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

Strong foreign promoters contribute to innate inflammatory responses induced by adenovirus transducing vectors

E1-deleted adenovirus (FG Ad) transducing vectors are limited for use in vivo by their induction of strong innate and adaptive inflammatory responses. We have examined the contribution of the transgene cassette, particularly the foreign promoter driving transgene expression, in the induction of innate inflammation using a mouse ear model in which swelling is measured as a sensitive surrogate marker of the total innate inflammatory response. The commonly used cytomegalovirus major immediate early (CMV) promoter led to high-level swelling that was independent of transgene expression, while the Rous sarcoma virus and human ubiquitin C promoters led to intermediate levels of swelling and the Ad E1A promoter or no promoter led to equally low levels of swelling. Significant swelling was induced by a virus in which the E1A promoter directed pIX expression, supporting the possibility that activation of expression of Ad genes retained in the vector plays an important role in the inflammatory response. Taken together, our findings support the idea that strong foreign promoters likely play the limiting role in the induction of innate and adaptive immune responses that limit the duration of transgene expression after transduction by FG Ad vectors.

Read-through activation of transcription in a cellular genomic context

Read-through transcription from the adjacent E1a gene region is required for wild-type (wt) activity of the downstream adenovirus E1b promoter early after infection (read-through activation). However, whether a cellular chromosomal template can support read-through activation is not known. To address this issue, read-through activation was evaluated in the context of stably expressed templates in transfected cells. Inhibition of read-through transcription by insertion of a transcription termination sequence between the E1a and E1b promoters reduced downstream gene expression from stably integrated templates. The results indicate that the mechanism of read-through activation does not depend on the structure of early adenovirus nucleoprotein complexes, a structure that is likely to be different from that of cellular chromatin. Accordingly, this regulatory interaction could participate in the coordinated control of the expression of closely linked cellular genes.

Default assembly of early adenovirus chromatin

In adenovirus particles, the viral nucleoprotein is organized into a highly compacted core structure. Upon delivery to the nucleus, the viral nucleoprotein is very likely to be remodeled to a form accessible to the transcription and replication machinery. Viral protein VII binds to intra-nuclear viral DNA, as do at least two cellular proteins, SET/TAF-Ibeta and pp32, components of a chromatin assembly complex that is implicated in template remodeling. We showed previously that viral DNA-protein complexes released from infecting particles were sensitive to shearing after cross-linking with formaldehyde, presumably after transport of the genome into the nucleus. We report here the application of equilibrium-density gradient centrifugation to the analysis of the fate of these complexes. Most of the incoming protein VII was recovered in a form that was not cross-linked to viral DNA. This release of protein VII, as well as the binding of SET/TAF-Ibeta and cellular transcription factors to the viral chromatin, did not require de novo viral gene expression. The distinct density profiles of viral DNA complexes containing protein VII, compared to those containing SET/TAF-Ibeta or transcription factors, were consistent with the notion that the assembly of early viral chromatin requires both the association of SET/TAF-1beta and the release of protein VII.

E1A and E1B proteins inhibit inflammation induced by adenovirus

Replication-defective human adenovirus (Ad) group C transducing vectors, most of which have the E1A, E1B, and E3 genes deleted, are highly inflammatory despite the fact that the parental viruses typically cause subclinical or mild infections. To investigate this paradox, the roles that the E1A, E1B, and E3 genes play in inflammation were tested by using replication-incompetent viruses carrying a deletion of the preterminal protein gene. The viruses were injected into BALB/c mouse ears, and edema was monitored as a sensitive surrogate marker of inflammation. A virus deleted for the E1A 289R (transcription activating) protein was noninflammatory, and inhibited edema induced by empty virus particles. The E1A 243R and E1B 55-kDa (p53 binding) proteins play the most important roles in inhibition of inflammation by the noninflammatory virus. The E1B 19-kDa antiapoptotic protein inhibited edema when both the E1A 243R and E1B 55-kDa proteins were expressed but strongly induced edema when only one was expressed. E3 proteins had their greatest effect on the inhibition of edema induced by the E1A 289R protein. The results support a model in which inflammation is countered through a mechanism that involves complex genetic interactions between Ad early region proteins and offer promise for the design and construction of noninflammatory Ad gene therapy vectors that are relatively easy to grow and purify.

Local character of readthrough activation in adenovirus type 5 early region 1 transcription control

Wild-type early activity of the adenovirus 5 E1b gene promoter requires readthrough transcription originating from the adjacent upstream E1a gene. This unusual mode of viral transcription activation was identified by genetic manipulation of the mouse beta(maj)-globin gene transcription termination sequence (GGT) inserted into the E1a gene. To facilitate further study of the mechanism of readthrough activation, the activities of GGT and a composite termination sequence CT were tested in recombinant adenoviruses containing luciferase reporters driven by the E1b promoter. There was a strict correlation between readthrough and substantial downstream gene expression, indicating that interference with downstream transcription was not a unique property of GGT. Blockage of readthrough transcription of E1a had no apparent effect on early expression of the major late promoter, the next active promoter downstream of E1b. A test for epistatic interaction between termination sequence insertions and E1a enhancer mutations suggested that readthrough activation and E1a enhancer activation of the E1b promoter are mechanistically distinct. In addition, substitution of the human cytomegalovirus major immediate-early promoter for the E1b promoter suppressed the requirement for readthrough. These results suggest that readthrough activation is a "local" effect of a direct interaction between the invading transcription elongation complex and the E1b promoter. DNase I hypersensitivity footprinting provided evidence that this interaction altered an extensive E1b promoter DNA-protein complex that was assembled in the absence of readthrough transcription.

