Mouse cells transformed by bovine papillomavirus contain only extrachromosomal viral DNA sequences

Evidence that replication initiates at only some of the potential origins in each oligomeric form of bovine papillomavirus type 1 DNA

In a subclone of ID13 mouse fibroblasts latently infected with bovine papillomavirus type 1 (BPV-1) DNA, the viral genome occurred as a mixture of extrachromosomal circular monomers and oligomers. Multiple copies were also associated with the host cell genome, predominantly at a single site in a head-to-tail tandem array. We examined the replicative intermediates of extrachromosomal forms of BPV-1 DNA by using two-dimensional gel electrophoresis. The results obtained indicate that initiation of DNA replication occurred near the center of the EcoRI-BamHI 5.6-kilobase fragment. In some molecules, however, this fragment was replicated from one end to the other by means of a single fork initiated elsewhere. Termination also occurred within this fragment. The EcoRI-BamHI 2.3-kilobase fragment replicated as a DNA molecule containing a termination site for DNA replication and also by means of a single fork traversing the fragment from one end to the other. Thus, replication forks proceeded through these fragments in different manners, apparently depending on whether they were part of a monomer, a dimer, a trimer, or higher oligomers. These observations lead to the conclusion that initiation of DNA replication in BPV-1 DNA takes place at or close to plasmid maintenance sequence 1. From this point, replication proceeds bidirectionally and termination occurs approximately 180 degrees opposite the origin. The results obtained are consistent with one or more replication origins being quiescent in BPV-1 DNA oligomers.

Evidence that replication initiates at only some of the potential origins in each oligomeric form of bovine papillomavirus type 1 DNA

In a subclone of ID13 mouse fibroblasts latently infected with bovine papillomavirus type 1 (BPV-1) DNA, the viral genome occurred as a mixture of extrachromosomal circular monomers and oligomers. Multiple copies were also associated with the host cell genome, predominantly at a single site in a head-to-tail tandem array. We examined the replicative intermediates of extrachromosomal forms of BPV-1 DNA by using two-dimensional gel electrophoresis. The results obtained indicate that initiation of DNA replication occurred near the center of the EcoRI-BamHI 5.6-kilobase fragment. In some molecules, however, this fragment was replicated from one end to the other by means of a single fork initiated elsewhere. Termination also occurred within this fragment. The EcoRI-BamHI 2.3-kilobase fragment replicated as a DNA molecule containing a termination site for DNA replication and also by means of a single fork traversing the fragment from one end to the other. Thus, replication forks proceeded through these fragments in different manners, apparently depending on whether they were part of a monomer, a dimer, a trimer, or higher oligomers. These observations lead to the conclusion that initiation of DNA replication in BPV-1 DNA takes place at or close to plasmid maintenance sequence 1. From this point, replication proceeds bidirectionally and termination occurs approximately 180 degrees opposite the origin. The results obtained are consistent with one or more replication origins being quiescent in BPV-1 DNA oligomers.

A bovine papillomavirus type-1 (BPV-1) containing plasmid replicates extrachromosomally in Xenopus embryos

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Modulation of HPV18 and BPV1 transcription in human keratinocytes by simian virus 40 large T antigen and adenovirus type 5 E1A antigen

Transcription of early open reading frames initiated from the long control region (LCR) of HPV18 and BPV1 is known to be modulated by homologous and heterologous papillomarvirus E2 gene products. Using CAT constructs transfected into normal human keratinocytes, we show that SV40 large T antigen activates transcription from the LCR of both viruses, whereas Ad5-E1a antigen represses transcription from the HPV18-LCR but activates transcription from BPV1-LCR. Experiments using constructs containing subfragments of the HPV18-LCR cloned in enhancer configuration ahead of the SV40 early promoter or the HSV1-Tk promoter suggest that the effect of Ad5-E1a antigen on HPV18 transcription is probably due to a repression of the enhancer function of the LCR. The mechanism of transcription stimulation by SV40 large T antigen is less clear. The 230 bp Rsa1-Rsa1 central domain of the HPV18-LCR seems involved both in transcriptional stimulation by SV40 large T antigen and transcriptional inhibition by adenovirus E1a antigen.

Phenotypic analysis of bovine papillomavirus type 1 E2 repressor mutants

The bovine papillomavirus type 1 (BPV-1) E2 open reading frame encodes three proteins: the E2 transcriptional transactivator, the E2 transcriptional repressor (E2-TR), and the E8/E2 fusion peptide. In this study, we describe the phenotypes of BPV-1 mutants which are disrupted in their capacity to encode either the E2 transcriptional repressor or the E8/E2 fusion peptide. We also describe experiments which demonstrate that the E8/E2 gene product functions similarly to E2-TR. In the context of the entire viral genome, disruption of E8/E2 expression had little effect on viral processes, whereas disruption of E2-TR expression resulted in a complex phenotype marked by a 10- to 20-fold increase in viral DNA plasmid copy number as well as increased transformation potential. A double mutant, defective in the expression of both E8/E2 and E2-TR proteins, had high levels of E2 transactivation activity yet had reduced plasmid replication capacity and a delayed capacity to transform rodent cells.

Transforming growth factor-beta expression in fibropapillomas induced by bovine papillomavirus type 1, in normal bovine skin, and in BPV-1-transformed cells

There is substantial evidence to suggest that transforming growth factor-beta (TGF-beta) plays an important role in wound healing and tissue repair as well as in carcinogenesis. It has also been observed that naturally occurring bovine papillomavirus type 1 (BPV-1)-induced bovine fibropapillomas occur predominantly at traumatized sites of the body, suggesting that humoral factors released in wounds might be important for papillomavirus infection. We have therefore investigated the possible role of TGF-beta 1 in BPV-1 infections. Two antipeptide antibodies which recognize different epitopes in the N-terminus of TGF-beta 1 were used to localize TGF-beta 1 in bovine fibropapillomas and normal bovine skin using immunohistochemical methods. Staining by anti-LC(1-30) is intracellular in suprabasal keratinocytes of the epidermis as well as the hair follicles and sebaceous glands and correlates with known sites of TGF-beta 1 mRNA synthesis. Anti-CC(1-30) staining is extracellular in the immediately underlying dermis. Neither the pattern nor intensity of TGF-beta 1 staining was affected by BPV-1 infection. C127 cells and BPV-1-transformed C127 cells were compared for TGF-beta 1 mRNA expression and secretion of TGF-beta 1 peptide. Although the levels of messenger RNA and secreted TGF-beta 1 peptide were similar in both cell types, five- to six-fold greater amounts of TGF-beta-like activity per cell was detected in media conditioned by the uninfected cells. TGF-beta 1 treatment had no effect on the growth rate of either cell type or on BPV-1 gene expression in the transformed cells.

