Persistence of non-integrated viral DNA in bovine cells transformed in vitro by bovine papillomavirus type 2

The bovine papillomavirus replicon

The bovine papillomavirus genome contains two cis-acting sequences which can serve as signals for replication. At least three virally encoded genes seem to be involved in plasmid replication: E6, E6/7 and E1. Mutations in either the E6 or the E7 open reading frame create plasmids that are maintained at a low copy number per cell. Mutations in the E1 open reading frame are absolutely lethal to replication. Complementation experiments show that these mutations define separate genes. Experiments are described which show that cells harbouring plasmids with mutations in either the E6 or the E7 open reading frame acquire an immunity to high copy-number plasmids. We suggest that either the cell or the virus encodes a repressor. The positive action of E6 and E6/7 modulates the activity of this repressor to allow for the high copy-number state. Though the viral oncogenes are capable of transforming cells separately when they are expressed as part of certain recombinant DNA expression systems, it is clear that, in the context of the entire viral replicon, interactions between the transforming functions and replication functions must exist.

Organization and expression of the genome of bovine papillomavirus type 1

The viral mRNAs present in C127 cells transformed by bovine papillomavirus type 1 (BPV-1) have been mapped by a variety of techniques, including S1 nuclease analysis, Northern blot analysis, primer extension and electron microscopic heteroduplex analysis. The results reveal a very complex mRNA pattern, comprising at least five types of spliced cytoplasmic mRNAs. Both unspliced and partially processed nuclear RNA species have also been identified. The transforming region of BPV-1 contains several promoter regions. A major cap site is located at coordinate 1 and another putative cap site at coordinate 31. A third candidate cap site maps around coordinate 39.

Random-choice replication of extrachromosomal bovine papillomavirus (BPV) molecules in heterogeneous, clonally derived BPV-infected cell lines

Using fluorescence in situ hybridization and Southern blot analysis, we show that three clonally derived cell lines transformed with bovine papillomavirus (BPV), including ID13, the cell line commonly employed for BPV replication studies, are heterogeneous populations having extensive cell-to-cell variation in both the distribution and amount of BPV DNA. Different subclones of ID13 were found to differ in the form and amount of BPV DNA they contain. Most subclones showed no detectable BPV sequences; some contained either extrachromosomal BPV molecules distributed throughout the nucleus or BPV sequences integrated at discrete chromosomal sites, while others contained both integrated and plasmid forms. The results of density gradient analysis of BPV DNA from individual homogeneous subclones showed replication of the extrachromosomal BPV plasmids in a random-choice mode. In all cell lines studied, the presence after one round of chromosomal DNA replication of unreplicated BPV DNA and of BPV DNA having two postreplicative strands was independent of the presence of high-BPV-copy-number ("jackpot") cells. Our results substantiate the earlier conclusion that extrachromosomal BPV molecules replicate randomly and not according to a once-per-cell-cycle mechanism.

Induction of bovine papillomavirus E2 gene expression and early region transcription by cell growth arrest: correlation with viral DNA amplification and evidence for differential promoter induction

The bovine papillomavirus type 1 (BPV-1) genome replicates as a latent plasmid in mouse C127 cells transformed in vitro by the virus. However, we have recently shown that BPV-1 DNA amplification can be induced in a subpopulation of cells under culture conditions which suppress cell proliferation, a finding which led us to hypothesize that expression of a viral replication factor was regulated by cell growth stage. In this report, we describe the detection in these cells of abundant BPV-1 nuclear E2 antigen by immunofluorescence analysis. Expression of E2 antigen in fibropapilloma tissue was similarly localized to nonproliferating epidermal cells of the lower spinous layers--the natural site of induction of vegetative viral DNA replication. Immunoprecipitation analysis showed that the previously characterized 48-kilodalton (transactivator) and 31-kilodalton (repressor) E2 proteins were both induced in growth-arrested cell cultures. In parallel with E2 antigen synthesis under conditions of serum-deprivation in vitro, we observed a significant increase in levels of BPV-1 early region mRNAs. Furthermore, we present evidence for preferential induction of the P2443 promoter, in addition to specific induction of the P7940 promoter in response to serum deprivation. These observations indicate a central role for E2 transcription factors in the induction of viral DNA amplification in division-arrested cells in vitro and in vivo and suggest that this process is associated with a qualitative switch in the expression of viral early region genes.

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.

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).

