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

Regulation of bovine papillomavirus type 1 gene expression by RNA processing

Bovine papillomavirus type 1 (BPV-1) has served as a prototype for studying the molecular biology and pathogenesis of papillomaviruses. The expression of BPV-1 early and late genes is highly regulated at both transcription and post-transcriptional levels and strictly tied to the differentiation of keratinocytes. BPV-1 infects keratinocytes in the basal layer of the skin and replicates in the nucleus of infected cells in a differentiation-dependent manner. Although viral early genes begin to be expressed from the infected, undifferentiated basal cells, viral late genes are not expressed until the infected cells enter the terminal differentiation stage. Both BPV-1 early and late transcripts are intron-containing bicistronic or polycistronic RNAs, bearing more than one open reading frame and are polyadenylated at either an early or late poly (A) site. Nuclear RNA processing of these transcripts by RNA splicing and poly (A) site selection has been extensively analyzed in the past decade and various viral cis-elements and cellular factors involved in regulation of viral RNA processing were discovered, leading to our better understanding of the gene expression and biology of human papillomaviruses.

Repression of bovine papillomavirus type 1 transcription by the E1 replication protein

Bovine papillomavirus type 1 (BPV-1) is the prototype virus for the study of papillomavirus gene regulation. The functions of the BPV-1 E2 proteins in transcriptional regulation have been well characterized. The BPV-1 E1 protein is required for viral DNA replication and can bind to the origin of replication alone or in a complex with the E2 transactivator protein. In this study, we demonstrated that the BPV-1 E1 protein is also involved in transcriptional regulation. The E1 protein significantly repressed E2-transactivated transcription from the major early promoter P89. This activity is consistent with the elevated level of P89 transcription observed in BPV-1 E1 open reading frame mutants. Transcriptional repression by E1 correlated with the ability of an E1-E2 protein complex to bind the replication origin but was not dependent on viral DNA replication. These studies identify a new mechanism involved in the regulation of papillomavirus transcription which has implications regarding expression of the viral transforming functions.

Rolling-circle replication of a high-copy BPV-1 plasmid

We investigated the replicating form of a bovine papillomavirus type 1 (BPV-1) deletion mutant by direct electron-microscopic analysis of low molecular weight cellular DNA fractions. The detection of viral plasmid DNA replication intermediates was facilitated by the isolation of a spontaneously transformed mouse cell subclone containing an unusually high viral genome copy number (approx. 1000 per cell), and by employing a slight modification of the Hirt fractionation procedure to reduce the level of contaminating linear chromosomal DNA fragments. We observed exclusively rolling-circle-type viral DNA replication intermediates, at a frequency of detection of approximately one replication intermediate per 200 monomeric circular viral DNA molecules. The demonstration of rolling-circles with longer-than-genome-length tails indicated that this high-copy viral plasmid was not subject to a strict once-per-cell-cycle mode of DNA replication. Our observations provide further evidence in favour of an alternative replication mode of the BPV-1 genome, and may help to explain earlier conflicting findings concerning the mechanism of stable BPV-1 plasmid copy-number-control.

Localization of bovine papillomavirus type 1 E5 protein to transformed basal keratinocytes and permissive differentiated cells in fibropapilloma tissue

We examined expression of the E5 transforming protein of bovine papillomavirus type 1 (BPV-1) in naturally and experimentally infected bovine cells. Bovine conjunctival fibroblasts transformed in vitro by experimental infection with purified BPV-1 virions expressed significantly higher amounts of the 7-kDa E5 protein than BPV-1-transformed murine C127 cells. Indirect immunofluourescence analysis revealed a cytoplasmic, predominantly juxtanuclear, localization of E5 protein in the in vitro virus-transformed bovine cells. In naturally infected bovine skin fibropapilloma tissue, two widely separated sites of E5 protein synthesis were identified within the epithelial layers. Transformed basal layer keratinocytes throughout the tumor tissue expressed cytoplasmic E5 protein at a low uniform level. In addition, abundant amounts of cytoplasmic E5 protein with a granular staining pattern were detected in highly differentiated keratinocytes in close association with sites of viral capsid protein synthesis. These observations imply roles for the viral E5 oncogene in the growth transformation of basal epidermal keratinocytes as well as in the differentiation-linked process of viral maturation. Detection of a papillomavirus protein in the basal cell population of warts lends support to the hypothesis that these cells are maintained in a transformed state by continuous expression of a viral transforming gene.

An E1M--E2C fusion protein encoded by human papillomavirus type 11 is asequence-specific transcription repressor

We have isolated a putative, spliced E5 cDNA of human papillomavirus type 11 (HPV-11) by polymerase chain reaction amplification of cDNAs from an experimental condyloma. Using retrovirus-mediated gene transfer, we isolated two novel HPV-11 cDNAs, one of which had a splice linking nucleotides 1272 and 3377. This transcript also existed in experimental condylomata and in cervical carcinoma cells transfected with cloned genomic HPV-11 DNAs. The 5' end of the transcript in transfected cells originated upstream of the initiation codon of the E1 open reading frame (ORF). It could conceptually encode a fusion protein consisting of the amino-terminal 23% of the E1 ORF and the carboxy-terminal 40% of the E2 ORF. This E1M--E2C fusion protein contained both the DNA replication modulator domain E1M, as defined in the bovine papillomavirus system, and the DNA binding domain of the E2 protein, which regulates viral transcriptional activities. Indirect immunofluorescence with polyclonal antibodies raised against the bacterially expressed TrpE-HPV-11 E2 protein demonstrated nuclear localization of the E1M--E2C protein in cells transiently transfected with an expression plasmid. Immunoprecipitation revealed a specific protein with an apparent molecular weight of 42,000 in transfected cells. The chloramphenicol acetyltransferase assay established that the putative E1M--E2C protein was a potent transcriptional repressor of both E2-dependent and E2-independent HPV-11 enhancer/promoter activities. Northern (RNA) blot hybridization indicated the repression was on the transcriptional level. Mutational analysis suggested that the E1M--E2C protein is an E2-binding site-specific repressor. The fusion protein also repressed bovine papillomavirus type 1 (BPV-1) E2 protein-dependent BPV-1 enhancer activity. When constitutively expressed in mouse C127 cells, the E1M--E2C protein inhibited BPV-1 transformation and episomal DNA replication, consistent with a role in the modulation of replication.

