation, or steady-state levels of late mRNAs. All of the five mutant sequences which could not be placed in virus contained multiple transitions both up- and downstream of the TATA box. Two of these apparently lethal mutant sequences were used in promoter fusion experiments to test their ability to promote transcription of rabbit beta-globin sequences placed in the dispensable E1 region of the virus. Both sequences showed diminished ability compared with wild-type sequences to promote transcription in this context. Comparisons between these two sequences and the viable mutant sequences suggest a role for the string of G residues located between -38 and -33 in promoting transcription from the major late promoter. The data as a whole also demonstrate that the specific nucleotide sequence of this region of the major late promoter, which overlaps transcription elements of the divergent IVa2 transcription unit and coding sequences of the adenovirus DNA polymerase, is not rigidly constrained but can mutate extensively without loss of these several functions.

Sequence-specific activation of transcription by adenovirus EIa products is observed in HeLa cells but not in 293 cells

The adenovirus EIa gene products activate transcription from the viral EIII and EIIaE promoters. We studied the mechanism of this stimulation by constructing a series of chimeric promoter recombinants containing the upstream regions of the EIII and EIIaE promoters linked to the TATA box-start-site regions of the viral major late and EIIa late promoters. By introducing these recombinants into HeLa cells together with recombinants producing the EIa gene products, we demonstrated that the induction of EIII and EIIaE transcription by EIa 13S and 12S mRNA products is dependent on sequences located in the upstream region (approximately -40 to -250) of these promoters. In addition, we showed that the major late and EIIa late upstream promoter regions do not contain such EIa-responsive sequence elements. In contrast, after transfection of these chimeric promoter recombinants into 293 cells (which constitutively express the EIa proteins), we found that their relative levels of transcription are similar and markedly different from those observed when they are cotransfected into HeLa cells with EIa protein-producing recombinants. We conclude that the efficiency of transcription from a given promoter in 293 cells is not necessarily related to the presence of a specific EIa-responsive element.

Adenovirus-2 E1a and E1b gene products regulate enhancer mediated transcription

We have shown that adenovirus-2 early region 1 gene products in trans and the SV40 enhancer in cis have an additive effect in stimulating transcription from adenovirus IVa2 and major late promoters (Natarajan, V. and Salzman, N. P., Nucleic Acids Research 13, 4067, 1985). In the present study, we show that both the E1a and E1b gene products are necessary for this stimulatory effect on enhancer mediated transcription. In the absence of E1b region, the transcription is strongly suppressed by E1a. Transcription from E1a promoter is also stimulated 4-5 fold in the presence of E1b region. The data suggest that 21K protein coded by the E1b region modulates transcription from the E1a promoter and the action of E1a gene products on transcription from other promoters.

Sequence-specific activation of transcription by adenovirus EIa products is observed in HeLa cells but not in 293 cells

The adenovirus EIa gene products activate transcription from the viral EIII and EIIaE promoters. We studied the mechanism of this stimulation by constructing a series of chimeric promoter recombinants containing the upstream regions of the EIII and EIIaE promoters linked to the TATA box-start-site regions of the viral major late and EIIa late promoters. By introducing these recombinants into HeLa cells together with recombinants producing the EIa gene products, we demonstrated that the induction of EIII and EIIaE transcription by EIa 13S and 12S mRNA products is dependent on sequences located in the upstream region (approximately -40 to -250) of these promoters. In addition, we showed that the major late and EIIa late upstream promoter regions do not contain such EIa-responsive sequence elements. In contrast, after transfection of these chimeric promoter recombinants into 293 cells (which constitutively express the EIa proteins), we found that their relative levels of transcription are similar and markedly different from those observed when they are cotransfected into HeLa cells with EIa protein-producing recombinants. We conclude that the efficiency of transcription from a given promoter in 293 cells is not necessarily related to the presence of a specific EIa-responsive element.