Phosphorylation sites of the E2 transcriptional regulatory proteins of bovine papillomavirus type 1

The E2 open reading frame of bovine papillomavirus type 1 (BPV-1) encodes three transcriptional regulatory proteins. The full-length open reading frame encodes a protein of 410 amino acids which functions as a transcriptional transactivator. Two transcriptional repressor proteins, E2-TR and E8/E2, contain the C-terminal 249 and 204 amino acids, respectively. We have expressed both the full-length E2 protein and the E2-TR repressor protein in insect cells, by using recombinant baculoviruses, and in mammalian COS-1 cells, by using a chimeric simian virus 40/BPV-1 virus. Analysis of the E2 proteins revealed that both the transactivator and repressor forms are phosphorylated predominately on serine residues at similar sites in both expression systems. By a combination of peptide mapping and site-directed mutagenesis techniques, the serine residues at positions 298 and 301 were determined to be the major phosphorylation sites of the BPV-1 E2 proteins.

The human papillomavirus type 18 (HPV18) E2 gene product is a repressor of the HPV18 regulatory region in human keratinocytes

The human papillomavirus type 18 (HPV18) long control region (LCR) harbors transcriptional promoter and enhancer elements. Recombinant plasmids bearing all or part of the HPV18 LCR cloned in enhancer or promoter configuration upstream of the chloramphenicol acetyltransferase (CAT) gene were transfected into human fibroblasts and keratinocytes. Although the HPV18 enhancer can function in the absence of E2 gene products in both fibroblasts and keratinocytes, the promoter activity of the HPV18 LCR is detectable in keratinocytes but not in fibroblasts, suggesting that it is tissue specific. This promoter activity was repressed in human keratinocytes not only by the bovine papillomavirus type 1 E2 gene product but also by the homologous HPV18 E2 gene product. The promoter involved in the HPV18 E2 repression is located within a 230-base-pair domain directly upstream of the E6 open reading frame of the HPV18 LCR and is probably the previously identified E6 promoter. Although one cannot rule out the possibility that this repressing effect is mediated by a truncated form of HPV18 E2 protein, as was previously demonstrated for bovine papillomavirus type 1, a more likely explanation would be that the full-length HPV18 E2 protein behaves as a repressor. Indeed, at the same doses at which it inhibits transcription from the homologous HPV18 LCR, the HPV18 E2 gene product activates transcription from constructs bearing E2-binding palindromes cloned in enhancer configuration upstream of a heterologous promoter. The fact that the homologous HPV18 E2 gene product acts as a transcriptional repressor of the HPV18 LCR suggests a possible explanation for the overexpression of E6 and E7 open reading frames in cervical carcinoma cells and in cell lines derived from them.

Transcriptional termination between bovine papillomavirus type 1 (BPV-1) early and late polyadenylation sites blocks late transcription in BPV-1-transformed cells

Bovine papillomavirus type 1 (BPV-1) is a small DNA tumor virus which induces fibropapillomas in cattle and transforms rodent cells in culture. Transcripts are derived from a single strand of the circular viral genome, which has multiple promoters and two polyadenylation sites. In the transformed cell, the first (early) polyadenylation site is utilized exclusively and, therefore, only the early region is expressed. Transcription of the late genes, which requires use of the second (late) polyadenylation site, is seen only in the fully differentiated keratinocytes of the fibropapilloma. In this study, nascent RNA chain analysis of BPV-1-transformed C127 cells was used to demonstrate that at least 90% of the RNA polymerases which transcribe past the early polyadenylation site terminate transcription within the late region before reaching the late polyadenylation site. Therefore, transcription termination is at least partially responsible for the absence of late transcription in the BPV-1-transformed cell and is likely to be an important mechanism for regulation of papillomavirus late transcription during keratinocyte differentiation.

Stably expressed FIPV peplomer protein induces cell fusion and elicits neutralizing antibodies in mice

We have established bovine papilloma virus (BPV)-transformed mouse C127 cell lines that synthesize the peplomer protein of the feline infectious peritonitis virus (FIPV) strain 79-1146. For this purpose, a new cassette expression vector pHSL, which carries the Drosophila HSp70 promotor and the polyadenylation signal of the Moloney murine leukemia virus long terminal repeat, was constructed. Cocultivation of the BPV-transformed cell lines with FIPV-permissive feline fcwf-D cells resulted in polykaryocyte formation. Since it depended on the presence of fcwf-D cells, binding of E2 to the cell receptor may be required for membrane fusion. E2 was synthesized as a core-glycosylated protein of 180K which was only slowly transported from the endoplasmic reticulum to the medial Golgi: of the E2-molecules labeled during a 1-hr pulse about half was still completely sensitive to endoglycosidase H after a 2-hr chase, while the remaining E2 had been chased into multiple, partially endoglycosidase H-resistant forms. Immunofluorescence studies also indicated that most E2 was retained intracellularly. Mice immunized with whole lysates of the transformed cells produced FIPV-neutralizing antibodies as shown by plaque reduction.

Genetic assignment of multiple E2 gene products in bovine papillomavirus-transformed cells

The E2 open reading frame of bovine papillomavirus type 1 has been shown genetically to encode at least three transcriptional regulatory factors, and three E2 specific proteins have been recently identified in virally transformed rodent cells. In this study, the genes encoding these E2 specific proteins have been determined. The 48-kilodalton (kDa) protein was identified as the product of a full-length E2 open reading frame cDNA, which confirmed that this polypeptide is the E2 transactivator. The 31-kDa E2 protein species, which is the most abundant E2 specific polypeptide, was identified by analysis of both bovine papillomavirus type 1 mutants and cDNAs to be the previously identified E2 transcriptional repressor, E2-TR, which results from translation initiation at an internal E2 ATG codon. The smallest E2 protein species, the 28-kDa polypeptide, was identified as the product of the E8/E2 fusion gene which results from translation of a spliced mRNA species.