Proteins encoded by the bovine papillomavirus E1 open reading frame: expression in heterologous systems and in virally transformed cells

The E1 open reading frame (ORF) of bovine papillomavirus type 1 is required for the persistence of viral genomes as multicopy plasmid molecules in transformed rodent fibroblasts. E1 has been reported to contain two separate complementation groups (M and R, corresponding to N- and C-terminal domains, respectively) which regulate viral replication. However, E1 behaves as a single gene with respect to cell transformation and viral transcription. We examined the proteins translated from the entire ORF by using three antisera raised against E1 peptide or bacterial fusion proteins. The capacity of the whole ORF to encode a 72-kDa protein was demonstrated by translation of synthetic RNA in a reticulocyte lysate system, by microinjection of RNA into Xenopus oocytes, and by expression in recombinant baculoviruses and vaccinia viruses. In eucaryotic cells, this protein was found to be phosphorylated and targeted to the cell nucleus. In vitro translation also produced shorter peptides, containing only the E1 C-terminal domain, because of internal translation starts on the third and fourth methionine codons within E1 ORF. On the other hand, mammalian cells infected by vaccinia E1 recombinant virus contained additional larger E1 phosphoproteins (transient 85-kDa and stable 88-kDa species), likely representing processed forms of the 72-kDa species. The E1 72-kDa nuclear phosphoprotein was detected in bovine papillomavirus type 1-transformed cells. We report the biochemical characteristics of full-sized and truncated E1 proteins: (i) the C-terminal half of E1 ORF contains a phosphorylation site(s); (ii) the full-sized E1, but not the C-terminal protein, binds DNA, without indication for recognition of defined sequences, and critical determinants for this activity are likely confined to an N-terminal domain of the protein; (iii) covalent affinity labeling experiments performed on vaccinia virus-encoded E1 proteins with an ATP analog confirmed our previous observation of sequence similarities between the E1 C-terminal domain and the ATPase domain of simian virus 40 large T antigen.

A bovine papillomavirus constitutive enhancer is negatively regulated by the E2 repressor through competitive binding for a cellular factor

The bovine papillomavirus type 1 long control region (LCR) contains DNA sequence elements involved in the regulation of viral transcription and replication. Differences in the levels of transcription have previously been noted between bovine papillomavirus type 1-infected rodent cell lines and bovine cells. To investigate these differences, fragments of the LCR were cloned into an enhancer-deleted chloramphenicol acetyltransferase expression vector and assayed for enhancer activity. A strong constitutive enhancer was found in the 5' portion of the LCR that was most active in primary bovine fibroblasts and had little activity in other cell types. Deletion mapping localized most of the activity to a 113-bp fragment from nucleotides (nt) 7162 to 7275, a region of the viral sequence that also contains the P7185 promoter and an E2-binding site at nt 7203. The enhancer activity of this element could be positively modulated by the full-length E2 transactivator or negatively modulated by the E2 repressor. Site-directed mutagenesis defined two cis elements, CE1 and CE2, which were both necessary for enhancer activity. The CE1 element was required for P7185 activity, whereas the CE2 element was dispensable for P7185 activity. The CE1 and CE2 elements both overlap the E2-binding site at nt 7203. In vitro DNA-binding studies revealed (i) a specific gel retardation complex associated with cellular factor binding at the CE1 element, (ii) a correlation between enhancer activity and the binding of factors to the CE1 element, and (iii) competitive binding between the E2 repressor and the cellular factor at the CE1 element.

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.

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.

A novel spontaneous mutation of the bovine papillomavirus-1 genome

A cell clone (cl.2) having an atypical transformed morphology was isolated from a murine C127 cell culture experimentally infected with a bovine papillomavirus type 1 (BPV-1) virion preparation. cl.2 cells exhibited minimal transformed characteristics and contained multiple copies of a BPV-1 plasmid with a molecular size slightly less than that of the wild type viral genome. A simple deletion of 277 bp was mapped to the distal portion of the viral 69% transforming fragment where the early gene region merges with the late gene region. None of the recognized early open reading frames were affected by the deletion but sequences including the common early gene mRNA polyadenylation (poly(A] signal and several base pairs of the "distal" enhancer element were deleted. Transfection of C127 cultures with low molecular weight (Hirt) DNA prepared from cl.2 cells led to the appearance of transformed cell foci, and Southern blotting analysis of a cl.2 Hirt DNA-transformed cell line confirmed that the deletion did not destroy the ability to replicate as a high copy plasmid. Removal of the natural early poly(A) signal did not obligate use of the alternative natural viral poly(A) signal located towards the end of the late region. Instead, a new major early mRNA polyadenylation site was mapped close to the unique BamHI recognition sequence at the distal end of the transforming region. Our results underline previous observations that there is a block to the production of stable mRNAs from the late gene region in BPV-1-transformed C127 cells, yet this is not necessarily explained by premature termination of transcription within this region.