Isolation of episomal bovine papillomavirus chromatin and identification of a DNase I-hypersensitive region

The cellular bromodomain protein Brd4 has multiple functions in E2-mediated papillomavirus transcription activation

The cellular bromodomain protein Brd4 functions in multiple processes of the papillomavirus life cycle, including viral replication, genome maintenance, and gene transcription through its interaction with the viral protein, E2. However, the mechanisms by which E2 and Brd4 activate viral transcription are still not completely understood. In this study, we show that recruitment of positive transcription elongation factor b (P-TEFb), a functional interaction partner of Brd4 in transcription activation, is important for E2’s transcription activation activity. Furthermore, chromatin immunoprecipitation (ChIP) analyses demonstrate that P-TEFb is recruited to the actual papillomavirus episomes. We also show that E2’s interaction with cellular chromatin through Brd4 correlates with its papillomavirus transcription activation function since JQ1(+), a bromodomain inhibitor that efficiently dissociates E2-Brd4 complexes from chromatin, potently reduces papillomavirus transcription. Our study identifies a specific function of Brd4 in papillomavirus gene transcription and highlights the potential use of bromodomain inhibitors as a method to disrupt the human papillomavirus (HPV) life cycle.

Papillomavirus interaction with cellular chromatin

High-risk human papillomavirus (HPV) infection is the primary risk factor for cervical cancer. HPVs establish persistent infection by maintaining their genomes as extrachromosomal elements (episomes) that replicate along with host DNA in infected cells. The productive life cycle of HPV is intimately tied to the differentiation program of host squamous epithelium. This review examines the involvement of host chromatin in multiple aspects of the papillomavirus life cycle and the malignant progression of infected host cells. Papillomavirus utilizes host mitotic chromosomes as vehicles for transmitting its genetic materials across the cell cycle. By hitchhiking on host mitotic chromosomes, the virus ensures accurate segregation of the replicated viral episomes to the daughter cells during host cell division. This strategy allows persistent maintenance of the viral episome in the infected cells. In the meantime, the virus subverts the host chromatin-remodeling factors to promote viral transcription and efficient propagation of viral genomes. By associating with the host chromatin, papillomavirus redirects the normal cellular control of chromatin to create a cellular environment conducive to both its own survival and malignant progression of host cells. Comprehensive understanding of HPV-host chromatin interaction will offer new insights into the HPV life cycle as well as chromatin regulation. This virus-host interaction will also provide a paradigm for investigating other episomal DNA tumor viruses that share a similar mechanism for interacting with host chromatin.

Nuclear matrix attachment regions of human papillomavirus type 16 point toward conservation of these genomic elements in all genital papillomaviruses

The gene functions, transcriptional regulation, and genome replication of human papillomaviruses (HPVs) have been extensively studied. Thus far, however, there has been little research on the organization of HPV genomes in the nuclei of infected cells. As a first step to understand how chromatin and suprachromatin structures may modulate the life cycles of these viruses, we have identified and mapped interactions of HPV DNAs with the nuclear matrix. The endogenous genomes of HPV type 16 (HPV-16) which are present in SiHa, HPKI, and HPKII cells, adhere in vivo to the nuclear matrixes of these cell lines. A tight association with the nuclear matrix in vivo may be common to all genital HPV types, as the genomes of HPV-11, HPV-16, HPV-18, and HPV-33 showed high affinity in vitro to preparations of the nuclear matrix of C33A cells, as did the well-known nuclear matrix attachment region (MAR) of the cellular beta interferon gene. Affinity to the nuclear matrix is not evenly spread over the HPV-16 genome. Five genomic segments have strong MAR properties, while the other parts of the genome have low or no affinity. Some of the five MARs correlate with known cis-responsive elements: a strong MAR lies in the 5' segment of the long control region (LCR), and another one lies in the E6 gene, flanking the HPV enhancer, the replication origin, and the E6 promoter. The strongest MAR coincides with the E5 gene and the early-late intergenic region. Weak MAR activity is present in the E1 and E2 genes and in the 3' part of L2. The in vitro map of MAR activity appears to reflect MAR properties in vivo, as we found for two selected fragments with and without MAR activity. As is typical for many MARs, the two segments with highest affinity, namely, the 5' LCR and the early-late intergenic region, have an extraordinarily high A-T content (up to 85%). It is likely that these MARs have specific functions in the viral life cycle, as MARs predicted by nucleotide sequence analysis, patterns of A-T content, transcription factor YY1 binding sites, and likely topoisomerase II cleavage sites are conserved in similar positions throughout all genital HPVs.