Loss of bovine papillomavirus DNA replication control in growth-arrested transformed cells

The bovine papillomavirus type 1 (BPV-1) genome replicates as a plasmid within the nuclei of BPV-1-transformed murine C127 cells at a constant multiple copy number, and spontaneous amplification of the viral DNA is rarely observed. We report here that a mutant BPV-1 plasmid within a contact-inhibited C127 cell line replicated as a stable multicopy plasmid in exponentially growing cells but amplified to a high level in confluent cell culture. In situ hybridization analysis revealed that most of the mutant viral DNA amplification occurred in a minor subpopulation of cells within the culture. These consisted of giant nondividing cells with greatly enlarged nuclei, a cell form which was specifically induced in stationary-phase cultures. These observations indicated that expression of a viral DNA replication factor was cell growth stage specific. Consistent with this hypothesis, considerable amplification of wild-type BPV-1 DNA associated with characteristic giant cell formation was observed in typical wild-type virus-transformed C127 cultures following a period of growth arrest achieved by serum deprivation. Further observations indicated that induction of the giant-cell phenotype was dependent on BPV-1 gene expression and implicated a viral E1 replication factor in this process. Moreover, heterogeneity in virus genome copy numbers within the giant-cell population suggested a complex regulation of induction of DNA synthesis in these cells. It appears that this process represents a mechanism employed by the virus to ensure maximal viral DNA synthesis within a growth-arrested cell. Fundamental questions concerning the integration of the virus-cell control circuitry in proliferating and resting cells are discussed.

Bovine papillomavirus type 1 encodes two forms of a transcriptional repressor: structural and functional analysis of new viral cDNAs

Genetic and biochemical evidence has established that the E2 open reading frame (ORF) of bovine papillomavirus type 1 encodes at least two different site-specific DNA-binding proteins, one which activates and the other which represses expression from a viral promoter (P. F. Lambert, B. A. Spalholz, and P. M. Howley, Cell 50:69-78, 1987). We have obtained data which show that a second form of the repressor gene is expressed in transformed cells harboring stable viral plasmids. The structural details of this gene have been discerned by cDNA cloning, by RNase protection, and by primer extension analysis of in vivo RNA. Moreover, data from in vitro transcription experiments support the notion that this form of the E2 repressor is expressed from a novel viral promoter and that a small exon from another ORF is linked to an active repressor domain in E2. Thus, two different forms of the repressor are expressed from different promoters and might be independently regulated either in the cell cycle or in different tissue types. We show by functional in vivo assays utilizing a cDNA vector encoding this gene that the trans-acting factor has in vivo activities similar to those of the known repressor. Our screen of a cDNA library for cDNA clones representing bovine papillomavirus transcripts has also revealed a number of other novel structures defining new donor and acceptor RNA-processing sites. Notably, clones which conceptually can be translated to yield an E7 protein, the viral M gene, and the entire E2 ORF have been characterized. Finally, truncated versions of putative E8 cDNAs were also obtained.

Identification of bovine papillomavirus E1 mutants with increased transforming and transcriptional activity

The E1 open reading frame of bovine papillomavirus type 1 (BPV) has been shown previously to encode trans-acting functions, M and R, that are involved in extrachromosomal replication of the viral genome. We have determined that several E1 mutants mapping in both the M and R regions and a single mutant of the upstream regulatory region have a higher transforming activity on mouse C127 cells than the wild-type genome does. A representative mutant in M, a mutant in R, and the upstream regulatory region mutant were complemented in trans by the wild-type genome, but the two E1 mutants did not complement each other, suggesting that they affect the same inhibitory function. A long terminal repeat-activated clone constructed to express the intact E1 open reading frame reversed the high-transformation phenotype of the mutants. In contrast to the high-copy-number autonomous replication of the wild-type genome, the genomes of the E1 mutants were, as previously described for other E1 mutants, integrated at lower copy numbers in the transformed cells. Relative to the viral genome copy number, both the E1 M and R mutant transformed cells contained an average of 10-fold more BPV-specific transcripts than did the wild-type transformed cells. Cycloheximide treatment of the cells transformed by the E1 mutants did not lead to the rapid 10-fold increase in the accumulation of viral transcripts observed with the wild-type genome. These results suggest either that integration of the BPV genome makes it unresponsive to a labile repressor or that an E1 gene product, containing both M and R sequences, is a repressor of BPV transcription.

Presence of catenated human papillomavirus type 16 episomes in a cervical carcinoma cell line

Human papillomavirus (HPV) is frequently associated with cervical carcinoma and derived cell lines. In primary tissues of the carcinoma, the viral genome may be present in episomal or integrated configuration. In cell lines, however, only integrated HPV sequences have been reported. In this article, we describe the presence of episomal type 16 HPV (HPV16), demonstrated by electron microscopy and two-dimensional agarose gel electrophoresis, in a cervical carcinoma cell line, CC7T/VGH, established in 1980 in Taiwan. In CC7T/VGH, the HPV16 sequences are transcriptionally active, and at least three major HPV16 RNA species were detected in Northern blots. Results from restriction enzyme and S1 nuclease analysis suggest a composition of oligomeric HPV16 molecules in dimeric repeats. In addition, the HPV16 oligomers exist as catenated molecules of interlocking rings instead of concatemers. A monomeric copy of the HPV16 episome was cloned from a Hirt supernatant of CC7T/VGH by using a plasmid vector. Mapping and partial sequencing studies revealed an internal deletion of 163 base pairs within the L1 open reading frame. However, insertion of an A.C nucleotide pair at the deletion junction restored the otherwise frame-shifted L1 open reading frame. Two base transitions were also found within the E7 and the E1 open reading frames. Our findings suggest the need for closer examination for HPV episomal catenation in other cervical carcinoma cell lines as well as in primary carcinoma tissues of the uterine cervix and the anogenital tract. With CC7T/VGH, a way is now available for studies of many important aspects of the biology of HPV such as replication and gene expression of the extrachromosomal viral genome.

Human papillomaviruses and carcinomas

The recognition of multiple types of human papillomaviruses has resulted in remarkable progress in the detection of persisting viral nucleic acid sequences in carcinomas. The consistent transcription in tumors of two early open reading frames, E6 and E7, with few exceptions (Lehn et al., 1985), indicates a role for the products of these genes in the induction and/or maintenance of the transformed state. A number of studies have shown that in vitro transformation can be achieved by transfection of E6/E7 DNA, and proteins encoded by these DNA sequences can be demonstrated in primary human keratinocytes immortalized by this DNA (Kaur et al., 1989). Mutagenesis experiments are needed to determine the absolute requirement for and function of these genes in transformation. A preferential association of some types with benign lesions while others may be frequently found in malignant tumors has been observed. HPV types 5 and 8 in epidermodysplasia verruciformis patients and types 16, 18, 31, 33, etc. in genital lesions are most frequently associated with progression to malignancy, whereas other types, such as HPV-6,-10, -11, and -20, are regularly identified in benign warts. Such distinctions are not absolute but provide the initial steps toward establishing a causal role for some human papillomaviruses in carcinomas. The need for well-designed epidemiological studies in concert with optimum molecular and serologic evaluations is evident (Armstrong et al., 1988). The data from human and animal studies indicate that papillomaviruses contribute significantly to the development of many, if not all, carcinomas, but we do not yet have a clear understanding of the importance of other interacting viral, chemical, or cellular factors. The application of gene cloning and non-stringent hybridization (Law et al., 1979) has provided us with an apparently ever-increasing catalog of human papillomaviruses. More effort is now required to establish their prevalence, the natural history of infection, and the mechanism of neoplastic transformation.