Characterization of Fischer rat embryo (CREF) cells transformed by bovine papillomavirus type 1

Transformation of an established Fischer rat embryo (CREF) cell line by bovine papillomavirus type 1 (BPV-1), in contrast to transformation by type 5 adenovirus or the T24 (Ha-ras) oncogene, resulted in transformants which did not exhibit a major increase in saturation density or a decrease in 125I-epidermal growth factor binding. BPV-1-transformed CREF clones did, however, grow in agar suspension culture and were tumorigenic in both nude mice and Fischer rats. The majority of transformed clones contained multiple extrachromosomal copies of BPV DNA. One transformed CREF clone contained integrated BPV-1 DNA which underwent sequence rearrangements following tumor formation in a Fischer rat and reestablishment in cell culture. BPV copy number varied in subclones of transformants isolated from the same monolayer focus, in agar-derived subclones of the same transformed focus, in tumors and in tumor-derived BPV-transformed CREF subclones. The degree of expression of specific transformation-related phenotypes, i.e., saturation density, growth in agar, and tumorigenicity, was not correlated with BPV copy number in transformed clones. Analysis of the biological properties of tumor-derived BPV-1-transformed CREF subclones indicated that certain transformants developed a stable increase in expression of transformation-related properties, a process termed "progression." TPA did not enhance the frequency of BPV-1 transformation or BPV DNA copy number in transformed CREF cells. The present study demonstrates that the CREF- transformation system can be utilized to study the molecular basis of BPV-1 transformation and should prove useful in studying the role of specific BPV-1 transforming proteins in regulating expression of the transformed phenotype and the mechanism by which transformed cells undergo progression of the transformed state.

Messenger RNAs from the transforming region of bovine papilloma virus type I

Messenger RNAs present in C127 mouse cells transformed by bovine papilloma virus type 1 (BPV-1) were studied by the S1 nuclease protection technique, Northern blotting, and electron microscopic heteroduplex analysis. The results revealed at least five classes of spliced mRNAs which we designate types 1 to 5. They had a common poly(A) addition site located at co-ordinate 53 and all mRNAs, except the type 3 mRNAs, contained an exon located between co-ordinates 41 and 53. In the type 1 mRNAs this exon was connected to a very short leader sequence located around co-ordinate 31. The type 2 mRNAs contained 220 to 400-nucleotide long leaders which were located approximately 1.5 X 10(3) base-pairs further upstream. Two different subclasses of type 2 molecules (2A and 2B) were identified and these had slightly different leaders. The type 4 mRNAs contained a bipartite leader, whereas the type 5 mRNAs carried an approximately 900-nucleotide long leader. The type 3 mRNAs consisted of a main exon located between co-ordinates 32 and 53, linked to the same leader as is present in the type 2A mRNAs. A cap site which presumably is utilized by the type 2A, type 3, type 4 and type 5 mRNAs was mapped at nucleotide 89 in the BPV-1 sequence. A putative cap site for the type 1 mRNAs was mapped at co-ordinate 31.

Cellular transformation by human papillomavirus DNA in vitro

Molecularly cloned DNA's of human papillomaviruses HPV-5 and HPV-l induced morphological transformation of mouse C127 cells in culture. Single-cell clones of cells transformed by papillomavirus contained multiple persistent episomal copies of the transfected DNA species and were analyzed for growth characteristics indicating malignant potential.

A novel bovine papillomavirus (BPV-6) causing true epithelial papillomas of the mammary gland skin: a member of a proposed new BPV subgroup

A papillomavirus has been isolated from frond epithelial papillomas of the bovine udder. It is clearly distinguishable from all other bovine papillomaviruses (BPVs) based on DNA sequence homology and antigenic properties and is thus characterised as a new entity, designated BPV-6. BPV-6 does not possess the interspecific papillomavirus antigen, its genomic DNA (7.2 kb) is smaller than, and does not show any sequence homology to BPV-1, BPV-2, or BPV-5, whereas it is approximately the same length as BPV-3 or BPV-4, with which it shares some sequence homology. The bovine papillomaviruses have been classified into two subgroups: subgroup A, composed of BPV-1, BPV-2, and BPV-5, all of which induce fibropapillomas, and subgroup B, composed of BPV-3, BPV-4, and BPV-6, all of which cause true epithelial papillomas.

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.

