Analysis of an activatable promoter: sequences in the simian virus 40 late promoter required for T-antigen-mediated trans activation

Molecular dissection of cis-acting regulatory elements from 5'-proximal regions of a vaccinia virus late gene cluster

Promoter elements responsible for directing the transcription of six tightly clustered vaccinia virus (VV) late genes (open reading frames [ORFs] D11, D12, D13, A1, A2, and A3) from the HindIII D/A region of the viral genome were identified within the upstream sequences proximal to each individual locus. These regions were identified as promoters by excising them from the VV genome, abutting them to the bacterial chloramphenicol acetyl transferase gene, and demonstrating their ability to drive expression of the reporter gene in transient-expression assays in an orientation-specific manner. To delineate the 5' boundary of the upstream elements, two of the VV late gene (A1 and D13) promoter: CAT constructs were subjected to deletion mutagenesis procedures. A series of 5' deletions of the ORF A1 promoter from -114 to -24 showed no reduction in promoter activity, whereas additional deletion of the sequences from -24 to +2 resulted in the complete loss of activity. Deletion of the ORF A1 fragment from -114 to -104 resulted in a 24% increase in activity, suggesting the presence of a negative regulatory region. In marked contrast to previous 5' deletion analyses which have identified VV late promoters as 20- to 30-base-pair cap-proximal sequences, 5' deletions to define the upstream boundary of the ORF D13 promoter identified two positive regulatory regions, the first between -235 and -170 and the second between -123 and -106. Background levels of chloramphenicol acetyltransferase expression were obtained with deletions past -88. Significantly, this places the ORF D13 regulatory regions within the upstream coding sequences of the ORF A1. A high-stringency computer search for homologies between VV late promoters that have been thus far characterized was carried out. Several potential consensus sequences were found just upstream from RNA start sites of temporally related promoter elements. Three major conclusions are drawn from these experiments. (i) The presence of promoters preceding each late ORF supports the hypothesis that each is expressed as an individual transcriptional unit. (ii) Promoter elements can be located within the coding portion of the upstream gene. (iii) Sequence homologies between temporally related promoter elements support the notion of kinetic subclasses of late genes.

Both upstream and intron sequence elements are required for elevated expression of the rat somatic cytochrome c gene in COS-1 cells

To investigate the transcriptional control of nuclear-encoded respiratory genes in mammals, we have performed a deletional analysis of cis-acting regulatory sequences in the rat somatic cytochrome c gene. Three major regions are required for maximal expression of the transfected gene in kidney cell lines CV-1 and COS-1. One of these, region III (+71 to +115 from the transcription initiation site), is an unusual intragenic controlling element found in the 5' end of the first intron, while the other two, region I (-191 to -165) and region II (-139 to -84), define the upstream promoter. Region II contains two consensus CCAAT boxes and mediates a constitutive level of expression in both cell lines. In contrast, regions I and III are both required for the increased promoter activity observed in COS-1 cells compared with promoter activity observed in CV-1 cells, and the regions function individually as competitors with the full promoter for trans-acting factors or complexes. Region III contains a perfect octanucleotide homology with region I in addition to a consensus Sp1-transcription-factor-binding site. Promoter stimulation in COS-1 cells can be duplicated in CV-1 cells by cotransfecting with a T-antigen-producing vector, but purified T antigen does not bind anywhere in the cytochrome c promoter. A control promoter from the mouse metallothionein I gene is similarly activated in T-antigen-producing cells only in the presence of zinc, which activates its upstream regulatory sites. We conclude that T antigen stimulates these cellular promoters through the activation or induction of cellular factors or complexes that mediate their effects through promoter-specific regulatory elements. Cytochrome c promoter regions activated in this system may play a physiological role in controlling gene expression.