Two cellular single-strand-specific DNA-binding proteins interact with two regions of the bovine papillomavirus type 1 genome, including the origin of DNA replication

We have identified and purified to near homogeneity two specific single-stranded DNA-binding factors (SPSF I and II) with molecular masses of 42 and 39 kDa, respectively, from calf thymus. Gel retention analysis and competition experiments demonstrate that the ubiquitous proteins SPSF I and II specifically interact with single-stranded DNA derived from the minimal in vitro origin of replication of bovine papillomavirus type 1 and a region of the viral genome proposed to be involved in plasmid maintenance. Bovine papillomavirus type 1 proteins do not interfere with DNA binding of SPSF I and II. The exact location of the binding domains of SPSF I and II on the DNA has been determined by methylation interference and T4 DNA polymerase footprinting. A potential cellular binding site for SPSF I and II is the major promoter (P2) of the human c-myc gene.

Reconstitution of an episomal mouse aprt gene as a consequence of recombination

When a functional murine adenine phosphoribosyltransferase (aprt) gene linked to bovine papilloma virus (BPV) DNA is transfected into Aprt- L cells, the cells are rendered Aprt+ and the aprt gene persists as an episome. Cotransfection with two BPV vectors, one containing the 5' half of the aprt gene and the other the 3' half of the gene, that share about 300 bp of common sequence in intron 2, produces Aprt+ cells with functional aprt as an episome. Southern blot analysis of low molecular weight DNA derived from Hirt extracts revealed the regeneration of a diagnostic SmaI fragment, consistent with establishment of an episome with functional aprt that was reconstituted as a consequence of recombination. To establish cells with an episomal target for recombination, BPV vectors containing a G418 resistance marker and either the 5' half or 3' half of aprt were transfected into Aprt- L cells. Stably transfected cells, selected by their growth in G418, were in turn transfected with DNA containing the other half of the aprt gene. Following selection of Aprt+ cells, Southern blot and polymerase chain reaction (PCR) analysis of low molecular weight DNA confirmed the presence of a complete episomal aprt gene. The region of DNA shared by the episomal aprt fragment and the transfected aprt half was sequenced after PCR amplification of the reconstituted, episomal gene and was found to be wild type. The region of overlap that serves as the substrate for recombination lies entirely within an intron and can, therefore, tolerate nucleotide substitutions and deletions. The absence of such errors in the sequences examined is consistent with recombination events that are not error prone.

Topological properties of bovine papillomavirus type 1 (BPV-1) DNA in episomal nucleoprotein complexes: a model system for chromatin organization in higher eukaryotes

Sedimentation analysis of isolated episomal bovine papillomavirus type 1 (BPV-1) nucleoprotein complexes in sucrose gradients and subsequent separation of the purified DNA in chloroquine gels revealed different classes of molecules, varying in their degree of superhelicity. Since torsionally stressed DNA favors the adoption of secondary structures, we employed the single-strand-specific S1 nuclease to detect such structural alterations in both naked DNA and native chromatin. Direct examination of nuclease digestion products in chloroquine gels showed that neither the naked DNA nor the BPV-1 nucleoprotein complexes in isolated nuclei were cleaved randomly by the enzyme. Instead, there was a strict dependence on nuclease susceptibility and the degree of supercoiling, strongly suggesting that the structural features detected by S1 nuclease are due to the occurrence of torsionally stressed viral chromatin. Mapping analysis using the indirect end-labeling method demonstrated an S1-nuclease cleavage site adjacent to 20 homopurine residues known to be hypersensitive to S1 attack. Furthermore, direct methylation experiments with viral chromatin in isolated nuclei indicated that only circular, covalently closed nucleoprotein complexes served as substrate, whereas linearized BPV-1 chromatin was not susceptible to exogenously added Hhal methylase. This observation raises the possibility that the modulation of topology in nucleosomally organized DNA might also play a role in eukaryotic DNA methylation.

The E2 protein of human papillomavirus type 16. Over-expression and purification of an active transcriptional regulator

The E2 open reading frame of human papillomavirus type 16 was inserted into the Escherichia coli vector pKK223-3, and expressed to greater than 15% of total cellular protein when induced with isopropyl beta-D-thiogalactopyranoside. The highest expressing clone was grown in bulk and the E2 protein purified to homogeneity by the following procedure: (a) isolation of the insoluble protein fraction; (b) extraction with urea; (c) quaternary amino-ethyl-Sepharose ion-exchange chromatography and (d) renaturation and chromatography on dextran sulphate. That the purified protein was fully functionally active was confirmed by its specific DNA-binding properties and its ability to activate gene transcription by over two orders of magnitude in an in vivo assay.

