A TEF-1-independent mechanism for activation of the simian virus 40 (SV40) late promoter by mutant SV40 large T antigens

A TEF-1-element is required for activation of the promoter of pseudorabies virus glycoprotein X gene by IE180

The pseudorabies virus (PRV) immediate-early regulatory protein IE180 is able to transactivate the viral early and late genes. Using chloramphenicol acetyltransferase (CAT) assay, we investigated the transactivation function of IE180 to the promoter of PRV glycoprotein X (gX) gene, and our results showed that IE180 could significantly increase the expression of CAT gene which was under the control of gX promoter. To further identify the activation domains of IE180 protein that interact with the gX promoter sequences, various truncated mutants of IE180 gene and gX promoter gene were constructed and analyzed by CAT and gel retardation assay. Results revealed that the N-terminal amino acid residues from 133 to 736 of IE180 could interact with the binding site of transcriptional enhancer factor-1 (TEF-1) that resides in the gX promoter. Formation of protein-DNA complexes between the IE180 protein and the TEF-1 element of the gX promoter was observed using electrophoretic mobility shift assay (EMSA) as well as Southwestern blot analysis. These results indicated that a direct interaction occurred between IE180 and the TEF-1 element; and this interaction was abolished if the TEF-1 element was mutated. The association of IE180 with the TEF-1 element was further confirmed by the supershift of EMSA complexes using IE180 specific antibody. Taken together, our results suggested that formation of a complex between the IE180 protein and TEF-1 element in the gX promoter region was involved in the transcriptional regulation of the gX gene.

Cell type-specific replication of simian virus 40 conferred by hormone response elements in the late promoter

The late genes of SV40 are not expressed at significant levels until after the onset of viral DNA replication. We previously identified two hormone response elements (HREs) in the late promoter that contribute to this delay. Mutants defective in these HREs overexpress late RNA at early, but not late, times after transfection of CV-1PD cells. Overexpression of nuclear receptors (NRs) that recognize these HREs leads to repression of the late promoter in a sequence-specific and titratable manner, resulting in a delay in late gene expression. These observations led to a model in which the late promoter is repressed at early times after infection by NRs, with this repression being relieved by titration of these repressors through simian virus 40 (SV40) genome replication to high copy number. Here, we tested this model in the context of the viral life cycle. SV40 genomes containing mutations in either or both HREs that significantly reduce NR binding without altering the coding of any proteins were constructed. Competition for replication between mutant and wild-type viruses in low-multiplicity coinfections indicated that the +1 HRE offered a significant selective advantage to the virus within a few cycles of infection in African green monkey kidney cell lines CV-1, CV-1P, TC-7, MA-134, and Vero but not in CV-1PD' cells. Interestingly, the +55 HRE offered a selective disadvantage in MA-134 cells but had no effect in CV-1, CV-1P, TC-7, Vero, and CV-1PD' cells. Thus, we conclude that these HREs are biologically important to the virus, but in a cell type-specific manner.

Hormone response element in SV40 late promoter directly affects synthesis of early as well as late viral RNAs

We previously demonstrated that the presence of a hormone response element surrounding the transcription initiation site of the SV40 major late promoter (+1 HRE) confers a replication advantage to the virus in a cell-type-specific manner. We determine here the mechanism by which the +1 HRE confers this advantage by analyzing in detail the various stages of the viral life cycle of wild-type versus a +1 HRE mutant in MA-134 cells. We show that the mutant overexpresses late genes at the expense of early genes at early times after infection. This initial underproduction of early RNA leads, subsequently, to an underproduction of large T-antigen, viral DNA, and infectious virions. We conclude that the +1 HRE is necessary for the proper initial regulation of transcription from the early as well as late promoter so the cascade of subsequent events can be executed for the optimal production of virions.

Regulated expression of an MHC class II gene from a promoter-inducible retrovirus

Specific immune tolerance to fully allogeneic kidney grafts can be achieved in a miniature swine transplantation model by retrovirus-mediated transfer of allogeneic MHC class II genes into bone marrow cells (BMCs) of recipient animals. Graft survival correlated with transient expression of the somatic transgene (Tg) in the induction phase of tolerance. With the aim of investigating the effects of timing and threshold levels of Tg expression on induction of hyporesponsiveness to the grafted tissues, two recombinant retrovirus constructs containing the tetracycline binary regulatory system were used to achieve conditional expression of either the green fluorescent protein (tetGFP) as a control, or the porcine MHC class II DRbeta chain (tetDRB). Effective downregulation of GFP gene transcription was demonstrated in transduced murine fibroblasts after doxycycline treatment, leading to a > 90% reduction of GFP fluorescence. Similar diminution of the DRB gene transcription was achieved in transduced pig endothelial cells (ECs). Drug-dependent downregulation of DRBc gene expression in SLAd pig ECs coincided with complete inhibition of allogeneic activation of anti-class IIc-primed SLAd T cells. These in vitro results suggest that the binary tetracycline retrovirus system may also be adequate to regulate MHC class II Tg expression in vivo.