Efficient transactivation and morphologic transformation by bovine papillomavirus genes expressed from a bovine papillomavirus/simian virus 40 recombinant virus

To efficiently introduce bovine papillomavirus type 1 genes into cultured cells, we constructed a hybrid viral genome in which the simian virus 40 early region is replaced with a segment of the bovine papillomavirus type 1 transforming region. High-titer stocks of simian virus 40 virions containing the recombinant genome were produced in monkey cells that express simian virus 40 large tumor antigen. Cells infected with this virus efficiently expressed the bovine papillomavirus type 1 E2 and E5 genes. Expression of the E2 gene caused transactivation of genes linked to the bovine papillomavirus type 1 control region, resulting in up to a 1000-fold induction. At high multiplicity of infection of a cell line containing an integrated reporter gene, most cells were infected and responded to transactivation. Within 48 hr of infection with wild-type virus but not with an open reading frame E5 mutant, mouse C127 cells displayed dramatic changes in morphology and growth characteristics similar to those seen in tumorigenic transformation. This system can be used to determine the acute cellular response to introduction of bovine papillomavirus type 1 transforming and regulatory genes; it can also be used to induce foreign genes stably incorporated into cultured mammalian cells.

Bovine papillomavirus type 1 E1 replication-defective mutants are altered in their transcriptional regulation

Bovine papillomavirus type 1 (BPV-1) is capable of replicating as a stable, high-copy-number plasmid in transformed rodent cells. The BPV-1 E1 open reading frame (ORF) encodes multiple functions involved in viral DNA replication. Mutations which disrupt the translational integrity of the E1 ORF disable the viral genome from replicating as a stable plasmid and result in the integration of the viral genome into the host chromosome generally at a low copy number. Despite the low copy number of the integrated genomes, BPV-1 E1 mutants transform rodent cells to anchorage independence very efficiently, at levels equal to or greater than that of wild-type (wt) BPV-1. Studies were performed to provide insight into why these low-copy-number, replication-defective mutants are capable of expressing an equal or greater transformation potential than wt BPV-1. Analysis of viral RNA revealed higher rates of transcription per viral genome in cells harboring E1 mutated BPV-1 DNA than in cells containing wt BPV-1 DNA. Furthermore, the levels of viral RNA mapping the P89 promoter were found to be 15- to 35-fold higher in cells transformed by E1 mutated DNAs compared with wt BPV-1 transformants. This promoter controls expression of the BPV-1 E6 transforming gene and is regulated by the viral E2 gene products. The studies presented in this report determined that the E1 mutants were perturbed in their E2 transcriptional regulation, suggesting a possible explanation for the observed P89 induction. Mutations throughout the E1 ORF, in either of the two regions previously identified as encoding distinct replication functions, were altered in viral transcription.

Evidence for cooperativity between E2 binding sites in E2 trans-regulation of bovine papillomavirus type 1

The long control region of bovine papillomavirus type 1 (BPV-1) can function in an orientation- and position-independent manner as an E2-dependent enhancer. Dissection of the long control region has revealed two E2-responsive elements, E2RE1 and E2RE2, which map, respectively, between nucleotides 7611 and 7806 and between nucleotides 7200 and 7386 of the BPV-1 genome. In this study, we have carried out a detailed analysis of E2RE1, which has previously been shown to be involved in the regulation of the BPV-1 promoters P89 and P7940. One characteristic of E2RE1 is the presence of a pair of ACCN6GGT motifs (E2 binding sites) at each end of the element. To determine the contribution of these sites, as well as other sequences within E2RE1, to enhancer function, specific mutations and deletions were generated by oligonucleotide reconstruction. The functional analysis of these mutations confirmed that a pair of E2 binding sites was essential for E2-dependent enhancer activity but also indicated that cooperativity between the motifs at each end of E2RE1 creates a highly responsive element. Isolated ACCN6GGT motif pairs could also act as E2-dependent enhancers but at a significantly reduced level in comparison to the intact element. The sequences between the E2 binding sites in E2RE1 were not required for enhancer function and could actually block the enhancer activity of an isolated pair of E2 binding sites when positioned between the binding sites and the enhancer-deleted simian virus 40 early promoter.

The product of the bovine papillomavirus type 1 modulator gene (M) is a phosphoprotein

The M gene of bovine papillomavirus type 1 has been genetically defined as encoding a trans-acting product which negatively regulates bovine papillomavirus type 1 replication and is important for establishment of stable plasmids in transformed cells. The gene for this regulatory protein has been mapped in part to the 5' portion of the largest open reading frame (E1) in the virus. We constructed a trpE-E1 fusion gene and expressed this gene in Escherichia coli. Rabbits were immunized with purified fusion protein, and antisera directed against the product were used to identify the M gene product in virus-transformed cells. In this way a polypeptide with an apparent molecular mass of 23 kilodaltons was detected. The virus-encoded product is phosphorylated and can be readily detected by immunoprecipitation assays from cells transformed by the virus. Cells that harbor viral DNA without M as integrated copies do not produce this protein, whereas cells that harbor integrated viral genomes which are defective for another E1 viral gene important for plasmid replication, R, do produce this protein. The protein has an anomalously low electrophoretic mobility. An in vitro translation product of an SP6 RNA product of a sequenced cDNA predicts a molecular mass of 16 kilodaltons for the protein, and this in vitro translation product has an electrophoretic mobility identical to that of the in vivo immunoprecipitated protein. The results of these studies confirm our previous genetic studies which indicated that part of the E1 open reading frame defined a discrete gene product distinct from other putative products which may be encoded by this open reading frame.

Interaction of the bovine papillomavirus type 1 E2 transcriptional control protein with the viral enhancer: purification of the DNA-binding domain and analysis of its contact points with DNA

The E2 gene of bovine papillomavirus type 1 positively and negatively regulates the transcriptional enhancer located in the long control region of the viral genome. The DNA-binding domain of the E2 gene product was suspected to interact with the DNA sequence motif ACCN6GGT. We have shown that the carboxy-terminal 126 amino acids of the E2 protein constitute the DNA-binding domain. In this paper we described the expression of the E2 carboxy terminus in Escherichia coli and its subsequent purification. We provide definitive evidence that the protein recognizes the ACCN6GGT motifs in the viral enhancer. We show by methylation protection, methylation interference, and ethylation interference that the E2 protein contacts the DNA at the GG residues of the consensus sequence on both DNA strands. A gel retardation-DNase I footprint assay has revealed that the E2 DNA-binding domain exhibits different affinities for different ACCN6GGT motifs, indicating that nucleotides other than the conserved ACC and GGT sequences probably modulate the affinity of the DNA sequence for the E2 protein.