Characterization of DNA binding proteins released from sarcoma-180 chromatin during brief digestion with DNase-I

Brief digestion of sarcoma-180 chromatin (less than or equal to 5%) by pancreatic DNase-I releases six non-histone proteins with Mol. wt. 21.5K, 26K, 72.5K, 77K, 90K and 120K dalton and pI values ranging from 4.7 to 12.4. The protein of Mol. wt. 77K dalton was obtained at high alkaline range of pH = 12.4. The antibodies against these proteins induce dose dependent inhibition in transcription of native chromatin. Results suggest a role of these proteins in positive control of gene transcription in sarcoma-180 cells.

Bovine papillomavirus transcriptional regulation: localization of the E2-responsive elements of the long control region

The long control region (LCR) of the bovine papillomavirus type 1 genome can function as a conditional transcriptional enhancer which can be specifically trans-activated by the viral E2 gene product. To precisely map the target(s) of this trans-activation, BAL 31 exonuclease was used to generate two overlapping series of deleted DNA segments through the LCR. These fragments were assayed for their ability to activate transcription from the enhancer-deleted simian virus 40 early promoter of pA10CAT in the presence or absence of the viral E2 gene product. Two different E2-responsive elements were localized within the LCR. The major target for E2 trans-activation (E2-responsive element 1) was mapped to a 196-base-pair fragment between nucleotides 7611 and 7806, just upstream from promoters P7940 and P89. Further deletions which destroyed or impaired enhancer function revealed that the ACCN6GGT sequence motifs at each end of E2-responsive element 1 are critical components of this element. Primer extension analysis of RNA extracted from acute transfections with plasmids containing the bovine papillomavirus type 1 LCR driving the CAT gene revealed that each of the P7940 and P89 promoters is responsive to E2 trans-activation.

Nucleosomal organization of a BPV minichromosome containing a human H4 histone gene

To address the relationship between chromatin structure and histone gene expression, the nucleosomal organization of a cell cycle-dependent human H4 histone gene in a bovine papilloma virus (BPV) minichromosome was examined. The nucleosome repeat length of the human H4 histone gene, maintained as a stable episome in a C127 mouse cell line designated I-8, was compared with that of the chromosomal copy of the H4 gene in human (HeLa) cells. In both cell lines, the H4 histone gene is predominantly expressed during the S phase of the cell cycle. The nucleosome repeat length of total HeLa cell and C127 mouse cell chromatin was similarly examined. Nuclei were digested with micrococcal nuclease and the DNA was fractionated electrophoretically, transferred to nitrocellulose filters and hybridized with radiolabelled (32P) cloned DNA probes. The nucleosome repeat length of the H4 gene, as an episome in the C127 mouse cell (153 +/- 8) and as an integrated copy in a HeLa cell (163 +/- 10) was considerably shorter than total genomic host cell (C127) (190 +/- 5) or HeLa cell chromatin (183 +/- 7). Our results indicate that the episomal H4 histone gene is packaged as chromatin. Moreover, the shortened nucleosome repeat length of the H4 gene, both as an episome or integrated chromosome sequence, suggests that the repeat length is characteristic of the gene and may be functionally related to its cell cycle regulated expression.

In vitro methylation of bovine papillomavirus alters its ability to transform mouse cells

Bovine papillomavirus (BPV) was methylated in vitro at either the 29 HpaII sites, the 27 HhaI sites, or both. Methylation of the HpaII sites reduced transformation by the virus two- to sixfold, while methylation at HhaI sites increased transformation two- to fourfold. DNA methylated at both HpaII and HhaI sites did not differ detectably from unmethylated DNA in its efficiency of transformation. These results indicate that specific methylation sites, rather than the absolute level of methylated cytosine residues, are important in determining the effects on transformation and that the negative effects of methylation at some sites can be compensated for by methylation at other sites. BPV molecules in cells transformed by methylated BPV DNA contained little or no methylation, indicating that the pattern of methylation was not faithfully retained in these extrachromosomally replicating molecules. Methylation at the HpaII sites (but not the HhaI sites) in the cloned BPV plasmid or in pBR322 also inhibited transformation of the plasmids into Escherichia coli HB101 cells.