Evidence for integrated EBV genomes in Raji cellular DNA

Human lymphoid cell lines cannot be grown in long-term tissue culture, as a rule, unless the cells have been transformed by Epstein-Barr virus (EBV). The latent EBV DNA in established cell lines, is mainly present as free covalently closed circles but viral DNA sequences with properties of integrated DNA also seem to be present. We have extended the studies on the physical state of the EB viral DNA sequences in the cell line Raji which appear at a lower density than that for free EB viral DNA during fractionation on CsCl density gradients. In such material a novel EcoRI EBV DNA fragment is present, which hybridizes to viral sequences homologous to EcoRI A. This fragment is not present in free covalently closed circular EBV DNA. When this EcoRI fragment is further analysed with HindIII a smaller fragment than expected, which contains BamHI W sequences, is detected. The demonstration of this HindIII fragment and its characteristics as a joint, viral-host chromosome fragment will be discussed.

Presence of human papillomavirus DNA sequences in cervical intraepithelial neoplasia

Twenty two patients referred to a district colposcopy clinic because of an abnormal cervical cytology report or a suspicious cervix and found to have a cervical epithelial abnormality were studied. The techniques of cytology, histology, immunohistochemistry, and DNA-DNA hybridisation were used to detect infection by human papillomavirus. Using an indirect immunoalkaline phosphatase technique human papillomavirus antigen was found in biopsy specimens from six of the 22 patients and DNA of papillomavirus type 6 in biopsy specimens from 13 of these women, including four out of six whose histological diagnosis was cervical intraepithelial neoplasia grade 3. In eight cases where cytological, colposcopical, and histological investigations all indicated the presence of wart virus infection, papillomavirus type 6 DNA was found in seven. Papillomavirus type 6 DNA was found in more than half of the proved cases of cervical intraepithelial neoplasia. The presence of this viral DNA in women with no cervical abnormality is to be studied.

Transcriptional organization of bovine papillomavirus type 1

Multiple bovine papillomavirus type 1 (BPV-1)-specific polyadenylated RNA species in a BPV-1-infected bovine fibropapilloma were identified and mapped. All of the RNA species were transcribed from the same DNA strand of the BPV-1 genome. Five RNA species previously identified in BPV-1-transformed mouse cells were also present in the bovine fibropapilloma. These five species measured 1,050, 1,150, 1,700, 3,800, and 4,050 bases, mapped within the 69% transforming segment of the BPV-1 genome, and shared a 3' coterminus at 0.53 map units (m.u.). The 5' ends of the bodies of these distinct transcripts were located at ca. 0.03, 0.09, 0.34, 0.39, and 0.41 m.u. Additional polyadenylated RNA species not present in BPV-1-transformed mouse cells were specific for the BPV-1-infected bovine fibropapilloma and measured 1,700, 3,700, 3,800, 6,700, and 8,000 bases. These wart-specific species shared a 3' coterminus at 0.90 m.u. The 5' termini of the bodies of the 1,700- and 3,800-base species mapped at 0.71 and 0.42 m.u., respectively. Exonuclease VII analysis failed to reveal any internal splicing in these two species; however, the presence of small remote 5' leader sequences could not be ruled out. The 3,700-base species hybridized to DNA fragments from the 69% transforming segment as well as from the 31% nontransforming segment of the BPV-1 genome; however, this species was not precisely mapped. The 5' termini of the two largest RNA species (6,700 and 8,000 bases in size) were located at ca. 0.01 and 0.90 m.u., respectively. Since the 5' ends of these mapped adjacent to a TATAAA sequence which could possibly serve as an element of a transcriptional promoter, it is possible that one or both of these species represent nonspliced precursor RNA molecules.

Introduction of cloned human papillomavirus genomes into mouse cells and expression at the RNA level

The entire DNA genomes of five different human papillomaviruses (HPVs) were cloned into the BamHI site of pBR322 (HPV-1a, HPV-3, HPV-4, and HPV-9) or the EcoRI site of pBR325 (HPV-2), using as starting materials virus preparations isolated from papillomas of individual patients. Under stringent hybridization conditions (Tm-28 degrees), the five cloned HPVs exhibited less than 10% homology with one another. To establish model cell systems that may be useful for the identification of HPV genes and HPV gene products, mouse thymidine kinase negative (tk-) cells were cotransformed to the tk+ phenotype with the herpesvirus thymidine kinase gene and each of the five HPV cloned DNAs (either as intact recombinants or excised HPV DNA without removal of pBR). In most tk+ cell clones, a complex pattern of multiple high molecular weight inserts of HPV DNA were present in high copy number. Most of the HPV DNA sequences in the cotransformed cells were not present as unit-length episomal viral DNA. Analyses of the integration pattern (DNA blot) and RNA expression (RNA blot) of several HPV-1a and HPV-3 transformed cell lines suggest that some copies of the viral genome are integrated in a similar manner in different cell lines leading to the expression of identical viral RNA-containing species. Two of the cell lines transformed by the intact HPV-1a/pBR322 recombinant synthesized substantial amounts of four discrete viral polyadenylated cytoplasmic RNA species of 1.9, 3.2, 3.8, and 4.5 kb. Two cell lines transformed by the intact HPV-3/pBR322 recombinant synthesized 4-5 polyadenylated cytoplasmic viral RNA species ranging from 0.8 to 4.6 kb. The analysis shows that each viral RNA species appears to be a hybrid RNA molecule containing both HPV and pBR322 sequences. Based on these findings and the molecular organization of the HPV-1a genome (O. Danos, M. Katinka, and M. Yaniv (1982). EMBO J. 1, 231-237), it is possible that transcription of each of the HPV-1a RNA species is initiated using the HPV early promoter and terminated in pBR322.