Contribution of different GC-motifs to the control of simian virus 40 late promoter activity

During the course of lytic infection the 21-bp repeat region regulates differentially the late gene expression; a mutant deleted for this region expresses late genes either to a higher level in the absence of T antigen or to a lower level in the late phase of infection as compared to wild type (23). By analysing a series of clustered point mutations generated within the GC-motifs we show that i) mutations within motifs I and II stimulate late transcription two to three-fold, suggesting that competition for transcription machinery between early-early and late promoters is mediated by these two motifs, ii) after viral replication, simultaneous mutations within motifs IV, V and VI decrease to 23% the efficiency of late transcription, indicating that these motifs are elements of the late promoter. Moreover comparison of results presented in this paper with results published by Barrera-Saldana et al. strongly suggest that late-early and late promoters are regulated in a similar manner.

Specific stimulation of simian virus 40 late transcription in vitro by a cellular factor binding the simian virus 40 21-base-pair repeat promoter element

We have identified a cellular transcription factor from uninfected HeLa cells that stimulates the simian virus 40 (SV40) late mode of transcription and specifically binds the SV40 21-base-pair repeat promoter element. In particular, the late SV40 transcription factor (LSF) stimulates transcription at initiation sites L325 and L264 of the SV40 late promoter, which are the major transcription sites utilized after DNA replication during the SV40 lytic cycle. In addition, LSF appears to stimulate transcription to a lesser extent from the late-early initiation site of the early promoter. LSF binds specifically to the 21-base-pair repeats of the SV40 promoters, forming specific protein-DNA complexes, which migrate more rapidly through nondenaturing polyacrylamide gels than that formed by the previously identified transcription factor Sp1. Thus, LSF is distinguishable from Sp1 in both its transcriptional and DNA-binding properties. These findings suggest a potential role of LSF in the early to late transcriptional switch during a SV40 lytic infection.

Oligomerization and origin DNA-binding activity of simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (T antigen) exists in multiple molecular forms, some of which are separable by zone velocity sedimentation of soluble extracts from infected monkey cells. Three subclasses of this antigen from SV40-infected monkey cells have been separated and characterized: the 5S, 7S, and 14S forms. Newly synthesized T antigen occurs primarily in the 5S form. Chemical cross-linking provided evidence that the 14S form is primarily a tetramer, whereas the 5S and 7S forms could not be cross-linked into oligomers. The DNA-binding properties of each subclass were investigated after immunopurification. The affinities of the three forms for SV40 DNA and for a synthetic 19-base-pair sequence from binding site I are very similar (equilibrium dissociation constant [KD], 0.3 to 0.4 nM). The specific activity of DNA binding was greatest for the 5S and 7S subclasses and least for the 14S subclass. Moreover, the specific activity of the 5S and 7S subclasses increased sharply at about 40 h after infection, whereas the activity of the 14S subclass was maintained at a constant low level throughout infection. A model relating oligomerization and DNA binding of T antigen in infected cells is presented.

Identification of p40x-responsive regulatory sequences within the human T-cell leukemia virus type I long terminal repeat

Distinct transcriptional regulatory sequences located within the upstream sequences required for p40x trans-activation of the human T-cell leukemia virus type I (HTLV-I) long terminal repeat (LTR) were chemically synthesized and cloned upstream of the basal HTLV-I LTR promoter. Plasmids containing a single 21-base-pair (bp) repeat were weakly inducible by p40x. The level of trans-activation by p40x was increased when two (30-fold) or three (40-fold) 21-bp repeats were present in the upstream control region. In the mutant containing two 21-bp repeats, the upstream 21-bp repeat could be positioned in either the sense (30-fold) or the antisense (16-fold) orientation. Plasmids containing a 51-bp repeat element, which included a single 21-bp repeat, were induced to levels similar to that obtained with the 21-bp repeat sequence alone. Template DNAs containing a single copy of the HTLV-I sequences between -117 and -160 were stimulated approximately 10-fold by p40x when one copy of the 21-bp element was located downstream.

