Requirements for species-specific papovavirus DNA replication

Host-specific replication of BK virus DNA in mouse cell extracts is independently controlled by DNA polymerase alpha-primase and inhibitory activities

The activation of the human polyomavirus BK causes polyomavirus-associated nephropathy in immunocompromised humans. Studies of the virus have been restricted since the virus DNA replication is species specific. Cell-based and cell-free DNA replication systems, including the BK virus (BKV) monopolymerase DNA replication system using purified proteins, reproduce the species specificity (28). Therefore, the major host proteins comprising this assay, DNA polymerase alpha-primase (Pol-prim) and replication protein A (RPA), were intensively studied here. We demonstrate that Pol-prim plays a major role in the species specificity of BKV DNA replication. Both large subunits p180 and p68 of the enzyme complex have central functions in modulating the host specificity. Recently, an inhibitory activity of BKV DNA replication was described (C. Mahon, B. Liang, I. Tikhanovich, J. R. Abend, M. J. Imperiale, H. P. Nasheuer, and W. R. Folk, J. Virol. 83:5708-5717, 2009), but neither mouse Pol-prim nor mouse RPA diminishes cell-free BKV DNA replication. However, the inhibition of BKV DNA replication in mouse extracts depends on sequences flanking the core origin. In the presence of human Pol-prim, the inhibitory effect of mouse cell factors is abolished with plasmid DNAs containing the murine polyomavirus early promoter region, whereas the late enhancer region and the core origin are supplied from BKV. Thus, BKV replication is regulated by both Pol-prim, as a core origin species-specific factor, and inhibitory activities, as origin-flanking sequence-dependent factor(s).

Restriction of human polyomavirus BK virus DNA replication in murine cells and extracts

BK virus (BKV) causes persistent and asymptomatic infections in most humans and is the etiologic agent of polyomavirus-associated nephropathy (PVAN) and other pathologies. Unfortunately, there are no animal models with which to study activation of BKV replication in the human kidney and the accompanying PVAN. Here we report studies of the restriction of BKV replication in murine cells and extracts and the cause(s) of this restriction. Upon infection of murine cells, BKV expressed large T antigen (TAg), but viral DNA replication and progeny were not detected. Transfection of murine cells with BKV TAg expression vectors also caused TAg expression without accompanying DNA replication. Analysis of the replication of DNAs containing chimeric BKV and murine polyomavirus origins revealed the importance of BKV core origin sequences and TAg for DNA replication. A sensitive assay was developed with purified BKV TAg that supported TAg-dependent BKV DNA replication with human but not with murine cell extracts. Addition of human replication proteins, DNA polymerase alpha-primase, replication protein A, or topoisomerase I to the murine extracts with BKV TAg did not rescue viral DNA replication. Notably, addition of murine extracts to human extracts inhibited BKV TAg-dependent DNA replication at a step prior to or during unwinding of the viral origin. These findings and differences in replication specificity between BKV TAg and the TAgs of simian virus 40 (SV40) and JC virus (JCV) and their respective origins implicate features of the BKV TAg and origin distinct from SV40 and JCV in restriction of BKV replication in murine cells.

Cross-species virus transmission and the emergence of new epidemic diseases

Host range is a viral property reflecting natural hosts that are infected either as part of a principal transmission cycle or, less commonly, as "spillover" infections into alternative hosts. Rarely, viruses gain the ability to spread efficiently within a new host that was not previously exposed or susceptible. These transfers involve either increased exposure or the acquisition of variations that allow them to overcome barriers to infection of the new hosts. In these cases, devastating outbreaks can result. Steps involved in transfers of viruses to new hosts include contact between the virus and the host, infection of an initial individual leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host. Here we review what is known about host switching leading to viral emergence from known examples, considering the evolutionary mechanisms, virus-host interactions, host range barriers to infection, and processes that allow efficient host-to-host transmission in the new host population.

Initiation of JC virus DNA replication in vitro by human and mouse DNA polymerase alpha-primase

Host species specificity of the polyomaviruses simian virus 40 (SV40) and mouse polyomavirus (PyV) has been shown to be determined by the host DNA polymerase alpha-primase complex involved in the initiation of both viral and host DNA replication. Here we demonstrate that DNA replication of the related human pathogenic polyomavirus JC virus (JCV) can be supported in vitro by DNA polymerase alpha-primase of either human or murine origin indicating that the mechanism of its strict species specificity differs from that of SV40 and PyV. Our results indicate that this may be due to differences in the interaction of JCV and SV40 large T antigens with the DNA replication initiation complex.

