Sequences in the polyomavirus DNA regulatory region involved in viral DNA replication and early gene expression

A B-lymphocyte activation molecule related to the nerve growth factor receptor and induced by cytokines in carcinomas

B cells and primary carcinomas express a surface molecule, Bp50 (CDw40), absent from other hematopoietic cells and from normal epithelium, and thought to play a regulatory role in B-cell maturation and epithelial neoplasia. In this work the sequence of a cDNA clone encoding Bp50 was analyzed by a newly derived transition matrix method. Among several interesting relationships with known receptors was found an extensive homology with the nerve growth factor receptor. The mRNA is induced by gamma-interferon in both B cells and epithelial neoplasms, suggesting a role for the molecule in the development of carcinomas at sites of chronic inflammation.

Optimal replication of plasmids carrying polyomavirus origin regions requires two high-affinity binding sites for large T antigen

The efficiency of replication of plasmids containing the control region of polyomavirus DNA including one, two, or all three of the strong binding sites for large T antigen was measured in COP 8 cells which provide polyomavirus T antigen in trans. It was found that plasmids carrying only binding site A (the one closest to the origin core region) exhibited only 10% of the replication competence of plasmids with binding sites A and B or A and C. Plasmids containing all three binding sites, A, B, and C, did not replicate more efficiently than those with only two strong T-antigen-binding sites. We conclude, therefore, that optimal T-antigen-dependent replication of polyomavirus DNA requires two high-affinity T-antigen-binding sites.

Asp-286----Asn-286 in polyomavirus large T antigen relaxes the specificity of binding to the polyomavirus origin

We isolated revertants of a polyomavirus whose origin of DNA replication contains a point mutation in the palindrome to which large T antigen binds. Four independent second-site revertants contain an Asp-286----Asn-286 substitution in large T antigen. This mutant large T antigen activates replication of DNAs containing the mutant polyomavirus origin as well as replication of DNAs containing the wild-type origin; however, replication of DNAs with enhancer mutations is not activated by this large T antigen. The Asn-286 mutation occurs in a positively charge region of large T antigen near the location of several mutations which inactivate DNA replication. We suggest that this region of large T antigen is responsible for recognition of specific DNA sequences at the origin and that ionic forces are important for this interaction.

Transduction of the human insulin gene via retroviral vectors fails to yield spliced transcripts

Previous reports on retroviral vectors have shown them to be useful for transferring genes into animal cells. Genes placed under the retroviral long terminal repeat (LTR) act as dominant loci in recipient cells and can permanently alter their genotype and phenotype. Previous reports have shown that recombinant retroviruses containing genomic sequences with both introns and exons display a high frequency of deletion and abnormal kinetics of splicing of intron sequences. We report here our findings when a 2.9-kb fragment containing the entire human insulin gene was inserted into a Moloney-derived retroviral vector in the same transcriptional orientation as the LTRs. RNA transcripts synthesized in cells containing such constructs remain unspliced, as assessed by both RNA blot analysis and S1 mapping. Ten subclones derived following viral passage showed no splicing, and failure to splice was observed regardless of cell type or species of origin, or number of viral passages. Thus, genomic sequences containing introns when situated within the context of a retroviral transcript do not in all instances exhibit expected kinetics of splicing.

Analysis of origin of DNA replication of human papovavirus BK

We have analyzed the cis-acting sequence elements and properties of the origin of DNA replication of human papovavirus BK (BKV). The precise boundaries of the origin varied, depending on the cell type and the viral T antigen used for assay. The BKV minimal origin of replication consisted of an inverted repeat, T-antigen-binding site II, and a 20-base-pair AT block when assayed in monkey kidney CV1 and HeLa cells by using the BKV T antigen. This 76-base-pair minimal origin did not replicate in COS cells in the presence of the simian virus 40 (SV40) T antigen. Unlike that from the SV40 minimal origin, replication from the BKV minimal origin was not enhanced by BKV ori-flanking sequences in CV1 or HeLa cells, using the BKV T antigen. BKV ori-flanking sequences did activate the SV40 minimal origin of replication in COS cells and relieved the orientation-dependent property of this origin. Finally, the BKV T antigen was found to autoregulate activity of the BKV early transcriptional regulatory region. The BKV origin of replication shows similarities to and differences from those of the related viruses SV40 and polyomavirus, suggesting that the proteins involved in the initiation of replication interact with origin sequences differently in these viruses.