The sequence motifs that are involved in SV40 enhancer function also control SV40 late promoter activity

The simian virus 40 (SV40) enhancer element is constituted of two domains which contain sequences important for late transcription (M. Ernoult-Lange, F. Omilli, D. O'Reilly and E. May, J. Virol. 61, 167-176, 1987). By analysing a series of clustered point mutations generated throughout the enhancer region we mapped domain I from nt 232 to 272 and domain II from nt 184 to 216. These two domains which are required for late promoter activity both in the presence and in the absence of T antigen correspond closely to the domains B and A respectively, identified for enhancer function (M. Zenke, T. Grundström, H. Matthes, M. Wintzerith, C. Schatz, A. Wildeman and P. Chambon, EMBO J., 5, 387-397, 1986). Similarly to the enhancer function the late promoter elements defined by these two domains contain multiple sequence motifs. Moreover there is a striking overlap between the sequence motifs within domain A, active for early enhancer function and those within domain II involved in efficient late transcription.

Immortalization of rat embryo fibroblasts by mutant polyomavirus large T antigens deficient in DNA binding

We have identified a putative DNA-binding domain in polyomavirus large T antigen. Mutations introduced into the gene between amino acids 290 and 310 resulted in proteins that no longer bound to the high-affinity binding sites on the polyomavirus genome, showed no detectable nonspecific DNA binding, and were not able to initiate DNA replication from the viral origin. These mutant T antigen genes were introduced into rat embryo fibroblasts together with the neomycin resistance gene to allow selection for growth in the presence of G418. All the mutations tested facilitated the establishment of these cells in long-term culture at an efficiency indistinguishable from that of the wild-type protein.

Bidirectional promoter elements of simian virus 40 are required for efficient replication of the viral DNA

Mutants of simian virus 40 (SV40) lacking parts of the 72- and 21-base-pair repeat regions were made deficient in large T antigen by recombination with dlA 4000, a mutant containing a frameshift deletion near the amino terminus of the T antigen genes. These double mutants were transfected into COS cells, and the amounts of replicated viral DNA were measured at various times thereafter. It was found that deletion of either the 72- or 21-base-pair repeat region did not significantly reduce the accumulation of viral DNA. However, cells transfected with mutants lacking both of these promoter elements accumulated 100-fold less viral DNA than cells transfected with wild-type SV40. This indicates that the 72- and 21-base-pair repeat regions are each sufficient for supplying a function required for efficient replication of SV40 DNA. In addition, the ability of either of these regions to support efficient replication was gradually reduced as the number of promoter elements within each was decreased. Since the 72- and 21-base-pair repeat regions bidirectionally induce transcription, our results indicate that bidirectional promoter elements play a role in the replication of viral DNA. However, fewer of these elements are required for efficient replication than for efficient transcription.

trans Activation of the simian virus 40 enhancer

We describe experiments which demonstrated that the simian virus 40 (SV40) enhancer affects certain transcriptional units differently. We also found that a specific enhancer-transcriptional unit interaction can be regulated by trans-acting factors. Using transient assays, we examined the effects of the SV40 enhancer on herpesvirus thymidine kinase (tk) RNA levels when transcription was initiated either by the herpesvirus tk promoter or by an SV40 early promoter-tk fusion. We were unable to detect any effect of the enhancer on transcription from the tk promoter in CV-1 or HeLa cells. However, we found that the addition of T-antigen in trans allowed the enhancer to stimulate expression from the tk promoter. This induction by T-antigen did not require T-antigen-binding sites in cis and appeared to be an indirect effect. In contrast, tk expression from the SV40 early promoter fusion was greatly stimulated by the enhancer in CV-1 cells. Furthermore, in 293 cells the SV40 enhancer had only a marginal effect on the SV40 promoter-tk fusion, whereas it strongly stimulated tk expression from the tk promoter. Our results raise the possibility that the enhancer function may not show cell specificity per se; rather, the interaction between the enhancer and a specific gene may be responsible for cell specificity. We discuss these observations in terms of the SV40 early gene-to-late gene switch that occurs during SV40 lytic growth.