Stimulation of DNA replication from the polyomavirus origin by PCAF and GCN5 acetyltransferases: acetylation of large T antigen

The PCAF and GCN5 acetyltransferases, but not p300 or CBP, stimulate DNA replication when tethered near the polyomavirus origin. Replication stimulation by PCAF and GCN5 is blocked by mutational inactivation of their acetyltransferase domains but not by deletion of sequences that bind p300 or CBP. Acetylation of histones near the polyomavirus origin assembled into chromatin in vivo is not detectably altered by expression of these acetyltransferases. PCAF and GCN5 interact with polyomavirus large T antigen in vivo, PCAF acetylates large T antigen in vitro, and large T-antigen acetylation in vivo is dependent upon the integrity of the PCAF acetyltransferase domain. These data suggest replication stimulation occurs through recruitment of large T antigen to the origin and acetylation by PCAF or GCN5.

Gene transfer to salivary glands

This article provides a review of the application of gene transfer technology to studies of salivary glands. Salivary glands provide an uncommon target site for gene transfer but offer many experimental situations likely of interest to the cell biologist. The reader is provided with a concise overview of salivary biology, along with a general discussion of the strategies available for gene transfer to any tissue. In particular, adenoviral vectors have been useful for proof of concept studies with salivary glands. Several examples are given, using adenoviral-mediated gene transfer, for addressing both biological and clinical questions. Additionally, benefits and shortcomings affecting the utility of this technology are discussed.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

A replication-deficient rSV40 mediates liver-directed gene transfer and a long-term amelioration of jaundice in gunn rats

BACKGROUND & AIMS

In the quest for a recombinant viral vector for liver-directed gene therapy that would permit both prolonged and efficient transgene expression in quiescent hepatocytes in vivo and repeated administration, we evaluated a recombinant simian virus 40 (rSV40).

METHODS

The rSV40 was generated through replacement of the DNA encoding for the T antigens (Tag) by the coding region of human bilirubin-uridine 5'-diphosphate-glucuronosyl-transferase (BUGT) complementary DNA (SV-hBUGT). Helper-free rSV40 units were generated at infectious titers of 5 x 10(9) to 1 x 10(10) infectious units (IU)/mL in a Tag-producing packaging cell line (COS-7 cells).

RESULTS

After 1, 3, or 7 daily infusions of 3 x 10(9) IU of SV-hBUGT through an indwelling portal vein catheter in bilirubin-UGT-deficient jaundiced Gunn rats, mean serum bilirubin concentrations decreased by 40%, 60% and 70%, respectively, in 3 weeks and remained at those levels throughout the duration of the study (40 days). Results of liver biopsies from SV-hBUGT-treated Gunn rats, but not from controls, were positive for human BUGT DNA, messenger RNA, and protein. Bilirubin-UGT activity in liver homogenates was 8%-12% of normal, and bilirubin glucuronides were excreted in bile. Immunostaining showed that >50%-60% of hepatocytes stably expressed the transgene. Portal vein infusion of an rSV40 expressing hepatitis B surface antigen (HBsAg) in a naive Gunn rat and a Gunn rat that had received 7 injections of SV-BUGT resulted in approximately equal levels of hepatic expression of HBsAg, indicating that multiple inoculations of SV-BUGT did not elicit neutralizing antibodies. Plasma alanine aminotransferase levels and liver histology remained normal despite repeated injections of rSV40.

CONCLUSIONS

rSV40 vectors may represent a significant advance toward gene therapy for metabolic diseases.

Marek's disease virus (MDV) homologues of herpes simplex virus type 1 UL49 (VP22) and UL48 (VP16) genes: high-level expression and characterization of MDV-1 VP22 and VP16

Genes UL49 and UL48 of Marek's disease virus 1 (MDV-1) strain RB1B, encoding the respective homologues of herpes simplex virus type 1 (HSV-1) genes VP22 and VP16, were cloned into a baculovirus vector. Seven anti-VP22 MAbs and one anti-VP16 MAb were generated and used to identify the tegument proteins in cells infected lytically with MDV-1. The two genes are known to be transcribed as a single bicistronic transcript, and the detection of only one of the two proteins (VP22) in MSB-1 lymphoma and in chicken embryo skin cells infected with MDV-1 prompted the study of the transcription/translation of the UL49-48 sequence in an in vivo and in vitro expression system. VP16 was expressed in vitro at detectable levels, whereas it could only be detected at a lower level in a more controlled environment. It was demonstrated that VP22 is phosphorylated in insect cells and possesses the remarkable property of being imported into all cells in a monolayer. VP22 localized rapidly and efficiently to nuclei, like its HSV-1 counterpart. The DNA-binding property of VP22 is also reported and a part of the region responsible for this activity was identified between aa 16 and 37 in the N-terminal region of the protein.