Expression and amplification in transgenic mice of a polyoma virus mutant regulatory region

Two hybrid gene constructs consisting of wild-type and mutant polyoma regulatory regions fused to a bacterial reporter gene were inserted in the mouse germline. Both transgenes were expressed in a large number of different organs. However, marker gene expression controlled by the polyoma wild-type regulatory region was not detectable in the early embryo and remained low throughout the life of the animal while expression controlled by the polyoma F9-1 mutation was detectable in blastocysts and was significantly higher at later stages of development. The F9-1 hybrid gene was also amplifiable when large T-antigen was supplied in trans to mice or to kidney cells derived from these transgenic mice. Amplification resulted in the appearance of several hundred copies of episomal transgenes and a marked increase of marker gene RNA and protein. Our results suggest that the F9-1 mutation does not alter the target spectrum of gene expression in vivo but does create a more efficient enhancer element in the polyoma early control region. Transgene amplification based upon use of the polyoma regulatory elements may be a means of increasing expression of genes in transgenic mice.

RNA polymerases stall and/or prematurely terminate nearby both early and late promoters on polyomavirus DNA

Levels of transcription within the E and L strands of the five major PstI fragments of polyomavirus (strain AT3) were measured by pulse-labeling RNA both in infected cells and in isolated nuclei or viral transcription complexes during the late phase of infection. Quantification was assured by hybridization to single-stranded DNAs in solution followed by collection of hybrids on nitrocellulose filters and ribonuclease treatment. The level of in vivo transcription in the region of the early (E strand) promoter was two- to threefold higher than that in all other E-strand regions, suggesting that most RNA polymerases prematurely terminate transcription shortly downstream from this promoter during the late phase. In vitro transcription levels in this region were five- to tenfold higher than in the remainder of the E strand, suggesting that many RNA polymerases 'stall' shortly after initiation in vivo but can be reactivated and continue transcription in vitro upon exposure to detergents and high salt solution. Some premature termination nearby the late (L strand) promoter was also detected by the same method. Strikingly, many RNA polymerases also stalled on the L strand in the region of the early promoter, some 5 x 10(3) bases downstream from the late promoter. Treatment of cells with dichlororibofuranosylbenzimidazole did not affect polymerases that stalled or terminated prematurely, but strongly reduced the presence of polymerases that normally transcribed throughout the entire E or L strand. Examination of the size of RNA chains produced during in vitro incubations showed that many polymerases stalled in vivo within 50 to 100 nucleotides downstream from the initiation sites on both DNA strands. The number of polymerases active in vitro at the E strand promoter was similar to the number of polymerases at the L strand promoter. However, in contrast to L-strand transcription, most of the polymerases that initiated at the E-strand promoter were incapable of extended transcription in vivo. These results suggest that large T antigen-mediated repression of E-strand transcription is not simply due to the exclusion of RNA polymerases from the early promoter. Stalling and/or premature termination by RNA polymerases shortly downstream from the early promoter appears to be a mechanism by which temporal regulation of polyomavirus gene expression can be effected.

Construction and characterization of a retroviral vector demonstrating efficient expression of cloned cDNA sequences

We describe the construction and properties of a retroviral expression vector, designated pMV-7, designed to transfer unselected cDNAs and produce their encoded proteins in recipient cells. The vector is flanked by the long terminal repeats (LTRs) of the Moloney murine sarcoma virus (MSV) and contains the selectable drug resistance gene neo under the regulation of the herpes simplex virus (HSV) thymidine kinase (tk) promoter. Unique Eco RI and Hind III sites facilitate the introduction of sequences whose transcription is regulated by the 5' LTR. We have inserted cDNAs encoding: (i) the human lymphocyte antigen T4, (ii) the human lymphocyte antigen T8, and (iii) the murine hypoxanthine-guanine phosphoribosyl transferase (HPRT), into the pMV-7 vector. These constructions were used to transduce recipient cells to the neo+ phenotype. In each case, functional assays demonstrated that 65-92% of the neo+ clones produced the appropriate protein encoded by its corresponding cDNA. These clones were characterized further by analyzing the expression of vector-regulated transcripts. The neo+T4+ clones expressed a single full-length LTR-to-LTR transcript as detected by a T4 probe. The neo+T8+ clones, however, expressed both a full-length LTR-to-LTR transcript and an additional smaller transcript as detected by a T8 probe. This smaller transcript probably resulted from the utilization of cryptic signals which control 3' RNA processing. Furthermore, all of the neo+ clones expressed a transcript that initiated from the tk promoter, contained the neo gene, and used polyadenylation signals provided by the 3' LTR. Thus, the pMV-7 vector is capable of high-efficiency transfer and high-frequency expression of the cDNA-encoded protein.