Characterization of the simian virus 40 late promoter: relative importance of sequences within the 72-base-pair repeats differs before and after viral DNA replication

We examined sequences involved in the simian virus 40 (SV40) late promoter in vivo, by using quantitative S1 nuclease analysis of a series of deletion mutants within the SV40 regulatory region. These mutants were constructed so as to place the altered promoter region in its normal position relative to the SV40 late genes. The effects of the deletions on late transcriptional activity were analyzed before and after viral DNA replication, by omitting or including SV40 large T antigen. The data show that (i) in the absence of large T antigen, the deletion of the 21-base-pair (bp) repeats results in a fourfold increase in late transcription, and (ii) the sequences within the 72-bp repeats are a component of the SV40 late promoter, acting not only before, but also after viral DNA replication. We identified two domains which contain sequences important for efficient late transcription. Domain I, at the late proximal end of each 72-bp repeat, was found to function before replication and was possibly also involved after replication. The contribution of domain II, at the late distal end of each 72-bp repeat, was much more significant after replication but only of minor importance before replication.

Stimulation of the adenovirus E2 promoter by simian virus 40 T antigen or E1A occurs by different mechanisms

We have examined the ability of simian virus 40 T antigen to stimulate transcription from the adenovirus E2 promoter. T antigen, produced from a cotransfected plasmid, stimulated chloramphenicol acetyltransferase enzyme and mRNA production from an E2 promoter-chloramphenicol acetyltransferase fusion plasmid (pEC113) in monkey kidney CV-1 cells. The level of stimulation of E2 transcription by simian virus 40 T antigen was equal to that observed in cotransfections of pEC113 and the adenovirus E1A gene product. Deletion mutations from the 5' end of the E2 promoter were examined for their ability to express basal, T-antigen, or E1A trans-activated promoter activity. In each case, deletion of upstream promoter sequences to -70 base pairs reduced chloramphenicol acetyltransferase expression to approximately 30% of the level observed with the intact E2 promoter. Deletion to -59 base pairs resulted in chloramphenicol acetyltransferase expression that was 3 to 5% of that observed with the intact E2 promoter. At saturating levels of the stimulatory proteins, the chloramphenicol acetyltransferase levels obtained in response to T antigen and adenovirus E1A were additive. COS-1 cells, which are derived from CV-1 cells and constitutively express simian virus 40 T antigen, do not support E2 promoter trans activation by T antigen. E1A trans activation of the E2 promoter is efficient in COS-1 cells. These results suggest that although promoter sequence requirements are similar, T antigen and E1A trans activate the E2 promoter by different mechanisms.

Binding of polyomavirus large T antigen to the human hsp70 promoter is not required for trans activation

Polyomavirus large T antigen binds to two sites located between positions -110 and -170 of a human heat shock protein 70 (hsp70) promoter. Methylation interference studies show that binding for each site is determined by two GPuGGC pentanucleotide sequences. The specificity of this binding interaction is similar to that observed for large T binding to the viral genome. The existence of sequences that bind a viral protein in a cellular promoter raises the possibility that these sequences play a role in gene expression in an uninfected cell. We show that hsp70 large T antigen binding site 1 is capable of functioning as an upstream promoter element in cells that do not contain any viral T antigen. Genetic analysis of this effect suggests that a cellular factor exists that has a binding specificity that overlaps but is not identical to that of polyomavirus large T antigen. To determine whether binding of polyomavirus large T antigen can regulate expression of the intact human hsp70 promoter, we have introduced the promoter into mouse cells with plasmids that express the polyomavirus early proteins. These proteins stimulate the level of correctly initiated hsp70 transcripts, but surprisingly the degree of stimulation remains unchanged for promoter constructs in which the large T antigen binding sites have been deleted. These observations suggest that trans activation of the hsp70 promoter by the polyomavirus early proteins occurs through protein-protein interactions and not through sequence-specific DNA binding.