CBP-dependent and independent enhancing activity of steroid receptor coactivator-1 in thyroid hormone receptor-mediated transactivation

Full-length of steroid receptor coactivator-1 (F-SRC-1) has been shown to interact with thyroid hormone receptors (TRs) in a ligand-dependent manner and to stimulate receptor-dependent transcription. To identify functional domains of F-SRC-1, several internal deletion mutants of F-SRC-1 were constructed. Although in vitro pull down assay with TR showed interaction of all of these mutants with TR, lack of mid legion (amino acids 398-1172) lost enhancing activity of TR-mediated transcription in a transient transfection assay. However, F-SRC-1 mutant lacking CBP-interacting domain still preserved enhancing activity. Surprisingly, F-SRC-1 mutants also increased basal level of viral promoter activity depending upon their deleted region. Yeast activation function assay revealed that these F-SRC-1 mutants had intrinsic activation function when bound to DNA. Analyses of small fragments of F-SRC-1 identified three separable activation domains. In vitro binding assay showed that TBP and TFIIB bound to C-terminal half of F-SRC-1. These results suggest that F-SRC-1 can function via both CBP-dependent and independent manners using various sets of activation domains and that direct interactions between F-SRC-1 and TBP or TFIIB may not be important for CBP-independent transcription.

Simian virus 40 large T antigen interacts with human TFIIB-related factor and small nuclear RNA-activating protein complex for transcriptional activation of TATA-containing polymerase III promoters

The TATA-binding protein (TBP) is common to the basal transcription factors of all three RNA polymerases, being associated with polymerase-specific TBP-associated factors (TAFs). Simian virus 40 large T antigen has previously been shown to interact with the TBP-TAFII complexes, TFIID (B. Damania and J. C. Alwine, Genes Dev. 10:1369-1381, 1996), and the TBP-TAFI complex, SL1 (W. Zhai, J. Tuan, and L. Comai, Genes Dev. 11: 1605-1617, 1997), and in both cases these interactions are critical for transcriptional activation. We show a similar mechanism for activation of the class 3 polymerase III (pol III) promoter for the U6 RNA gene. Large T antigen can activate this promoter, which contains a TATA box and an upstream proximal sequence element but cannot activate the TATA-less, intragenic VAI promoter (a class 2, pol III promoter). Mutants of large T antigen that cannot activate pol II promoters also fail to activate the U6 promoter. We provide evidence that large T antigen can interact with the TBP-containing pol III transcription factor human TFIIB-related factor (hBRF), as well as with at least two of the three TAFs in the pol III-specific small nuclear RNA-activating protein complex (SNAPc). In addition, we demonstrate that large T antigen can cofractionate and coimmunoprecipitate with the hBRF-containing complex TFIIIB derived from HeLa cells infected with a recombinant adenovirus which expresses large T antigen. Hence, similar to its function with pol I and pol II promoters, large T antigen interacts with TBP-containing, basal pol III transcription factors and appears to perform a TAF-like function.

Mechanisms of transcriptional regulation of cellular genes by SV40 large T- and small T-antigens

During the past decade a number of virus-encoded transcriptional trans-activators that regulate the expression of viral genes have been reported. These trans-activators may also affect the expression or activity of several cellular genes or gene products to create an optimal cellular environment that favors viral replication. Among the better-studied viral trans-activating proteins are the Simian virus 40 large T- and small t-antigens. During the last few years, mechanisms by which these two viral proteins influence cellular gene expression start to emerge. They are grouped provisionally and reflect the methods used to determine the effects of large T-antigen. Large T-antigen may influence cellular gene expression by: i. altering mRNA levels of cellular transcription factors; ii. interacting with and regulating the DNA-binding or transcriptional activity of specific transcription factors; iii. functionally substitution of eukaryotic transcription factors; iv. direct binding to DNA; or v. regulating components of signaling transduction pathways. Small t-ag seems to exert its effect mainly through inhibiting a cellular phosphatase, protein phosphatase 2A, thereby modulating components of signal transduction pathways and preventing dephosphorylation of several transcription factors. However, small t-ag may also control cellular gene expression by regulating mRNA levels of transcription factors or by interacting with other transcription factors.