DNA sequence requirements for replication of polyomavirus DNA in vivo and in vitro

Cell extracts of FM3A mouse cells replicate polyomavirus (Py) DNA in the presence of immunoaffinity-purified Py large T antigen, deoxynucleoside triphosphates, ATP, and an ATP-generating system. This system was used to examine the effects of mutations within or adjacent to the Py core origin (ori) region in vitro. The analysis of plasmid DNAs containing deletions within the early-gene side of the Py core ori indicated that sequences between nucleotides 41 and 57 define the early boundary of Py DNA replication in vitro. This is consistent with previously published studies on the early-region sequence requirements for Py replication in vivo. Deleting portions of the T-antigen high-affinity binding sites A and B (between nucleotides 57 and 146) on the early-gene side of the core ori led to increased levels of replication in vitro and to normal levels of replication in vivo. Point mutations within the core ori region that abolish Py DNA replication in vivo also reduced replication in vitro. A mutant with a reversed orientation of the Py core ori region replicated in vitro, but to a lesser extent that wild-type Py DNA. Plasmids with deletions on the late-gene side of the core ori, within the enhancer region, that either greatly reduced or virtually abolished Py DNA replication in vivo replicated to levels similar to those of wild-type Py DNA plasmids in vitro. Thus, as has been observed with simian virus 40, DNA sequences needed for Py replication in vivo are different from and more stringent than those required in vitro.

Major rearrangement of cellular DNA in the vicinity of integrated polyomavirus DNA

The Cyp cell line was produced by transforming mouse embryo cells at the restrictive temperature with an early thermosensitive mutant of polyomavirus (Py). Transfer of Cyp cells to the nonrestrictive temperature causes excision to occur at a single chromosomal site carrying viral DNA, and leads to the production of infectious virus. We have attempted to elucidate the recombination event that occurred during the integration of Py DNA in this inducible line. Physical characterization of two recombinant DNAs-one selected from a genomic library of normal mouse DNA and the other constructed from the unoccupied allele of the Cyp integration site-indicates that generation of the Cyp line has involved the joining of not only viral DNA to a cellular alpha site, but also the cellular alpha site to a cellular alpha site to cellular beta site. Hence, previously described hybrid excision products from the Cyp line were made of mouse DNA segments representing two distinct cellular sites. The alpha-beta joining may play a role in the expression of integrated Py DNA.

Cloning vectors for expression of cDNA libraries in mammalian cells

We have constructed a series of compound cloning vectors (lambda ZD vectors), each consisting of phage lambda arms carrying a modified version of the retroviral expression vector pZIP-neoSV (x)1. cDNA, inserted into a cloning site present in the retroviral vector component, is cloned with high efficiency using the lambda system. A cDNA library in plasmids is then released by homologous recombination between the retroviral long terminal repeats. Retroviral transduction is achieved by transient expression of the released library in a cell line containing a packaging mutant of Moloney murine leukemia virus, followed by cocultivation of these producers with recipient cells. Transcription of cDNAs in the recipient cells is driven by the strong long terminal repeat promoter, and the transcripts, even from truncated cDNAs, can be expressed because translational start sites have been provided in all three reading frames (tri-initiator). Sequences conferring a recognizable phenotype can be rescued by cell fusion. The functionality of the tri-initiator and the rescue of a rare cDNA have been successfully tested using model systems.

An LFA-3 cDNA encodes a phospholipid-linked membrane protein homologous to its receptor CD2

Recently the human T cell erythrocyte receptor CD2 has been shown to bind human erythrocytes through LFA-3, a heavily glycosylated surface protein of broad tissue distribution. CD2-LFA-3 interactions are important for cytolytic conjugate formation, for thymocyte adhesion, and for T cell activation. A complementary DNA clone encoding LFA-3 was isolated using a complementary DNA clone encoding LFA-3 was isolated using a novel transient expression system of mouse cells. The cDNA encodes a phospholipid-linked membrane protein whose extracellular domain shares significant homology with CD2. As CD2 is homologous with the neural cell adhesion molecule NCAM in immunoglobulin-like domains, cellular adhesion molecules in both neural and lymphoid tissues could have a common ancestor.