Establishment of composite DNA derived from L factor as a plasmid in mouse embryonal carcinoma (F9) cells

We have recently reported a mammalian cell plasmid (L factor) whose structure is related to that of polyomavirus (T. Kusano, H. Uehara, H. Saito, K. Segawa, and M. Oishi, Proc. Natl. Acad. Sci. USA 84:1789-1793, 1987). When composite DNA constructed from L factor and a foreign gene was introduced into mouse embryonal carcinoma (F9) cells by transfection, the DNA was reestablished in the cells as a plasmid. The reestablished plasmid DNA in F9 cells could be rescued in Escherichia coli. The plasmid-bearing cells underwent normal in vitro differentiation in response to retinoic acid. The efficiency of plasmid establishment of the L-factor-derived DNA and transcriptional and transient replicational activities were compared with those of similar composite DNA constructed from polyomavirus and an embryonal carcinoma mutant of polyomavirus which is permissive in F9 cells. The results suggest an inverse relationship between the efficiency of the plasmid establishment and the activity of gene expression controlled by the intrinsic enhancer-promoter of the DNA.

DNA bending is induced in an enhancer by the DNA-binding domain of the bovine papillomavirus E2 protein

The E2 gene of bovine papillomavirus type 1 has been shown to encode a DNA-binding protein and to trans-activate the viral enhancer. We have localized the DNA-binding domain of the E2 protein to the carboxyl-terminal 126 amino acids of the E2 open reading frame. The DNA-binding domain has been expressed in Escherichia coli and partially purified. Gel retardation and DNase I "footprinting" on the bovine papillomavirus type 1 enhancer identify the sequence motif ACCN6GGT (in which N = any nucleotide) as the E2 binding site. Using electrophoretic methods we have shown that the DNA-binding domain changes conformation of the enhancer by inducing significant DNA bending.

Phenotypic transformation of primary mouse fibroblasts by BPV 1 DNA

Cultures of primary fibroblasts of C57BL/6J mice were used as targets for transformation by bovine papillomavirus type 1 (BPV 1) DNA. Although no foci were observed, several lines of transformed cells were established by subculturing. These immortalized cell lines had in vitro growth characteristics in high and low serum media and saturation densities typical of transformed cells. Karyotype analyses revealed extensive aneuploidic changes. In two of the three cell lines analyzed, viral DNA was present in monomeric episomal form, in the third cell line all viral sequences were found in the high molecular weight region of a Southern blot. Despite the transformed phenotype, only one of the cell lines was tumorigenic in nude mice at a low level.

Human papillomavirus type 16 open reading frame E7 encodes a transforming gene for rat 3Y1 cells

Human papillomavirus type 16 (HPV 16) DNA is capable of morphologically transforming rat 3Y1 cells. The expression plasmids, constructed from the simian virus 40-based expression vector pSV2-0 and specific DNA fragments from the putative early region of the HPV 16 genome, were tested for their transforming capacity. Among the various pSV2 plasmids, only those containing the intact E7 coding region were found to produce foci of the transformed rat cells which could grow in a soft-agar medium. The data indicate that expression of the HPV 16 E7 open reading frame is sufficient to induce focal transformation of rat cells.

Progression of the phenotype of transformed cells after growth stimulation of cells by a human papillomavirus type 16 gene function

Alteration of the growth properties of the established murine fibroblast cell lines NIH 3T3 and 3Y1 was studied in monolayer cultures and in cells suspended in semisolid medium after introduction of a cloned human papillomavirus type 16 (HPV16) DNA. HPV 16 DNA stimulated both cell lines to grow beyond their saturation densities in monolayer cultures without any apparent morphological changes or tendency to pile up. These cells were also stimulated to grow in soft agar. Since essentially all the cells that received the viral gene were stimulated to grow, the growth-stimulatory activity of HPV16 appeared to be due to the direct effect of a viral gene function. The NIH 3T3 cells showed an additional change in growth properties upon prolonged incubation of dense monolayers of cells containing the HPV16 DNA; morphologically recognizable dense foci appeared at a frequency of about 10(-3). These cells, when cloned from the foci, grew more rapidly in soft agar than the parental cells and were morphologically transformed. In other words, there were two sequential steps in cell transformation induced by HPV16. Practically all the viral DNAs were present in the cells as large rearranged multimers and were integrated into host chromosomal DNA. There was no obvious difference in the state of viral DNA in the cells of the original clone or the three subclones derived from it as dense foci. There was no difference in the amount or the number of viral RNA species expressed in the cells at these two stages. The secondary changes in the growth properties of NIH 3T3 cells appear to be due to some cellular alterations.

Multiple cis-active elements in the long control region of bovine papillomavirus type 1 (BPV-1)

A 1.0 kb region of the BPV-1 genome (the long control region, LCR), contains controls for transcription and the origin of replication. Transcription directed by the LCR is activated by the viral encoded E2 protein. To define the essential cis acting elements that are required to control transcription we have constructed a series of deletions throughout the LCR. We have identified three important domains in the LCR, two of which respond to E2. We have analysed the ability of small subcloned regions of the E2 responsive domains to act as enhancers in a heterologous assay system. This has led to the identification of five independent E2 responsive elements. We have shown that a fragment of only 38 base pairs is sufficient to respond to activation by E2. We also present evidence to suggest two types of E2 responsiveness that result in strong or weak activation. Strong response is correlated with the presence of the sequence 5'-ACCG/TNNNC/TCGGTGC-3' whereas weak response is correlated with the presence of a related sequence 5'-ACC(N)6GGT-3'. The contribution of these multiple elements to viral transcription is discussed.

Stable expression of recombinant factor VIII molecules using a bovine papillomavirus vector

The bleeding disorder in hemophilia A results from a deficiency or abnormality of Factor VIII (FVIII), a member of the coagulation cascade. FVIII is a large glycoprotein (approximately 350,000 daltons) that is activated by a series of proteolytic cleavages. During activation, a large internal domain (B domain) is removed, resulting in an active complex comprised of the amino and carboxyl subunits of the parental molecule. Using a bovine papillomavirus expression vector system, we have established stable, genetically engineered cell lines harboring either full-length FVIII cDNA or variant FVIII cDNA (delta FVIII), the latter containing an extensive deletion in the region encoding the B domain. We demonstrate that the two recombinant FVIII molecules manifest the biological attributes of native FVIII. Relative to full-length FVIII transformants, cells harboring delta FVIII cDNA are five to eight times more efficient in expressing coagulant activity. This difference is due to a post-transcriptional event.