Tiny T antigen: an autonomous polyomavirus T antigen amino-terminal domain

Three mRNAs from the murine polyomavirus early region encode the three well-characterized tumor antigens. We report the existence of a fourth alternatively spliced mRNA which encodes a fourth tumor antigen, tiny T antigen, which comprises the amino-terminal domain common to all of the T antigens but is extended by six unique amino acid residues. The amount of tiny T antigen in infected cells is small because of its short half-life. Tiny T antigen stimulates the ATPase activity of Hsc70, most likely because of its DnaJ-like motif. The common amino-terminal domain may interface with chaperone complexes to assist the T antigens in carrying out their diverse functions of replication, transcription, and transformation in the appropriate cellular compartments.

TAF-like function of SV40 large T antigen

The simian virus 40 (SV40) early gene product large T antigen promiscuously activates simple promoters containing a TATA box or initiator element and at least one upstream transcription factor-binding site. Previous studies have suggested that promoter activation requires that large T antigen interacts with both the basal transcription complex and the upstream-bound factor. This mechanism of activation is similar to that proposed for TBP-associated factors (TAFs). We report genetic and biochemical evidence suggesting that large T antigen performs a TAF-like function. In the ts13 cell line, large T antigen can rescue the temperature-sensitive (ts) defect in TAF(II)250. In contrast, neither E1a, small t antigen, nor mutants of large T antigen defective in transcriptional activation were able to rescue the ts defect. These data suggest that transcriptional activation by large T antigen is attributable, at least in part, to an ability to augment or replace a function of TAF(II)250. In addition, we show that large T antigen interacts in vitro with the Drosophila TAFs (dTAFs) dTAF(II)150, dTAF(II)110, and dTAF(II)40, as well as TBP. The relevance of these in vitro results was established in coimmunoprecipitation experiments using extracts of SV40-infected alpha3 cells that express an epitope-tagged TBP. Large T antigen was coimmunoprecipitated by antibodies to epitope-tagged TBP, endogenous TBP, hTAF(II)100, hTAF(II)130, and hTAF(II)250, under conditions where holo-TFIID would be precipitated. In addition, large T antigen copurified and coimmunoprecipitated with phosphocellulose-purified TFIID from SV40-infected alpha3 cells. Large T antigen also coprecipitated with anti-TBP antibody from extracts of ts13 cells expressing wild-type large T antigen under conditions where the ts defect in TAF(II)250 was rescued. In contrast, a transactivation mutant of large T antigen, which was unable to rescue the ts defect, failed to coprecipitate. We conclude from these data that transcriptional activation of many promoters by large T antigen results from its performing a TAF-like function in a complex with TFIID.

The major transcriptional transactivation domain of simian virus 40 large T antigen associates nonconcurrently with multiple components of the transcriptional preinitiation complex

Simian virus 40 (SV40) large T antigen (Tag) is a promiscuous transcriptional transactivator; however, its mechanism of transactivation remains unknown. Recent studies have suggested the possible involvement of protein-protein interactions with TBP, the TATA box-binding protein of TFIID, and TEF-1, an enhancer-binding factor. We show here that (i) the Tag domain containing amino acids 133 to 249 directly interacts with the general transcription factor TFIIB, the activator protein Sp1, and the 140-kDa subunit of RNA polymerase II, as well as with TBP and TEF-1; (ii) these interactions can also occur when these transcription factors are present in their functional states in cellular extracts; (iii) binding of Tag to TBP is eliminated by preincubation of TBP either at 48 degrees C or with the adenovirus 13S E1a protein; (iv) this domain of Tag cannot bind concurrently to more than one of these transcription factors; and (v) the substitution of Tag amino acid residues 173 and 174 inactivates the ability of this Tag domain both to associate with any of these transcription factors and to transactivate the SV40 late promoter. Thus, we conclude that SV40 Tag probably does not transactivate via the concurrent interaction with multiple components of the preinitiation complex. Rather, we hypothesize that transactivation by Tag may primarily occur by removing or preventing the binding of factors that inhibit the formation of preinitiation complexes.