Efficient transcription of a Caenorhabditis elegans heat shock gene pair in mouse fibroblasts is dependent on multiple promoter elements which can function bidirectionally

A divergently transcribed pair of Caenorhabditis elegans hsp16 genes was introduced into mouse fibroblasts by stable transfection with vectors containing bovine papillomavirus plasmid maintenance sequences and a selectable gene. The hsp16 genes were transcriptionally inactive in the mouse cells under normal growth conditions and were strongly induced by heat shock or arsenite. In a cell line with 12 copies of the gene pair, there were estimated to be more than 10,000 hsp16 transcripts in each cell after 2 h of heat shock treatment. The hsp16 transcript levels were more than 100 times higher than those of a gene with a herpes simplex virus thymidine kinase gene promoter carried on the same vector. A single heat shock promoter element (HSE) could activate bidirectional transcription of the two hsp16 genes when placed between the two TATA elements, but the transcriptional efficiency was reduced 10-fold relative to that of the wild-type gene pair. Four overlapping HSEs positioned between the two TATA elements resulted in inducible bidirectional transcription at greater than wild-type levels. The number of HSEs can therefore be a major determinant of the promoter strength of heat-inducible genes in mammalian cells. Partial disruption of an alternating purine-pyrimidine sequence between the two hsp16 genes had no significant effect on their transcriptional activity.

The noncoding region of HPV-6vc contains two distinct transcriptional enhancing elements

HPV-6vc subgenomic fragments were inserted into an enhancer-dependent expression vector for chloramphenicol acetyltransferase (CAT) and assayed for the presence of transcriptional enhancing elements. A transcriptional enhancing element was detected in the noncoding region (NCR) of the HPV-6vc viral genome when the CAT assays were performed in viral transformed human kidney cell lines (293 and 324K), in human cervical carcinoma cell lines (HeLa and Siha), and in bovine papillomavirus type 1 (BPV-1) transformed mouse cells (C127-53). The NCR region of the HPV-6b genome was only capable of enhancing transcription of the CAT gene in the HeLa cell line at a level one-third that of the HPV-6vc NCR. The HPV-6vc NCR enhancing activity in C127-53 cells was further stimulated by the addition of sodium butyrate to the growth medium. Localization of the DNA sequences in the HPV-6vc NCR responsible for enhancing transcription revealed two distinct enhancer elements. One element (HPV-6vc position 7218-7544) was active in the 293, HeLa, Siha, and C127-53 cells. The second enhancer element (HPV-6vc position 7544-7971) was only capable of stimulating transcription in HeLa, C127-53, and Siha cells. When the HPV NCR-CAT expression vectors were cotransfected with a competitor plasmid (pNCR75) into C127-53 or HeLa cells then transcriptional enhancement decreased, indicating competition of cellular factors which affect both segments of the HPV-6vc NCR.

Efficient transcription of a Caenorhabditis elegans heat shock gene pair in mouse fibroblasts is dependent on multiple promoter elements which can function bidirectionally

A divergently transcribed pair of Caenorhabditis elegans hsp16 genes was introduced into mouse fibroblasts by stable transfection with vectors containing bovine papillomavirus plasmid maintenance sequences and a selectable gene. The hsp16 genes were transcriptionally inactive in the mouse cells under normal growth conditions and were strongly induced by heat shock or arsenite. In a cell line with 12 copies of the gene pair, there were estimated to be more than 10,000 hsp16 transcripts in each cell after 2 h of heat shock treatment. The hsp16 transcript levels were more than 100 times higher than those of a gene with a herpes simplex virus thymidine kinase gene promoter carried on the same vector. A single heat shock promoter element (HSE) could activate bidirectional transcription of the two hsp16 genes when placed between the two TATA elements, but the transcriptional efficiency was reduced 10-fold relative to that of the wild-type gene pair. Four overlapping HSEs positioned between the two TATA elements resulted in inducible bidirectional transcription at greater than wild-type levels. The number of HSEs can therefore be a major determinant of the promoter strength of heat-inducible genes in mammalian cells. Partial disruption of an alternating purine-pyrimidine sequence between the two hsp16 genes had no significant effect on their transcriptional activity.