Simian virus 40 DNA replication: functional organization of regulatory elements

The efficiency of simian virus 40 (SV40) DNA replication is dependent on the structural organization of the regulatory region. The enhancing effect of the G + C-rich 21-base-pair (bp) repeats on SV40 DNA replication is position and dose dependent and to some extent orientation dependent. The inverted orientation is about 50% as effective as the normal orientation of the 21-bp repeat region. Movement of the 21-bp repeat region 180 or 370 bp upstream of the ori sequence abolishes its enhancing effect, whereas no replication is detected if the 21-bp repeat region is placed downstream of the ori sequence. The dose-dependent enhancement of the 21-bp repeat of SV40 DNA replication as first described in single transfection by Bergsma et al. (D. J. Bergsma, D. M. Olive, S. W. Hartzell, and K. N. Subramanian, Proc. Natl. Acad. Sci. USA 79:381-385, 1982) is dramatically amplified in mixed transfection. In the presence of the 21-bp repeat region, the 72-bp repeat region can enhance SV40 DNA replication. In the presence of the 21-bp repeats and a competitive environment, the 72-bp repeat region exhibits a cis-acting inhibitory effect on SV40 DNA replication.

Immortalization of rat embryo fibroblasts by mutant polyomavirus large T antigens deficient in DNA binding

We have identified a putative DNA-binding domain in polyomavirus large T antigen. Mutations introduced into the gene between amino acids 290 and 310 resulted in proteins that no longer bound to the high-affinity binding sites on the polyomavirus genome, showed no detectable nonspecific DNA binding, and were not able to initiate DNA replication from the viral origin. These mutant T antigen genes were introduced into rat embryo fibroblasts together with the neomycin resistance gene to allow selection for growth in the presence of G418. All the mutations tested facilitated the establishment of these cells in long-term culture at an efficiency indistinguishable from that of the wild-type protein.

Activity of simian virus 40 late promoter elements in the absence of large T antigen: evidence for repression of late gene expression

We used chloramphenicol acetyltransferase transient expression to examine the activity of the promoter elements of the simian virus 40 late promoter in the absence of large T antigen. Since the experiments were done in permissive CV-1 cells, these conditions mimic the state which exists early in the viral lytic cycle before the onset of replication and T-antigen-mediated trans activation. Our data, using deletion analysis, indicate that removal of the 21-base-pair (bp) repeat region causes as much as a 10-fold increase in activity of the late promoter elements. This result suggests that the 21-bp repeat sequences may be involved in repression of the late promoter elements during the early phase of the lytic infection. This is supported by competition analysis which indicates that increasing amounts of competitor containing only the 21-bp repeat region results in increased activity of the intact promoter. A model for the activity of the late promoter through the course of lytic infection is presented.

Bidirectional promoter elements of simian virus 40 are required for efficient replication of the viral DNA

Mutants of simian virus 40 (SV40) lacking parts of the 72- and 21-base-pair repeat regions were made deficient in large T antigen by recombination with dlA 4000, a mutant containing a frameshift deletion near the amino terminus of the T antigen genes. These double mutants were transfected into COS cells, and the amounts of replicated viral DNA were measured at various times thereafter. It was found that deletion of either the 72- or 21-base-pair repeat region did not significantly reduce the accumulation of viral DNA. However, cells transfected with mutants lacking both of these promoter elements accumulated 100-fold less viral DNA than cells transfected with wild-type SV40. This indicates that the 72- and 21-base-pair repeat regions are each sufficient for supplying a function required for efficient replication of SV40 DNA. In addition, the ability of either of these regions to support efficient replication was gradually reduced as the number of promoter elements within each was decreased. Since the 72- and 21-base-pair repeat regions bidirectionally induce transcription, our results indicate that bidirectional promoter elements play a role in the replication of viral DNA. However, fewer of these elements are required for efficient replication than for efficient transcription.