The initiation of simian virus 40 DNA replication in vitro

DNA replication is a complicated process that is largely regulated during stages of initiation. The Siman Virus 40 in vitro replication system has served as an excellent model for studies of the initiation of DNA replication, and its regulation, in eukaryotes. Initiation of SV40 replication requires a single viral protein termed T-antigen, all other proteins are supplied by the host. The recent determination of the solution structure of the T-antigen domain that recognizes the SV40 origin has provided significant insights into the initiation process. For example, it has afforded a clearer understanding of origin recognition, T-antigen oligomerization, and DNA unwinding. Furthermore, the Simian virus 40 in vitro replication system has been used to study nascent DNA formation in the vicinity of the viral origin of replication. Among the conclusions drawn from these experiments is that nascent DNA synthesis does not initiate in the core origin in vitro and that Okazaki fragment formation is complex. These and related studies demonstrate that significant progress has been made in understanding the initiation of DNA synthesis at the molecular level.

Polyomavirus large T can support DNA replication in human cells

Human cells are generally thought to be nonpermissive for polyomavirus (Py) DNA replication. Using transient transfection, we show that Py large T-antigen (LT) was able to support replication of a Py origin-containing plasmid in two human cell lines. Replication supported by LT in human cells was specific for the Py origin and required its enhancer sequences, as well as the previously reported critical phosphorylation sites within LT. Py replication efficiency was comparable to that of papillomavirus E1 and E2 activated DNA replication in transient assays performed in human 293 and C-33A cells. Previous analysis of DNA replication in vitro has pointed to polymerase alpha-primase as a specificity determinant for polyomavirus. The data presented here imply that in certain cellular environments, Py LT must functionally interact with human polymerase alpha-primase to permit DNA replication.

Species-specific replication of simian virus 40 DNA in vitro requires the p180 subunit of human DNA polymerase alpha-primase

Human cell extracts efficiently support replication of simian virus 40 (SV40) DNA in vitro, while mouse cell extracts do not. Since human DNA polymerase alpha-primase is the major species-specific factor, we set out to determine the subunit(s) of DNA polymerase alpha-primase required for this species specificity. Recombinant human, mouse, and hybrid human-mouse DNA polymerase alpha-primase complexes were expressed with baculovirus vectors and purified. All of the recombinant DNA polymerase alpha-primases showed enzymatic activity and efficiently synthesized the complementary strand on an M13 single-stranded DNA template. The human DNA polymerase alpha-primase (four subunits [HHHH]) and the hybrid DNA polymerase alpha-primase HHMM (two human subunits and two mouse subunits), containing human p180 and p68 and mouse primase, initiated SV40 DNA replication in a purified system. The human and the HHMM complex efficiently replicated SV40 DNA in mouse extracts from which DNA polymerase alpha-primase was deleted, while MMMM and the MMHH complex did not. To determine whether the human p180 or p68 subunit was required for SV40 DNA replication, hybrid complexes containing only one human subunit, p180 or p68, together with three mouse subunits (HMMM and MHMM) or three human subunits and one mouse subunit (MHHH and HMHH) were tested for SV40 DNA replication activity. The hybrid complexes HMMM and HMHH synthesized oligoribonucleotides in the SV40 initiation assay with purified proteins and replicated SV40 DNA in depleted mouse extracts. In contrast, the hybrid complexes containing mouse p180 were inactive in both assays. We conclude that the human p180 subunit determines host-specific replication of SV40 DNA in vitro.