Natural history of human papillomavirus infection of the anogenital tract

A number of viruses, most notably herpes virus type 2, have been suggested as etiological agents of cervical neoplasia. Recent studies with human papillomaviruses, however, have demonstrated a remarkable association of a subgroup of these viruses with about 90% of benign, preinvasive and invasive lesions of the cervix and anogenital tract. The oncogenic potential of papillomaviruses has been demonstrated both in laboratory animals and in cultured cells. Furthermore, susceptibility to certain human papillomaviruses has been associated with a recessive genetic defect that results in squamous cell carcinoma of the skin. The human papillomaviruses are difficult to study, however, because of the lack of an animal model, difficulty in developing a tissue culture system permissive for their replication, and a lack of understanding of their biology. Current understanding of the natural history of anogenital neoplasia may provide insights into the mechanisms the host uses to cope with potentially oncogenic human papillomaviruses.

Messenger RNAs from the E1 region of bovine papillomavirus type 1 detected in virus-infected bovine cells

Bovine papillomavirus type 1 DNA replicated to a high copy number in virus-infected bovine fibroblasts. Infected bovine cells were therefore used as a source of RNA for Northern blotting analysis to search for viral transcripts hybridizing to the E1 gene region, implicated in viral DNA replication. Cytoplasmic polyadenylated RNA preparations contained at least five different E1-region transcripts, ranging from 1200 to approximately 4500 nucleotides in length. All of these species contained sequence information from the 5'-end of the E1 open reading frame, but only the largest species included sequences from its central portion. The latter transcript is a candidate mRNA for a stimulatory replication factor (R) previously mapped by genetic experiments (1).

Plasmidal maintenance of composite DNA derived from polyoma related plasmid, L factor

Recently, we reported a multicopy mammalian plasmid with a structure related to polyoma. The plasmid, named L factor, was found at a high copy number (5,000 or more per cell) in a subclone derived from mouse L cells. We attempted to utilize L factor as a plasmid vector for mammalian cells. A series of composite DNA consisting of L factor and a foreign (herpes simplex virus tk) were constructed. These DNA could be established as plasmids after transfection to several mouse cell lines, although the copy number of the re-established plasmids was considerably less than that observed for the original subclone. The composite DNA maintained the structure of the original DNA after prolonged culture and the copy number remained constant even with no selective pressure. A composite DNA, with no DNA sequence corresponding to polyoma T antigen, could also be established as a plasmid in a mouse L cell line in which polyoma T antigen is expressed. The potential use of the plasmid is discussed.

Chromosomal insertion of human papillomavirus 18 sequences in HeLa cells detected by nonisotopic in situ hybridization and reflection contrast microscopy

Genomic insertion of human papillomavirus (HPV) sequences is associated with the genesis of cervical carcinoma, and HPV-induced incipient cellular alterations may also present a requisite for the establishment of cell lines such as HeLa. Considering the theoretical importance of specific viral integration sites, we attempted to detect in HeLa cells the chromosomal location of DNA sequences homologous to HPV-16 and HPV-18 sequences by a nonisotopic high resolution in situ hybridization technique. Chromosome identification following in situ hybridization was possible by counterstaining of the same preparation with Chromomycin A3, Distamycin A, and DAPI. Using this approach, we have assigned HPV-18 integration in HeLa cells to band 8q24 (a site including the locus of the myc-protooncogene), to an abnormal chromosome 22, and to a not yet identified marker chromosome possibly neighboring other oncogenic or activating sites. The sensitive detection technique described in this study presents a new approach involving in situ chromosome hybridization with biotinylated DNA probes in combination with reflection contrast microscopy and subsequent fluorescent R- and C-banding. The method allowed the assignment of a 7-kb HPV-18 DNA probe to human chromosomal sites important in growth regulation and cancerogenesis. It should prove useful in a number of similar studies using other viral and oncogenic DNA probes.

Human cytomegalovirus (HCMV) enhances bovine papilloma virus (BPV) transformation in vitro

Infection of NIH 3T3 cells with a combination of HCMV and BPV resulted in more foci than infection with BPV alone. Foci were microscopically apparent at 4 days in the mixed infection and did not appear until 2 days later in the cultures infected with BPV alone. The enhancement was abolished by heat inactivation of the HCMV and also when the HCMV was replaced by a "mock inoculum." Southern blot analysis of cellular DNA from transformed cells showed a similar amount of extrachromosomal BPV DNA in cells infected by BPV alone and in cells co-infected with HCMV. No HCMV antigens could be found in these cells by immunofluorescence. The mechanisms of the enhancement are not known. Stimulation of host DNA synthesis by HCMV could possibly increase the transforming efficiency of BPV. Alternatively, the increase in BPV transforming efficiency could be due to a transient increase in BPV-1 transcription by an HCMV transcriptional transactivation factor. Since both HCMV and human papillomaviruses are commonly found in the uterine cervix, HCMV may play a role in human cervical cancer by enhancing the carcinogenic potential of human papillomavirus.

Isolation and characterization of minichromosome particles that contain a glucocorticoid-modulated promoter

A procedure for the enrichment of minichromosomes, composed of bovine papilloma virus and the long terminal repeat element of the mouse mammary tumor virus (MTV), from isolated nuclei is described. Up to 60% of the minichromosomes were extracted as nucleoprotein particles. These particles sediment in sucrose gradients as 160S complexes. Hormone-labeled glucocorticoid receptor co-purifies with these complexes in a specific fashion. Between four and six molecules of receptor are bound per minichromosome molecule. Analysis of DNase I hypersensitivity demonstrates that hypersensitive sites are preserved through the purification procedure in a manner that reflects the hormone-dependent in vivo pattern of digestion. These purified minichromosomes will allow features of chromatin structure that may be important for steroid hormone modulation of transcription to be studied in vitro without resorting to destructive nuclease digestion procedures.

Mouse cell lines that use heat shock promoters to regulate the expression of tissue plasminogen activator

The promoters from Drosophila and human 70,000-dalton heat shock protein (hsp70) genes were linked to human tissue plasminogen activator (tPA) cDNA. Mouse C127 cells were transformed with bovine papilloma virus (BPV) vectors carrying the hybrid hsp70/tPA genes. Stable BPV-transformed cell lines were selected and analyzed for tPA expression before and after heat shock. In most cell lines, there was a low level of tPA production even in the absence of heat shock or other obvious stress. After heat shock (42 degrees C, 2 hr), there was up to a 40-fold increase in tPA production. Production of tPA protein occurred within the first 5 h after the heat shock and was due to a burst of hsp70/tPA transcription during the heat shock. The hsp70/tPA transcripts appeared to have a short half-life. Thus, stable mouse cell lines carrying hsp70/tPA hybrid genes can be induced by a short heat shock to transcribe high levels of hsp70/tPA mRNAs and, subsequently, to produce elevated levels of tPA protein.