DNA sequence requirements for replication of polyomavirus DNA in vivo and in vitro

Cell extracts of FM3A mouse cells replicate polyomavirus (Py) DNA in the presence of immunoaffinity-purified Py large T antigen, deoxynucleoside triphosphates, ATP, and an ATP-generating system. This system was used to examine the effects of mutations within or adjacent to the Py core origin (ori) region in vitro. The analysis of plasmid DNAs containing deletions within the early-gene side of the Py core ori indicated that sequences between nucleotides 41 and 57 define the early boundary of Py DNA replication in vitro. This is consistent with previously published studies on the early-region sequence requirements for Py replication in vivo. Deleting portions of the T-antigen high-affinity binding sites A and B (between nucleotides 57 and 146) on the early-gene side of the core ori led to increased levels of replication in vitro and to normal levels of replication in vivo. Point mutations within the core ori region that abolish Py DNA replication in vivo also reduced replication in vitro. A mutant with a reversed orientation of the Py core ori region replicated in vitro, but to a lesser extent that wild-type Py DNA. Plasmids with deletions on the late-gene side of the core ori, within the enhancer region, that either greatly reduced or virtually abolished Py DNA replication in vivo replicated to levels similar to those of wild-type Py DNA plasmids in vitro. Thus, as has been observed with simian virus 40, DNA sequences needed for Py replication in vivo are different from and more stringent than those required in vitro.

Nucleotides in the polyomavirus enhancer that control viral transcription and DNA replication

The polyomavirus enhancer is required in cis for high-level expression of the viral early region and for replication of the viral genome. We introduced multiple mutations in the enhancer which reduced transcription and DNA replication. Polyomaviruses with these mutant enhancers formed very small plaques in whole mouse embryo cells. Revertants of the viral mutants were isolated and characterized. Reversion occurred by any of the following events: restoration of guanosines at nucleotide (nt) 5134 and nt 5140 within the adenovirus 5 E1A enhancer core AGGAAGTGACT; acquisition of an A----G mutation at nt 5258, which is the same mutation that enables polyomavirus to grow in embryonal carcinoma F9 cells; duplication of mutated sequences between nt 5146 and 5292 (including sequences homologous with immunoglobulin G, simian virus 40, and bovine papillomavirus enhancer elements). Reversion restored both the replicative and transcriptional functions of the viruses. Revertants that acquired the F9 mutation at nt 5258 grew at least 20-fold better than the original mutant in whole mouse embryo cells, but replicated only marginally better than the original mutant in 3T6 cells. Viruses with a reversion of the mutation at nt 5140 replicated equally well in both types of cells. Since individual nucleotides in the polyomavirus enhancer simultaneously altered DNA replication and transcription in specific cell types, it is likely that these processes rely upon a common element, such as an enhancer-binding protein.

Polyomavirus tumor induction in mice: influences of viral coding and noncoding sequences on tumor profiles

We determined the DNA sequences of the noncoding regions of two polyomavirus strains that differ profoundly in their abilities to induce tumors in mice. Differences between strains were found, both on the late side of the replication origin in the region containing known enhancer elements and on the early side of the origin, affecting the number and location of large-T-antigen-binding sites. By constructing and analyzing recombinant viruses between these high- and low-tumor strains, we attempted to localize determinants which affect the frequency and histotype of tumors. Seven recombinants were constructed and propagated in vitro, and the tumor profile of each was established by inoculation into newborn C3H mice. Recombinants containing noncoding sequences from the high-tumor strain and coding sequences from the low-tumor strain behaved like the latter, inducing tumors at a low frequency and strictly of mesenchymal origin. Reciprocal recombinants with noncoding sequences of the low-tumor strain linked to structural determinants from the high-tumor strain induced several types of epithelial tumors typical of the high-tumor strain but at reduced frequency, in addition to mesenchymal tumors. A high frequency and full diversity of epithelial tumors required, in addition to structural regions from the high-tumor strain, noncoding sequences on the early side of the origin also present in this strain. A high-tumor profile thus resulted from the combined effects of structural and regulatory determinants in the high-tumor strain, with the former affecting primarily the tissue tropism and the latter affecting the frequency of tumors. No differential effects of the enhancer regions from the late side of the origin in the two virus strains were seen in this study.