Promiscuous trans activation of gene expression by an Epstein-Barr virus-encoded early nuclear protein

We identified an Epstein-Barr virus (EBV) gene product which functions in transient-expression assays as a nonspecific trans activator. In Vero cells, cotransfection of the BglII J DNA fragment of EBV together with recombinant constructs containing the bacterial chloramphenicol acetyltransferase (CAT) gene gave up to a 100-fold increased expression of CAT activity over that in cells transfected with the recombinant CAT constructs alone. The BglII J fragment acted promiscuously, in that increased CAT synthesis was observed regardless of whether the promoter sequences driving the CAT gene were of EBV, simian virus 40, adenovirus, or herpes simplex virus origin. Cleavage of cloned BglII-J plasmid DNA before transfection revealed that activation was dependent upon the presence of an intact BMLF1 open reading frame. This was confirmed with subclones of BglII-J and with hybrid promoter-open reading frame constructs. This region of the genome is also present in the rearranged P3HR-1-defective DNA species, and defective DNA clones containing these sequences produced a similar activation of CAT expression in cotransfection experiments. The heterogeneous 45-60-kilodalton polypeptide product of BMLF1 may play an important regulatory role in expression of lytic-cycle proteins in EBV-infected lymphocytes.

Binding of polyomavirus large T antigen to the human hsp70 promoter is not required for trans activation

Polyomavirus large T antigen binds to two sites located between positions -110 and -170 of a human heat shock protein 70 (hsp70) promoter. Methylation interference studies show that binding for each site is determined by two GPuGGC pentanucleotide sequences. The specificity of this binding interaction is similar to that observed for large T binding to the viral genome. The existence of sequences that bind a viral protein in a cellular promoter raises the possibility that these sequences play a role in gene expression in an uninfected cell. We show that hsp70 large T antigen binding site 1 is capable of functioning as an upstream promoter element in cells that do not contain any viral T antigen. Genetic analysis of this effect suggests that a cellular factor exists that has a binding specificity that overlaps but is not identical to that of polyomavirus large T antigen. To determine whether binding of polyomavirus large T antigen can regulate expression of the intact human hsp70 promoter, we have introduced the promoter into mouse cells with plasmids that express the polyomavirus early proteins. These proteins stimulate the level of correctly initiated hsp70 transcripts, but surprisingly the degree of stimulation remains unchanged for promoter constructs in which the large T antigen binding sites have been deleted. These observations suggest that trans activation of the hsp70 promoter by the polyomavirus early proteins occurs through protein-protein interactions and not through sequence-specific DNA binding.

Simian virus 40 DNA replication: functional organization of regulatory elements

The efficiency of simian virus 40 (SV40) DNA replication is dependent on the structural organization of the regulatory region. The enhancing effect of the G + C-rich 21-base-pair (bp) repeats on SV40 DNA replication is position and dose dependent and to some extent orientation dependent. The inverted orientation is about 50% as effective as the normal orientation of the 21-bp repeat region. Movement of the 21-bp repeat region 180 or 370 bp upstream of the ori sequence abolishes its enhancing effect, whereas no replication is detected if the 21-bp repeat region is placed downstream of the ori sequence. The dose-dependent enhancement of the 21-bp repeat of SV40 DNA replication as first described in single transfection by Bergsma et al. (D. J. Bergsma, D. M. Olive, S. W. Hartzell, and K. N. Subramanian, Proc. Natl. Acad. Sci. USA 79:381-385, 1982) is dramatically amplified in mixed transfection. In the presence of the 21-bp repeat region, the 72-bp repeat region can enhance SV40 DNA replication. In the presence of the 21-bp repeats and a competitive environment, the 72-bp repeat region exhibits a cis-acting inhibitory effect on SV40 DNA replication.

Stimulation of the adenovirus E2 promoter by simian virus 40 T antigen or E1A occurs by different mechanisms