Human papillomavirus types 6 and 11 mRNAs from genital condylomata acuminata

We have identified and mapped a number of RNA species of human papillomavirus types 6 and 11 from condylomata acuminata by the electron microscopic R-loop technique. Each of the early (E)- and late (L)-region open reading frames (ORFs) deduced from the DNA sequences was represented in one or more transcripts. In addition, RNA species that could encode the modulator of DNA replication and the repressor of transcription, functions recently identified in the genetically similar bovine papillomavirus type 1, were also detected. Some ORFs were 5' proximal in one or more transcripts, whereas others were not 5' proximal in any species, suggesting that internal initiation of translation might be required to gain access to these latter ORFs. Virtually all transcripts had their 5' ends located in the E region and were polyadenylated at one of two sites, i.e., at the end of the E region or at the end of the L region. The great majority of the RNAs were derived from the E region of the genome, with one species approximately 50 to 100 times more abundant than the others. For most of the RNAs, the 5' end mapped near nucleotide 700; minor populations had 5' ends near nucleotide 100 or 1200. By correlating our mapping data with the genomic DNA sequences as well as available RNA structures and cDNA sequences of several papillomaviruses, we predict a number of mRNA splice donor and acceptor sites and suggest that the papillomaviruses have sophisticated usage of ORFs through alternative promoters, mRNA splice sites, and polyadenylation sites.

Promoters and processing sites within the transforming region of bovine papillomavirus type 1

The mRNAs present in bovine papillomavirus type 1 (BPV-1)-transformed C127 cells were studied by primer extension. The results show that two internal promoters are present in the E region of BPV-1 in addition to the previously identified promoter at coordinate 1 (H. Ahola, A. Stenlund, J. Moreno-López, and U. Pettersson, Nucleic Acids Res. 11:2639-2650, 1983). One, located at coordinate 31, generated a set of mRNAs with heterogeneous 5' ends, which may encode the major transforming protein of BPV-1, the E5 protein. The second promoter, which is located at coordinate 39, generates colinear mRNAs which encode either the E4 protein or a truncated form of the E2 protein. Unlike the cottontail rabbit papillomavirus (O. Danos, E. Georges, G. Orth, and M. Yaniv, J. Virol. 53:735-741, 1985), BPV-1 appears to lack a separate promoter for expression of the E7 protein. The major splice sites in the transforming region (E region) of the BPV-1 genome were also identified by nucleotide sequence analysis.

Identification and mapping of human papillomavirus type 1 RNA transcripts recovered from plantar warts and infected epithelial cell cultures

Multiple spliced transcripts of human papillomavirus type 1 were detected by electron microscopic analysis of R-loops formed with total RNA extracted from plantar warts and with poly(A)+ RNA isolated from cultured keratinocytes infected with human papillomavirus type 1. The 5' ends of the RNAs were mapped to sites in the E7 open reading frame (ORF), just upstream of the E6 ORF and in the upstream regulatory region. Species with 5' ends in E7 accounted for over 95% of all transcripts seen. Two polyadenylation sites were used, one at the end of the early (E) region of the viral DNA, the other at the end of the late (L) region. The most abundant species had a short 5' exon of approximately 100 nucleotides spanning the junction of the E7 and E1 ORFs spliced to a 3' exon of 800 nucleotides in the region with overlapping E2 and E4 ORFs; it was polyadenylated at the end of the E region. This species probably encodes the abundant E4 protein found in plantar warts (F. Breitburd, O. Croissant, and G. Orth, Cancer Cells, vol. 5, in press; J. Doorbar, D. Campbell, R. J. A. Grand, and P. H. Gallimore, EMBO J. 5:355-362, 1986). Other transcripts had exons spanning the E6-E7 ORFs, the E4-E5-L2-L1 ORFs, or the L1 ORF. The infrequent L1 transcript, probably the mRNA coding for the major capsid protein, had the same 5' exon in E7 as the abundant mRNA spliced from E1 and E4 ORFs, suggesting genetic regulation via the choice of the alternative polyadenylation sites or mRNA processing.

Episomal maintenance of a bovine papilloma virus vector in transgenic mice

We have used a bovine papillomavirus-based vector to generate transgenic mice. Transgenic mice result from either pronuclear or cytoplasmic injections of the vector into fertilized eggs. Of 30 mice generated by microinjection, 27 (90%) contained the vector in its episomal form, at less than one copy per cell. This represents a highly efficient means of gene transfer in which the transgene is in a controlled genetic environment.

Transcriptional trans-activation by the human papillomavirus type 16 E2 gene product

We identified a conditional transcriptional enhancer in the long control region (LCR) of human papillomavirus type 16 (HPV-16). This conditional enhancer requires activation in trans by a product of the viral early-region open reading frames (ORFs). Primer extension analysis of chloramphenicol acetyltransferase RNA isolated from transiently transfected CV-1 cells demonstrated that trans-activation of the HPV-16 LCR enhancer operated at the transcriptional level. Mutational analysis of the early ORFs demonstrated that the conditional enhancer of the LCR was trans-activated by the product of the E2 ORF. The E2 gene product of bovine papillomavirus type 1, which can trans-activate the conditional enhancer in the bovine papillomavirus type 1 LCR, was also capable of trans-activating the E2-responsive enhancer of HPV-16. The activity of the HPV-16 LCR enhancer was also assayed in two human cervical carcinoma cell lines, HeLa and SiHa, which harbor transcriptionally active, integrated HPV-18 and HPV-16 DNA sequences, respectively. No endogenous E2 or E2-like activity was detected in either cell line.