Replicative cis-advantage of polyomavirus regulatory region mutants in different murine cell lines

To determine the relative growth advantages of polyomavirus regulatory region mutants selected from Friend leukemic and neuroblastoma cells persistently infected with polyomavirus A2 wild type, different murine cell lines, some of which are capable of further differentiation in vitro, were used as hosts in mixed infection and transfection. The tests allowed the determination, through the measurement of cis-advantage in replication, of the most effective polyomavirus regulatory region constitutions for a given cell line and, in some cases, for specific stages of cell differentiation. Furthermore, different domains of the viral regulatory region were shown to have different effects--positive, neutral, or negative--depending on the host analyzed.

Analysis of the early regulatory region of the human papovavirus BK

BKV is a human papovavirus which latently infects a majority of the world population and whose DNA has been found in human tumor tissue. Along with simian virus 40 (SV40) and JCV, it is one of several highly homologous polyomaviruses which display distinct host ranges, tissue tropisms, and transformation potentials. Determination of these properties is thought to reside, in part, in the noncoding regulatory region of these viruses. We have studied the regulation of gene expression by the early promoter and enhancer of BKV. Our results show that the early promoter of BKV consists of elements found both to the early side of and within the proximal 18 bp of the first enhancer element itself. At least one BKV regulatory element appears to be downstream of the mRNA start sites. The BKV enhancer consists of two different types of elements, three direct repeats, and an element (denoted "c") found in the 30 bp to the late side of the distal repeat. When used with the BKV promoter the enhancer repeat elements were found to be redundant, optimal promoter activity requiring only two of the three repeats plus the c element. Using the heterologous SV40 promoter the optimal BKV enhancer consisted of either three repeats or two repeats plus the c element. Subfragments of the enhancer region were capable of partial activation of homologous and heterologous promoters. We conclude that the regulation of BKV early gene expression involves novel elements arranged in ways not previously described in other papovaviruses.

Nucleotides in the polyomavirus enhancer that control viral transcription and DNA replication

The polyomavirus enhancer is required in cis for high-level expression of the viral early region and for replication of the viral genome. We introduced multiple mutations in the enhancer which reduced transcription and DNA replication. Polyomaviruses with these mutant enhancers formed very small plaques in whole mouse embryo cells. Revertants of the viral mutants were isolated and characterized. Reversion occurred by any of the following events: restoration of guanosines at nucleotide (nt) 5134 and nt 5140 within the adenovirus 5 E1A enhancer core AGGAAGTGACT; acquisition of an A----G mutation at nt 5258, which is the same mutation that enables polyomavirus to grow in embryonal carcinoma F9 cells; duplication of mutated sequences between nt 5146 and 5292 (including sequences homologous with immunoglobulin G, simian virus 40, and bovine papillomavirus enhancer elements). Reversion restored both the replicative and transcriptional functions of the viruses. Revertants that acquired the F9 mutation at nt 5258 grew at least 20-fold better than the original mutant in whole mouse embryo cells, but replicated only marginally better than the original mutant in 3T6 cells. Viruses with a reversion of the mutation at nt 5140 replicated equally well in both types of cells. Since individual nucleotides in the polyomavirus enhancer simultaneously altered DNA replication and transcription in specific cell types, it is likely that these processes rely upon a common element, such as an enhancer-binding protein.

Regulation of polyomavirus late promoter activity by viral early proteins

To assess the effect of the polyomavirus (Py) early proteins, the large T (LT), middle T (MT), and small T (ST) antigens, on gene expression from the Py late promoter, replication-defective plasmid constructs with the bacterial chloramphenicol acetyltransferase (cat) gene linked to this promoter were cotransfected into mouse or rat cells with plasmids capable of producing either LT, MT, or all three early proteins. When target CAT plasmids contained a truncated early region and thus had the coding potential for MT and ST, base-line CAT activities were low, whereas cotransfection with an LT plasmid resulted in up to 70-fold stimulation of CAT activity that was also reflected in similar increases in the level of steady-state mRNA. Studies with target plasmids with deletions within the Py regulatory region indicated that at least the major LT-binding site C and a functional enhancer region were both required for maximal stimulation of CAT activity. However, although enhancer deletions totally suppressed the ability of target plasmids to be trans activated, a consistent two- to fourfold stimulation of CAT activity by LT was still observed with a plasmid in which all three major LT-binding sites were deleted. Of four mutant LTs incapable of binding Py DNA but retaining immortalization potential, only one showed a low but significant trans-activating ability. When the early coding region was completely eliminated from the target plasmid, base-line CAT activity was increased 10-fold. LT failed to stimulate CAT activity to the same levels observed with target plasmid containing the truncated early region, but this limited response could be enhanced by supplying, in addition, MT and ST. Our results suggest that LT trans activation may involve the formation of a complex of transcriptional factors which interacts with the enhancer, an interaction that is facilitated both by the binding of LT to the Py regulatory region and by the presence of MT or ST or both, and that a significant portion of LT stimulation of late gene expression is a result of the removal of the competing early transcriptional unit via autoregulation. In addition, our results suggest that LT trans activation involves a second indirect component acting independently of LT binding and that the immortalization and trans activation functions of LT can be dissociated.