We have examined the ability of simian virus 40 T antigen to stimulate transcription from the adenovirus E2 promoter. T antigen, produced from a cotransfected plasmid, stimulated chloramphenicol acetyltransferase enzyme and mRNA production from an E2 promoter-chloramphenicol acetyltransferase fusion plasmid (pEC113) in monkey kidney CV-1 cells. The level of stimulation of E2 transcription by simian virus 40 T antigen was equal to that observed in cotransfections of pEC113 and the adenovirus E1A gene product. Deletion mutations from the 5' end of the E2 promoter were examined for their ability to express basal, T-antigen, or E1A trans-activated promoter activity. In each case, deletion of upstream promoter sequences to -70 base pairs reduced chloramphenicol acetyltransferase expression to approximately 30% of the level observed with the intact E2 promoter. Deletion to -59 base pairs resulted in chloramphenicol acetyltransferase expression that was 3 to 5% of that observed with the intact E2 promoter. At saturating levels of the stimulatory proteins, the chloramphenicol acetyltransferase levels obtained in response to T antigen and adenovirus E1A were additive. COS-1 cells, which are derived from CV-1 cells and constitutively express simian virus 40 T antigen, do not support E2 promoter trans activation by T antigen. E1A trans activation of the E2 promoter is efficient in COS-1 cells. These results suggest that although promoter sequence requirements are similar, T antigen and E1A trans activate the E2 promoter by different mechanisms.

Allosteric control of simian virus 40 T-antigen binding to viral origin DNA

Simian virus 40 (SV40) large tumor antigen (T antigen) possesses several biochemical activities localized in different domains of the protein. These activities include sequence-specific binding to two major sites, I and II, in the SV40 control region, ATPase, and nucleotide-binding activity. In the present communication, we present evidence that specific binding of immunopurified T antigen to SV40 DNA is markedly inhibited by low concentrations of ATP, dATP, GTP, and dGTP. The inhibition is reversible after removal of the nucleotide, suggesting that simple nucleotide binding rather than a covalent modification of T antigen in the presence of ATP is responsible for the inhibition. The results suggest that T antigen may assume two conformations, one active and one inactive in binding to the SV40 origin of replication. In the presence of purine nucleoside triphosphates, the inactive conformation is favored.

Replication and transformation functions of in vitro-generated simian virus 40 large T antigen mutants

We used sodium bisulfite mutagenesis to introduce point mutations within the early region of the simian virus 40 genome. Seventeen mutants which contained amino acid changes in the amino-terminal half of the large T antigen coding sequence were assayed for their ability to replicate viral DNA and to induce transformation in the established rodent cell line Rat-3. The mutants fell into four basic classes with respect to these two biological functions. Five mutants had wild-type replication and transformation activities, six were totally defective, three were replication deficient and transformation competent, and two were replication competent and transformation deficient. Within these classes were mutants which displayed intermediate phenotypes, such as four mutants which were not totally deficient in viral replication or cellular transformation but instead showed reduced large T antigen function relative to wild type. Three large T mutants displayed transforming activity that was greater than that of wild type and are called supertransforming mutants. Of the most interest are mutants differentially defective in replication and transformation activities. These results both support and extend previous findings that two important biological functions of large T antigen can be genetically separated.

trans Activation of the simian virus 40 enhancer

We describe experiments which demonstrated that the simian virus 40 (SV40) enhancer affects certain transcriptional units differently. We also found that a specific enhancer-transcriptional unit interaction can be regulated by trans-acting factors. Using transient assays, we examined the effects of the SV40 enhancer on herpesvirus thymidine kinase (tk) RNA levels when transcription was initiated either by the herpesvirus tk promoter or by an SV40 early promoter-tk fusion. We were unable to detect any effect of the enhancer on transcription from the tk promoter in CV-1 or HeLa cells. However, we found that the addition of T-antigen in trans allowed the enhancer to stimulate expression from the tk promoter. This induction by T-antigen did not require T-antigen-binding sites in cis and appeared to be an indirect effect. In contrast, tk expression from the SV40 early promoter fusion was greatly stimulated by the enhancer in CV-1 cells. Furthermore, in 293 cells the SV40 enhancer had only a marginal effect on the SV40 promoter-tk fusion, whereas it strongly stimulated tk expression from the tk promoter. Our results raise the possibility that the enhancer function may not show cell specificity per se; rather, the interaction between the enhancer and a specific gene may be responsible for cell specificity. We discuss these observations in terms of the SV40 early gene-to-late gene switch that occurs during SV40 lytic growth.