In vitro methylation of bovine papillomavirus alters its ability to transform mouse cells

Bovine papillomavirus (BPV) was methylated in vitro at either the 29 HpaII sites, the 27 HhaI sites, or both. Methylation of the HpaII sites reduced transformation by the virus two- to sixfold, while methylation at HhaI sites increased transformation two- to fourfold. DNA methylated at both HpaII and HhaI sites did not differ detectably from unmethylated DNA in its efficiency of transformation. These results indicate that specific methylation sites, rather than the absolute level of methylated cytosine residues, are important in determining the effects on transformation and that the negative effects of methylation at some sites can be compensated for by methylation at other sites. BPV molecules in cells transformed by methylated BPV DNA contained little or no methylation, indicating that the pattern of methylation was not faithfully retained in these extrachromosomally replicating molecules. Methylation at the HpaII sites (but not the HhaI sites) in the cloned BPV plasmid or in pBR322 also inhibited transformation of the plasmids into Escherichia coli HB101 cells.

Transient replication of bovine papilloma virus type 1 plasmids: cis and trans requirements

A transient assay has been used to study bovine papilloma virus type 1 (BPV-1) replication. We show that BPV-1 early replication occurs faster than cellular DNA synthesis. Initial replication events are dependent on a gene product(s), encoded by the BPV-1 E1 open reading frame. Mutational analysis of the viral upstream regulatory region shows the requirement of two domains in cis for replication. Domain one, located outside of the viral 69% transforming fragment, is an enhancer-like activity and can be replaced by other known viral enhancers. Domain two lies within sequences previously defined as plasmid maintenance sequence 1. The apparent requirement for a proximal enhancer function for replication may explain why certain BPV-1 constructions, when linked to bacterial plasmid sequences, can be maintained extrachromosomally while others cannot.

Properties of intracellular bovine papillomavirus chromatin

Episomal nucleoprotein complexes of bovine papillomavirus type 1 (BPV-1) in transformed cells were exposed to DNase I treatment to localize hypersensitive regions. Such regions, which are indicative for gene expression, were found within the noncoding part of the genome, coinciding with the origin of replication and the 5' ends of most of the early mRNAs. However, there were also regions of hypersensitivity within the structural genes. These intragenic perturbations of the chromatin structure coincide with regulatory sequences at the DNA level. One of these regions maps in close proximity to a Z-DNA antibody-binding site which is located near the putative BPV-1 enhancer sequence.

Minichromosome assembly of non-integrated plasmid DNA transfected into mammalian cells

The nucleoprotein structures formed on various plasmid expression vectors transfected into mammalian cells by both the calcium phosphate and DEAE-dextran methods have been studied. We demonstrate by a variety of means that mammalian cells are capable of rapidly assembling non-integrated circular plasmids (both replicating and non-replicating) into typical "minichromosomes" containing nucleosomes with a 190 bp repetitive spacing. Treatment of recipient cells with sodium butyrate for a short period of time (12-16 h) immediately following transfection markedly increased the DNase I digestion sensitivity of the newly assembled plasmid chromatin. Furthermore, minichromosomes isolated from such butyrate-treated cells are depleted in histone H1 and contain highly acetylated forms of histone H4. These findings are entirely consistent with our earlier speculation (Gorman et al., Nucleic Acids Res. 11, 1044; 1983) that appropriate butyrate treatment might stimulate transient expression of newly transfected genes by facilitating their assembly into an "active" type of chromatin structure.

A common function for polyoma virus large-T and papillomavirus E1 proteins?

Nucleotide sequencing has revealed a common genetic organization for three papillomaviruses: BPV-1 (bovine papillomavirus type 1), HPV-1 (human papillomavirus type 1a) and HPV-6 (human papillomavirus type 6b). Several open reading frames, corresponding to as yet uncharacterized proteins, were observed in these genomes in the region that is required for oncogenic transformation by BPV-1 and for plasmidial maintenance of its genome. The longest of these frames, E1, is also the most conserved between the three viruses; we have compared the amino acid sequence of its putative product ('E1 protein') with those of the large-T proteins of three polyoma viruses and report here significant homologies in their carboxy-terminal halves, extending for over 200 amino acids. Moreover, similar secondary structures were predicted in this region, especially in two blocks of homologous residues, which correspond in the large-T proteins of polyoma and simian virus 40 (SV40) viruses to sites involved in the ATPase and nucleotide-binding activities. These observations suggest that the papillomavirus E1 proteins might have a function in common with the polyoma virus large-T proteins (which are required for the initiation of viral DNA replication). As it was suggested recently that the E1 gene product is involved in maintaining the BPV-1 genome as a plasmid in transformed cells, we speculate that the structural features conserved in these otherwise very different viruses are general characteristics of eukaryotic proteins involved in the control of DNA replication.