Species-specific in vitro synthesis of DNA containing the polyoma virus origin of replication

In vitro replication of DNA containing the polyoma (Py) virus origin of replication has been carried out with cell-free extracts prepared from mouse FM3A cells. The in vitro system required the Py virus-encoded large tumor (T) antigen, DNA containing the Py virus origin of replication, ATP, and an ATP-regenerating system. The replication reaction was inhibited by aphidicolin, suggesting the involvement of DNA polymerase alpha in this system. Simian virus 40 (SV40) T antigen could not substitute for the Py T antigen. Cell extracts prepared from HeLa cells, a source that replicates SV40 DNA in the presence of SV40 T antigen, replicated Py DNA poorly. The addition of purified DNA polymerase alpha-primase complex isolated from FM3A cells enabled HeLa cell extracts to replicate Py DNA with the same efficiency as FM3A cell extracts. Complementary experiments have shown that FM3A cell extracts do not support SV40 DNA replication unless supplemented with DNA polymerase alpha-primase complex from HeLa cells [Murakami, Y., Wobbe, C.R., Weissbach, L., Dean, F.B. & Hurwitz, J. (1986) Proc. Natl. Acad. Sci. USA 83, 2869-2873]. These results indicate that the host-cell source of the DNA polymerase alpha-primase complex plays an important role in discriminating between SV40 T antigen- and Py T antigen-dependent replication of their homologous DNA in vitro. This may explain the host-range specificity of these viruses in vivo.

Polyomavirus late leader region serves an essential spacer function necessary for viability and late gene expression

All three polyomavirus late mRNAs contain multiple tandem copies of the same nontranslated 57-nucleotide sequence, the late leader, at their 5' ends. We show here that a polyoma variant (ALM) lacking 48 central bases of the 57-base leader unit is nonviable by plaque assay and by a new method for testing virus viability, an immunofluorescence burst assay. ALM is, however, unaffected in early gene expression as measured both by indirect immunofluorescence of large T antigen and by transformation levels of rat F-111 cells. DNA replication in mouse cells is also as wild type, and the defect in ALM is complemented by an early-defective helper virus DNA. ALM does not make detectable levels of late viral proteins and is minimally 200-fold depressed in the accumulation of cytoplasmic polyadenylated late RNA. When the deleted leader sequence of ALM is replaced by a variety of procaryotic sequences, viability almost always returns. Some of the substituted leader variants produce plaques with the same apparent kinetics as wild-type viral DNA. The indication is that the sequence of the polyoma late leader is not important for late gene expression but that it has an essential spacer function on the RNA or DNA level. This spacer function is apparently necessary for late viral RNA transcription, processing, or stability.

An inducible eukaryotic host-vector expression system: amplification of genes under the control of the polyoma late promoter in a cell line producing a thermolabile large T antigen

We have taken advantage of the inherent instability of integrated polyoma (Py) DNA sequences in the presence of a functional viral large T antigen (LT) to develop a eukaryotic host-vector system where copy number is controlled by temperature. A mouse cell line WOP32-4, that constitutively expresses a temperature sensitive (ts) LT, was transfected with plasmids containing the Py origin of DNA replication (ori) and either a neomycin-resistance gene (neo) or chloramphenicol acetyl transferase gene (cat) linked to the Py late promoter. Stable transformants were selected at 39 degrees C, the non-permissive temperature for the ts LT function. Upon shift to 33 degrees C, the resident Py sequences present in the WOP32-4 cells cannot excise due to an ori deletion. However, excision of the transfected plasmid molecules and subsequent extrachromosomal replication occur at high rates leading in some cases to the production of 1000-2000 copies per cell (average) of the plasmid. Proportional increases in either neo-specific mRNA or CAT activity were also observed. In situ hybridization for one cell line indicated that about 20% of temperature-shifted cells contained amplified plasmid DNA.