Transcriptional control signals in the genome of bovine papillomavirus type 1

Papillomaviruses cause benign tumours in their natural hosts, which, in some cases, become foci for the appearance and spread of malignant carcinomas. It is therefore of considerable interest to analyse the strategy of gene expression of this group of viruses. Towards this end, we have used a generally applicable technique, in which we monitor the ability of DNA fragments to induce the expression of an inactive thymidine kinase (tk) gene, from which the promoter region has been deleted. Using this approach we now report for the first time the location of four transcriptional control sequences in the early region of the genome of bovine papillomavirus type 1 (BPV-1). Two of the sequences with the characteristics of 'promoter' elements are located proximal to the putative 5' leaders of the early genes. The third sequence is located within the region coding for the main body of the early transcripts, and the fourth within the region spanning the 3' end of the early open reading frames and the 5' end of the late ones. The fourth sequence displays the characteristics of an 'enhancer' element; however, unlike previously described 'enhancers', it is located three to five kilobases (kb) away from the first two promoters and to the 3' side of the third element.

Virus-induced gene mutations of eukaryotic cells

Most animal viruses studied so far induce chromosomal aberrations. In addition, adenoviruses, papovaviruses, and retroviruses are known to induce gene mutations like mutagenic bacteriophages. At least in one case studied retrovirus induced mutagenesis involves gene and/or scripton splitting analogous to the mutagenic mechanism of action of mutatorphage Mu and other movable DNA elements. On the contrary, several results obtained by independent means indicate that Simian virus 40, a papovavirus, does not act by splitting the affected gene but presumably by generation of base pair substitutions or of other minor DNA damages leading to amino acid substitutions. The mechanisms involved are still unknown. There a some hints, however, that these mechanisms might have some step(s) in common with processes leading to malignancy. In fact those viruses proved unequivocally so far to be capable of inducing gene mutations are oncogenic viruses.

Episomal DNA of a BK virus variant in a human insulinoma

BK virus (BKV) DNA was detected by blot hybridization in a human adenoma of pancreatic islets from patient I.R. BKV DNA was free, and no evidence was found of viral sequences integrated into cellular DNA. Virus was rescued by transfection of human embryonic fibroblasts with tumor DNA. The DNA from rescued virus (BKV-IR) was different from wild-type BKV DNA by restriction endonuclease mapping. The genome of BKV-IR is 235 base pairs (bp) shorter than the genome of wild-type BKV. This alteration originates from a deletion of approximately 300 bp involving HindIII fragments B and D, and an insertion of 70 bp in the region of HindIII fragment C. Transformation of hamster kidney cells was induced by total tumor DNA as well as by BKV-IR and BKV-IR DNA. No antibodies to BKV tumor (T) antigen were detected in the patient's serum by immunofluorescence. The significance of episomal BKV DNA in a human tumor is discussed.

Isolation of a human papillomavirus from a patient with epidermodysplasia verruciformis: presence of related viral DNA genomes in human urogenital tumors