Transcriptionally active chromatin

Eukaryotic chromatin has a dynamic, complex hierarchical structure. Active gene transcription takes place on only a small proportion of it at a time. While many workers have tried to characterize active chromatin, we are still far from understanding all the biochemical, morphological and compositional features that distinguish it from inactive nuclear material. Active genes are apparently packaged in an altered nucleosome structure and are associated with domains of chromatin that are less condensed or more open than inactive domains. Active genes are more sensitive to nuclease digestions and probably contain specific nonhistone proteins which may establish and/or maintain the active state. Variant or modified histones as well as altered configurations or modifications of the DNA itself may likewise be involved. Practically nothing is known about the mechanisms that control these nuclear characteristics. However, controlled accessibility to regions of chromatin and specific sequences of DNA may be one of the primary regulatory mechanisms by which higher cells establish potentially active chromatin domains. Another control mechanism may be compartmentalization of active chromatin to certain regions within the nucleus, perhaps to the nuclear matrix. Topological constraints and DNA supercoiling may influence the active regions of chromatin and be involved in eukaryotic genomic functions. Further, the chromatin structure of various DNA regulatory sequences, such as promoters, terminators and enhancers, appears to partially regulate transcriptional activity.

Origin of replication in episomal bovine papilloma virus type 1 DNA isolated from transformed cells

The origin of replication of bovine papilloma virus type 1 (BPV-1) has been determined by isolating replicative intermediates (RI) of BPV-transformed hamster embryo fibroblasts (HEF-BPV). These RI were treated with single cut restriction enzymes to determine the start-position (origin) of the extending replication eyes using electron microscopic techniques. 'Cairns'-type RI molecules were shown to contain one replication eye in monomeric as well as in dimeric molecules. The position of this eye was localized at 6940 +/- 5% bp in the physical map. In a second set of experiments BPV-1 DNA fragments cloned in pBR322 were tested for transient episomal replication. Transfected cells were harvested after increasing periods of time and screened for replication with isoschizomeric restriction enzymes to differentiate between input and replicated DNA. The part of the BPV genome harboring the replication origin spans the BPV ClaI-C restriction fragment corresponding to the non-coding region of the BPV genome and coincides with the DNase I-hypersensitive control region in the chromatin, isolated from transformed cells.

DNA and RNA virus species are inhibited by xanthates, a class of antiviral compounds with unique properties

Various DNA and RNA virus species are inhibited by xanthate compounds at concentrations that leave the mitotic activity of uninfected cells unimpaired. The concentration of tricyclodecan -9-yl- xanthogenate that reduces the yield of herpes simplex virus types 1 and 2 by 50% is between 4.5 and 33 microM. The replication of DNA viruses such as simian virus 40 can be blocked at the DNA and RNA level both early and late after infection. The xanthates are not incorporated into nucleic acids. Episomal bovine papilloma virus DNA replication and transcription are also inhibited in transformed cells. The treated cells revert to the normal phenotype by acquisition of contact inhibition and a flat morphology.

Characterization of the bovine papilloma virus plasmid maintenance sequences

Bovine Papilloma Virus (BPV-1) establishes itself as a multicopy nuclear plasmid in somatic mammalian cells in culture. We report here that two discontinuous regions within the viral genome can independently support extrachromosomal replication of the Tn5 neomycinr gene in cells that provide viral factors in trans. The viral plasmid maintenance sequences (PMS) act in cis and will integrate along with the marker gene in cell lines that do not provide BPV-1 gene products. PMS-1 is localized within a 521 bp region upstream of the BPV-1 early transcription unit; PMS-2 has been localized to a 140 bp region within the putative reading frame for the E1 protein of the viral genome. Recombinant plasmids carrying either of the PMS elements are unrearranged and stably maintained at a constant copy number supernumerary to the resident BPV-1 genomes even in the absence of selective pressure. Specific deletion mutants within the viral genome show that BPV-1 gene products required for morphological transformation are dispensable for plasmid maintenance. In mouse cells cotransformed with such deletion derivatives and an unlinked marker gene (neomycinr or Tk), the marker genes integrate into the host genome while the BPV molecules are nonselectively carried as nuclear plasmids. This result implies that the BPV-1 genome must have signals that specifically preclude integration in the presence of transacting factors.