Sequences flanking the pentanucleotide T-antigen binding sites in the polyomavirus core origin help determine selectivity of DNA replication

Replication of the genomes of the polyomaviruses requires two virus-specified elements, the cis-acting origin of DNA replication, with its auxiliary DNA elements, and the trans-acting viral large tumor antigen (T antigen). Appropriate interactions between them initiate the assembly of a replication complex which, together with cellular proteins, is responsible for primer synthesis and DNA chain elongation. The organization of cis-acting elements within the origins of the polyomaviruses which replicate in mammalian cells is conserved; however, these origins are sufficiently distinct that the T antigen of one virus may function inefficiently or not at all to initiate replication at the origin of another virus. We have studied the basis for such replication selectivity between the murine polyomavirus T antigen and the primate lymphotropic polyomavirus origin. The murine polyomavirus T antigen is capable of carrying out the early steps of the assembly of an initiation complex at the lymphotropic papovavirus origin, including binding to and deformation of origin sequences in vitro. However, the T antigen inefficiently unwinds the origin, and unwinding is influenced by sequences flanking the T antigen pentanucleotide binding sites on the late side of the viral core origin. These same sequences contribute to the replication selectivity observed in vivo and in vitro, suggesting that the inefficient unwinding is the cause of the replication defect. These observations suggest a mechanism by which origins of DNA replication can evolve replication selectivity and by which the function of diverse cellular origins might be temporally activated during the S phase of the eukaryotic cell cycle.

Murine polyomavirus and simian virus 40 large T antigens produce different structural alterations in viral origin DNA

Murine polyomavirus (Py) and simian virus (SV40) encode homologous large T antigens (T Ags) and also have comparable sequence motifs in their core replication origins. While the ability of SV40 T Ag to produce specific distortions within the SV40 core replication origin (ori) in a nucleotide-dependent fashion has been well documented, little is known about related effects of Py T Ag on Py ori DNA. Therefore, we have examined viral origin DNA binding in the presence of nucleotide and the resulting structural changes induced by Py and SV40 T Ags by DNase I footprinting and KMnO4 modification assays. The structural changes in the Py ori induced by Py T Ag included sites within both the A/T and early side of the core origin region, consistent with what has been shown for SV40. Interestingly, however, Py T Ag also produced sites of distortion within the center of the origin palindrome and at several sites within both the early and late regions that flank the core ori. Thus, Py T Ag produces a more extensive and substantially different pattern of KMnO4 modification sites than does SV40 T Ag. We also observed that both T Ags incompletely protected and distorted the reciprocal ori region. Therefore, significant differences in the interactions of Py and SV40 T Ags with ori DNA may account for the failure of each T Ag to support replication of the reciprocal ori DNA in permissive cell extracts.

The mouse DNA polymerase alpha-primase subunit p48 mediates species-specific replication of polyomavirus DNA in vitro

Mouse cell extracts support vigorous replication of polyomavirus (Py) DNA in vitro, while human cell extracts do not. However, the addition of purified mouse DNA polymerase alpha-primase to human cell extracts renders them permissive for Py DNA replication, suggesting that mouse polymerase alpha-primase determines the species specificity of Py DNA replication. We set out to identify the subunit of mouse polymerase alpha-primase that mediates this species specificity. To this end, we cloned and expressed cDNAs encoding all four subunits of mouse and human polymerase alpha-primase. Purified recombinant mouse polymerase alpha-primase and a hybrid DNA polymerase alpha-primase complex composed of human subunits p180 and p68 and mouse subunits p58 and p48 supported Py DNA replication in human cell extracts depleted of polymerase alpha-primase, suggesting that the primase heterodimer or one of its subunits controls host specificity. To determine whether both mouse primase subunits were required, recombinant hybrid polymerase alpha-primases containing only one mouse primase subunit, p48 or p58, together with three human subunits, were assayed for Py replication activity. Only the hybrid containing mouse p48 efficiently replicated Py DNA in depleted human cell extracts. Moreover, in a purified initiation assay containing Py T antigen, replication protein A (RP-A) and topoisomerase I, only the hybrid polymerase alpha-primase containing the mouse p48 subunit initiated primer synthesis on Py origin DNA. Together, these results indicate that the p48 subunit is primarily responsible for the species specificity of Py DNA replication in vitro. Specific physical association of Py T antigen with purified recombinant DNA polymerase alpha-primase, mouse DNA primase heterodimer, and mouse p48 suggested that direct interactions between Py T antigen and primase could play a role in species-specific initiation of Py replication.

Amplification mediated by polyomavirus large T antigen defective in replication

The polyomavirus large T antigen promotes homologous recombination at high rates when expressed in rat cells carrying the viral replication origin and two repeats of viral DNA sequences stably integrated into the cellular genome. Recombination consists of both reciprocal and nonreciprocal events and is promoted by mutants defective in the initiation of viral DNA synthesis (L. St-Onge, L. Bouchard, and M. Bastin, J. Virol. 67:1788-1795, 1993). We have extended our studies to a rat cell line undergoing amplification of the viral insert. We show that large T antigen promotes amplification independently of its replicative function but that its origin-specific DNA binding activity is not sufficient to promote homologous recombination.