The DNA genome of a human papillomavirus (HPV), tentatively designated HPV-EV, was molecularly cloned from hand to leg lesions of a patient with epidermodysplasia verruciformis, a chronic skin disease associated with a 30% risk of developing cancer. Using stringent hybridization conditions, we observed less than 5% homology between HPV-EV and the cloned genomes of HPV-1, HPV-4, HPV-5, and HPV-5a. HPV-EV DNA showed approximately 6% homology with HPV-2 and 36% homology with HPV-3. These data suggest that HPV-EV is partially related to HPV-3. Using 32P-labeled cloned HPV-EV as probe in Southern blot hybridization experiments, we detected HPV-EV-related DNA in the carcinoma in situ (Bowenoid lesion) of the vulva of the patient from which HPV-EV was isolated. HPV-EV-related DNA was detected in 2 of 10 vulva carcinomas and in 2 of 31 cervical carcinomas. Related DNA sequences were found in papillomas from each of two patients with condyloma acuminata (anogenital warts), which is of interest considering that condylomas have been reported to convert occasionally to carcinomas. The positive vulva DNAs were also probed with other cloned HPV DNAs: HPV-1, HPV-4, and HPV-5a-related sequences were not detected; HPV-3 and HPV-2 DNA probes detected strong and weak DNA bands, respectively, of the same size as found with HPV-EV. The HPV DNA sequences were present in the positive tumors mainly as free viral DNA molecules; no evidence for integration into cellular DNA was found. The emerging biological picture with papillomaviruses is that cells transformed by these viruses are maintained in a transformed state by free episomal genomes. Thus, our findings are consistent with the idea, but by no means establish, that HPVs play a role in human cancer by a similar mechanism.

Recombinant DNA molecules comprising bovine papilloma virus type 1 DNA linked to plasmid DNA are maintained in a plasmidial state both in rodent fibroblasts and in bacterial cells

Transformed cells obtained after transfecting FR3T3 rat fibroblasts with DNA of bovine papilloma virus type 1 ( BPV1 ) maintained only free copies of the viral genome. Transfection with BPV1 DNA inserted in a bacterial plasmid (pBR322 or pML2 ) did not produce transformants at a detectable rate, unless the viral sequences had been first excised from the plasmid. In contrast, transfer of the same plasmids by polyethylene glycol-induced fusion of bacterial protoplasts with FR3T3 rat or C127 mouse cells led to significant transformation frequencies. A total of eight cell lines were studied, three rat and five mouse transformants, obtained with various BPV1 - pML2 recombinants. In all cell lines, both BPV1 and plasmid sequences were maintained as non-integrated molecules, predominantly as oligomeric forms of the transforming DNA. In the three rat transformants and in two of the mouse lines, parts of the non-transforming viral region and some bacterial sequences were deleted. In the remaining three mouse lines, the monomeric repeat was a non-rearranged plasmid molecule which could be re-established as a plasmid in Escherichia coli after cleavage with "one-cut" restriction endonucleases and circularization of the molecule.

Human papillomavirus 1a complete DNA sequence: a novel type of genome organization among papovaviridae

The complete nucleotide sequence of human papillomavirus type 1a (7811 nucleotides) has been established. The overall organization of the viral genome is different from that of other related papovaviruses (SV40, BKV, polyoma). Firstly, genetic information seems to be coded by one strand. Secondly, no significant homology is found with SV40 or polyoma coding sequence for either DNA or deducted protein sequences. The relatedness of human and bovine papillomaviruses is revealed by a conserved coding sequence in the two species. Two regions can be defined on the viral genome: the putative early region contains two large open reading frames of 1446 and 966 nucleotides, together with several split ones, and corresponds to the transforming part of the bovine papillomavirus type 1 genome, and the remaining sequences, which include two open reading frames likely to encode structural polypeptide(s). The DNA sequence is analysed and putative signals for regulation of gene expression, and homologies with the Alu family of human ubiquitous repeats and the SV40 72-bp repeat are outlines.

Characterization of viral genomes in the liver and serum of chimpanzee long-term hepatitis B virus carriers: a possible role for supercoiled HBV-DNA in persistent HBV infection

In chimpanzee hepatitis B virus (HBV) carriers, the molecular mechanism for viral persistence has been examined by analyzing the properties of viral DNA molecules in liver and serum. Two extrachromosomal HBV-DNA molecules migrating on Southern blots at 4.0 kb and 2.3 kb were observed in chimpanzee liver DNA. There was no evidence for integration of HBV sequences into the host genome. The HBV-DNA molecule which migrated at 4.0 kb position represents a full-length "nicked," relaxed circular form, and the DNA molecules migrating at 2.3 kb position represents a supercoiled form of the HBV genome. Evidence for supercoiled HBV-DNA in serum was obtained by production of the relaxed circular intermediate upon digestion of Dane particle DNA with specific nucleases S1 and Bal 31. A possible role of these two extrachromosomal HBV-DNA molecules in the biology of hepatitis B virus infection and the mechanism for viral persistence are discussed.