Adenovirus type 5 DNA-protein complexes from formaldehyde cross-linked cells early after infection

We report here the properties of viral DNA-protein complexes that purify with cellular chromatin following formaldehyde cross-linking of intact cells early after infection. The cross-linked viral DNA fractionated into shear-sensitive (S) and shear- resistant (R) components that were separable by sedimentation, which allowed independent characterization. The R component had the density and sedimentation properties expected for DNA-protein complexes and contained intact viral DNA. It accounted for about 50% of the viral DNA recovered at 1.5 h after infection but less than 20% by 4.5 h. The proportion of R component was independent of multiplicity of infection, even at less than one particle per cell. Viral hexon and protein VII, but not protein VI, were detected in the fractions containing the R component. These properties are consistent with those of partially uncoated virions associated with the nuclear envelope. A substantial proportion of the S component viral DNA had the same density as cellular chromatin. Protein VII was the most abundant viral protein present in gradient fractions that contained the S component. Complexes containing USF transcription factor cross-linked to the adenovirus major late promoter were detected by viral chromatin immunoprecipitation of the fractions containing S component. The S component probably contained uncoated nuclear viral DNA that assembles into early viral transcription complexes.

Readthrough activation of early adenovirus E1b gene transcription

In cells productively infected with adenovirus type 5, transcription is not terminated between the E1a gene and the adjacent downstream E1b gene. Insertion of the mouse beta(maj)-globin transcription termination sequence (GGT) into the E1a coding region dramatically reduces early, but not late, E1b expression (E. Falck-Pedersen, J. Logan, T. Shenk, and J. E. Darnell, Jr., Cell 40:897-905, 1985). In the study described herein, we showed that base substitution mutations in the globin DNA that specifically relieved transcription termination also restored early E1b promoter activity in cis, establishing that maximal early E1b expression requires readthrough transcription originating from the adjacent upstream gene. To identify potential targets of readthrough activation, a series of recombinant viruses with double mutations was constructed. Each double-mutant virus strain had the transcription termination sequences in the first exon of E1a and a deletion within the transcription control region of E1b. Early E1b expression from the double-mutant strains was more defective than that from strains containing either mutation alone, indicating that the deleted regions (positions -362 to -35) are not the target for readthrough activation. Two findings suggested that a cis-dominant property of early viral templates is important for readthrough activation. First, the early E1b defect caused by the GGT insertion was not complemented in trans by factors present in late-infected cells. Second, restoration of E1b transcription at late times occurred concurrently with viral DNA replication. Readthrough activation may help convert virion DNA into a transcriptionally competent template prior to DNA replication and late transcription.

A viral function represses accumulation of transcripts from productive-cycle genes in mouse ganglia latently infected with herpes simplex virus

Latent infections of neurons by herpes simplex virus form reservoirs of recurrent viral infections that resist cure. In latently infected neurons, viral gene expression is severely repressed; only the latency-associated transcripts (LATs) are expressed abundantly. Using sensitive reverse transcriptase PCR assays, we analyzed the effects of a deletion mutation in the LAT locus on viral gene expression in latently infected mouse trigeminal ganglia. The deletion mutation, which reduced expression of the major LATs 10(5)-fold, resulted in a approximately 5-fold increase in accumulation of transcripts from the immediate-early gene encoding ICP4, an essential transactivator of viral gene expression. The LAT deletion also resulted in a >10-fold increase in the accumulation of transcripts from the early gene encoding thymidine kinase, whose expression during productive infection stringently depends on ICP4, and positively affected the correlation of the levels of these transcripts with the levels of ICP4 transcripts. We also detected transcripts antisense to ICP4 RNA, which were in substantial excess to ICP4 transcripts in ganglia latently infected with wild-type virus. In contrast to its effects on productive-cycle transcripts, the LAT deletion reduced the accumulation of these antisense transcripts approximately 15-fold. Thus, a viral function associated with the LAT locus represses the accumulation of transcripts from at least two productive-cycle genes in latently infected mouse ganglia. We discuss possible mechanisms and consequences of this repression.

The adenovirus E1A transforming protein activates the proliferating cell nuclear antigen promoter via an activating transcription factor site

The transforming region of adenovirus (E1) stimulates expression of a reporter construct linked to the promoter for the human proliferating cell nuclear antigen (PCNA) gene in a cotransfection assay (G. F. Morris and M. B. Mathews, J. Biol. Chem. 264:13856-13864, 1989). The major products of the E1 region were assessed individually for their contribution to transactivation of the PCNA promoter. The E1A 13S and 12S products and the E1B 19-kDa product elevated expression from the PCNA promoter, whereas the E1B 55-kDa product did not. Induction of the PCNA promoter by E1A differed from transcriptional activation of the adenovirus E3 promoter in that the PCNA promoter is activated by the E1A 12S product whereas the E3 promoter is repressed; furthermore, the PCNA promoter is activated upon E1A overexpression, whereas the E3 promoter responds less well to high amounts of E1A. A site for the activating transcription factor ATF located approximately 50 nucleotides upstream from the transcription initiation site in the PCNA promoter mediates a positive response to the E1A 12S and 13S products.