Specific transcription factors stimulate simian virus 40 and polyomavirus origins of DNA replication

The origins of DNA replication (ori) in simian virus 40 (SV40) and polyomavirus (Py) contain an auxiliary component (aux-2) composed of multiple transcription factor binding sites. To determine whether this component stimulated replication by binding specific transcription factors, aux-2 was replaced by synthetic oligonucleotides that bound a single transcription factor. Sp1 and T-antigen (T-ag) sites, which exist in the natural SV40 aux-2 sequence, provided approximately 75 and approximately 20%, respectively, of aux-2 activity when transfected into monkey cells. In cell extracts, only T-ag sites were active. AP1 binding sites could replace completely either SV40 or Py aux-2. Mutations that eliminated AP1 binding also eliminated AP1 stimulation of replication. Yeast GAL4 binding sites that strongly stimulated transcription in the presence of GAL4 proteins failed to stimulate SV40 DNA replication, although they did partially replace Py aux-2. Stimulation required the presence of proteins consisting of the GAL4 DNA binding domain fused to specific activation domains such as VP16 or c-Jun. These data demonstrate a clear role for transcription factors with specific activation domains in activating both SV40 and Py ori. However, no correlation was observed between the ability of specific proteins to stimulate promoter activity and their ability to stimulate origin activity. We propose that only transcription factors whose specific activation domains can interact with the T-ag initiation complex can stimulate SV40 and Py ori-core activity.

Specific transcription factors stimulate simian virus 40 and polyomavirus origins of DNA replication

The origins of DNA replication (ori) in simian virus 40 (SV40) and polyomavirus (Py) contain an auxiliary component (aux-2) composed of multiple transcription factor binding sites. To determine whether this component stimulated replication by binding specific transcription factors, aux-2 was replaced by synthetic oligonucleotides that bound a single transcription factor. Sp1 and T-antigen (T-ag) sites, which exist in the natural SV40 aux-2 sequence, provided approximately 75 and approximately 20%, respectively, of aux-2 activity when transfected into monkey cells. In cell extracts, only T-ag sites were active. AP1 binding sites could replace completely either SV40 or Py aux-2. Mutations that eliminated AP1 binding also eliminated AP1 stimulation of replication. Yeast GAL4 binding sites that strongly stimulated transcription in the presence of GAL4 proteins failed to stimulate SV40 DNA replication, although they did partially replace Py aux-2. Stimulation required the presence of proteins consisting of the GAL4 DNA binding domain fused to specific activation domains such as VP16 or c-Jun. These data demonstrate a clear role for transcription factors with specific activation domains in activating both SV40 and Py ori. However, no correlation was observed between the ability of specific proteins to stimulate promoter activity and their ability to stimulate origin activity. We propose that only transcription factors whose specific activation domains can interact with the T-ag initiation complex can stimulate SV40 and Py ori-core activity.

Amplification of polyomavirus DNA sequences stably integrated in rat cells

To investigate the mechanism by which the polyomavirus large T antigen (T-Ag) promotes amplification of integrated viral sequences, we constructed a rat cell line, Hy2-ts5, carrying two different inserts of polyomavirus DNA. The first insert, designated the middle T (pmt) locus, was devised to analyze homologous recombination between two defective copies of pmt lying 3.3 kb apart on the same chromosome. Reconstitution of a functional pmt by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. The second locus contained the polyomavirus large T (plt) gene carrying a temperature-sensitive mutation and producing a nonfunctional large T-Ag at 39 degrees C. A shift to the permissive temperature for as little as 24 h induced the production of a functional large T-Ag which, in turn, promoted homologous recombination in the pmt locus at a rate close to 1.0 per cell generation. The particularity of this system is that it allowed recombination products to be analyzed as early as a single cell doubling following the initial recombinational event. Amplification occurred by successive duplications of a discrete sequence in the viral insert. Unequal sister chromatid exchange was ruled out as the recombination mechanism promoted by large T-Ag. Instead, we proposed a model of nonconservative recombination involving mispairing between homologous sequences.