BK virus-transformed inbred hamster brain cells: status of viral DNA in subclones

We have recently reported that viral DNA sequences in inbred LSH hamster brain cells transformed by the GS variant of BK virus (LSH-BR-BK) are present predominantly in a free form (Beth et al., J. Virol. 40:276-284, 1981). In this report, we confirm that the presence of viral DNA sequences in these cells is not due to virus production, since viral capsid proteins were not detected by immunoprecipitation. Furthermore, we examined the status of viral DNA in 15 subclones of this cell line and detected free and integrated viral DNA sequences in only 5 of the subclones. The other 10 subclones contained exclusively integrated viral DNA sequences, as shown by the blot hybridization of high-molecular-weight cell DNA which was uncleaved or digested with HincII, for which there are no sites in viral DNA. The arrangement of viral DNA in these clones was further analyzed by cleavage of cellular DNA with HpaII and HindIII. Mitomycin (0.03 microgram/ml) treatment of subclones containing only integrated sequences resulted in the appearance of free viral DNA sequences in some of these cells. This result supports the postulation that free viral DNA in LSH-BR-BK cells is made up of excision products of observed tandemly repeated integrated sequences. In addition to the large T- and small t-antigens, LSH-BR-BK and all of its 15 subclones contained two antigen species which were larger than large T and one species which was smaller than small t. The number of tumor antigens in the LSH- BR-BK cell line and its subclones with a large copy number in a free form was not more than in the subclones with low copy number and integrated DNA. This suggests that free viral DNA is not a template for tumor antigen production in transformed cells.

Multicopy plasmid with a structure related to the polyoma virus genome

In a subclone derived from mouse L(tk-) cells, we found a plasmid present in a high copy number (greater than 5000 copies per cell) that was stably maintained extrachromosomally without any cytopathic effect to the host cells. This plasmid, termed L factor, has two forms: 5.3 and 5.5 kilobase pairs. DNA sequencing and restriction enzyme mapping showed that, although the structure contains DNA sequences common to polyoma virus, plasmid sequences belonging to the regulatory region (the enhancer region) and other regions are quite different from those in polyoma. In cells bearing the plasmid, we detected a low level of material that cross-reacts with antibody to polyoma tumor antigens, suggesting that the plasmids replicate and are maintained in the cells by a mechanism different from that functioning during propagation following infection of papovaviruses.

Differential promoter utilization by the bovine papillomavirus in transformed cells and productively infected wart tissues

Expression of the 'late' genes of bovine papillomavirus type 1 (BPV-1) occurs only in the differentiated keratinocytes of the productively infected fibropapilloma. A detailed analysis of viral transcription in the fibropapilloma was performed and compared to BPV-1 specific transcription in transformed C127 cells. A cDNA library was constructed from bovine fibropapilloma mRNA using the method of Okayama and Berg. Analysis of full length cDNAs showed that the majority of viral transcripts in the fibropapilloma have 5' termini near nt 7250 and utilize a common splice donor site at nt 7385. This mRNA start site was confirmed by the combination of primer extension and nuclease S1 analyses; it is not utilized in the BPV-1-transformed C127 cell, thus identifying it as a wart specific, 'late' promoter. Upstream of this mRNA start site is a tandemly repeated sequence element homologous to the SV40 late promoter sequence GGTACCTAACC, which has been shown to be important for the efficient utilization of the SV40 major late start site. Two additional mRNA start sites at nt 7185 and nt 7940 in the long control region (LCR) were identified and were found to be used in bovine warts as well as in BPV-1-transformed mouse cells. The promoter region upstream of the nt 7940 mRNA start site contains the E2 responsive enhancer mapping between nt 7611 and nt 7805 [Spalholz, B.A., Lambert, P.F., Yee, C. and Howley, P.M. (1987) J. Virol., in press].(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and transcriptional analysis of human papillomavirus type 16 sequences in cervical carcinoma cell lines

We cloned and analyzed the integrated human papillomavirus type 16 (HPV-16) genomes that are present in the human cervical carcinoma cell lines SiHa and CaSki. The single HPV-16 genome in the SiHa line was cloned as a 10-kilobase (kb) HindIII fragment. Integration of the HPV-16 genome occurred at bases 3132 and 3384 with disruption of the E2 and E4 open reading frames (ORFs). An additional 52-base-pair deletion of HPV-16 sequences fused the E2 and E4 ORFs. the 5' portion of the disrupted E2 ORF terminated immediately in the contiguous human right-flanking sequences. Heteroduplex analysis of this cloned integrated viral genome with the prototype HPV-16 DNA revealed no other deletions, insertions, or rearrangements. DNA sequence analysis of the E1 ORF, however, revealed the presence of an additional guanine at nucleotide 1138, resulting in the fusion of the E1a and E1b ORFs into a single E1 ORF. Sequence analysis of the human flanking sequences revealed one-half of an Alu sequence at the left junction and a sequence highly homologous to the human O repeat in the right-flanking region. Analysis of the three most abundant BamHI clones from the CaSki line showed that these consisted of full-length, 7.9-kb HPV-16 DNA; a 6.5-kb genome resulting from a 1.4-kb deletion of the long control region; and a 10.5-kb clone generated by a 2.6-kb tandem repeat of the 3' early region. These HPV-16 genomes were arranged in the host chromosomes as head-to-tail, tandemly repeated arrays. Transcription analysis revealed expression of the HPV-16 genome in each of these two cervical carcinoma cell lines, albeit at significantly different levels. Preliminary mapping of the viral RNA with subgenomic strand-specific probes indicated that viral transcription appeared to be derived primarily from the E6 and E7 ORFs.

Mutational analysis of open reading frame E4 of bovine papillomavirus type 1

Open reading frame (ORF) E4 is a 353-base-pair ORF of bovine papillomavirus type 1. To determine the biological activities of this ORF in mouse C127 cells, we analyzed the effects of two constructed mutations which are predicted to prevent synthesis of ORF E4 proteins while leaving the amino acid sequence encoded by the overlapping ORF E2 unchanged. Neither mutation interfered with the abilities of the mutants to efficiently induce focus formation, induce growth in soft agarose, or transactivate an inducible bovine papillomavirus type 1 enhancer. Also, neither mutation prevented establishment of the viral DNA as an extrachromosomal plasmid in transformed cells. These results suggest that ORF E4 proteins are not required for these biological activities, and they are consistent with the observation of others (J. Doorbar, D. Campbell, R. J. A. Grand, and P. H. Gallimore, EMBO J. 5:355-362, 1986) that the ORF E4 protein of a human papillomavirus is associated with late gene expression during papilloma formation.

The E2 "gene" of bovine papillomavirus encodes an enhancer-binding protein

The E2 early open reading frame (presumably gene) of bovine papillomavirus-1 was fused in frame with the collagen-beta-galactosidase-encoding region of the vector pJG200 and was expressed in and partially purified from Escherichia coli. The hybrid protein specifically bound to the enhancer region of bovine papillomavirus at several sites. DNase I-cleavage protection analysis of one such site revealed the protected sequence. A comparison of the protected sequence with the remainder of the DNA sequences that also have affinity for the protein revealed a consensus sequence having the motif AATTGGCGGNNCG, in which N is any nucleotide. The protected region also includes a sequence with 2-fold rotational symmetry--ATCGGTG/CACCGAT.