Determination of the origin-specific DNA-binding domain of polyomavirus large T antigen

To map the DNA-binding domain of polyomavirus large T antigen, we constructed a set of plasmids coding for unidirectional carboxy- or amino-terminal deletion mutations in the large T antigen. Analysis of origin-specific DNA binding by mutant proteins expressed in Cos-1 cells revealed that the C-terminal boundary of the DNA-binding domain is at or near Glu-398. Fusion proteins of large T antigen lacking the first 200 N-terminal amino acids bound specifically to polyomavirus origin DNA; however, deletions beyond this site resulted in unstable proteins which could not be tested for DNA binding. Testing of point mutants and internal deletions by others suggested that the N-terminal boundary of the DNA-binding domain lies between amino acids 282 and 286. Taken together, these results locate the DNA-binding domain of polyomavirus large T antigen to the 116-amino-acid region between residues 282 and 398.

Single strand binding protein specific for the polyoma early-coding strand of PEA1 (AP1) regulatory sequence

We have shown that nuclear and cytosolic proteins from embryonal carcinoma F9 cells are able to bind to the early-coding strand of polyoma enhancer A domain. As demonstrated by mobility shift specific competition experiments, DNase I footprinting, depurination and depyrimidation interference, and proteolytic clipping performed with single stranded oligonucleotides, some of these proteins bind specifically to the early-coding PEA1 (AP1) motif. In addition, 'Southwestern' analysis has made possible the identification of a 46 KD nuclear protein that binds to this sequence. These cellular proteins did not bind to the complementary single strand as demonstrated by mobility shift analysis, nor did they bind to RNA synthesized in vitro by using the complementary strand as template. They were also shown to be different from their corresponding double strand binding factors. This new dimension in the functional flexibility and complexity of the polyoma enhancer suggests new properties of the classic regulating sequences that could provide additional modulation of regulating activities.

The yeast GAL4 protein transactivates the polyomavirus origin of DNA replication in mouse cells

We have replaced the polyomavirus (Py) enhancer, which is an essential component of the Py origin of DNA replication (ori), with five repeats of a 17-bp oligonucleotide including the yeast GAL4 upstream activating sequence (5xGAL4 sites). Plasmids containing this modified Py ori, designated test plasmids, and plasmids encoding either the GAL4 transcriptional activator protein or various derivatives of this protein were cotransfected into mouse cells which constitutively synthesize a temperature-sensitive Py large tumor antigen (T-Ag). Replication of the test plasmids was monitored by Southern blot determinations of the amounts of plasmid DNA that became resistant to cleavage by the enzyme DpnI. These studies showed that in the presence of a functional T-Ag, the GAL4 protein, and hybrid proteins including the GAL4 DNA-binding domain and the activating domain of the adenovirus E1a or herpesvirus VP16 protein transactivated the modified Py ori. A truncated protein including just the GAL4 DNA-binding domain was inactive in these assays. The authentic GAL4 protein was found to be a more efficient replication transactivator than the hybrid proteins. In contrast, chloramphenicol acetyltransferase assays showed that the hybrid proteins were more efficient transcriptional activators than the GAL4 protein. The extent of the GAL4-dependent replication of a plasmid in which the Py early promoter was deleted was 55% lower than that of a plasmid including the promoter. However, the extents of replication of plasmids including two tandem repeats of the remaining Py origin core and 5xGAL4 sites or two origin cores flanking a single cluster of 5xGAL4 sites were 4.8- and 1.6-fold higher than that of the plasmid including a single copy of each element. The replication of a plasmid including two clusters of 5xGAL4 sites flanking a single origin core was below the limit of detection of our assays. These results indicate that the GAL4 and hybrid transactivators do not activate the Py ori by virtue of their interactions with transcription factors that bind promoter elements. Rather, it appears that these activator proteins may interact with the replication initiation complexes, thereby facilitating or inhibiting the initiation of replication.

Intrachromosomal recombination mediated by papovavirus large T antigens

To investigate the mechanism by which the large T antigen (T-Ag) of polyomavirus and simian virus 40 can promote recombination in mammalian cells, we analyzed homologous recombination events occurring between two defective copies of the polyomavirus middle T (pmt) oncogene lying in close proximity on the same chromosome in a rat cell line. Reconstitution of a functional pmt gene by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. Introduction of the polyomavirus large T (plt) oncogene into the cell line by DNA transfection promoted recombination very efficiently, with rates in the range of 10(-1) to 10(-2) per cell generation. Recombination was independent of any amplification of viral sequences and could even be promoted by the large T-Ag from simian virus 40, which cannot activate polyomavirus DNA replication. To explain the role of large T-Ag, we propose a novel mechanism of nonconservative recombination involving slipped-strand mispairing between the two viral repeats followed by gap repair synthesis.