Tumor viruses and early mouse embryos

A novel role of metal response element binding transcription factor 2 at the Hox gene cluster in the regulation of H3K27me3 by polycomb repressive complex 2

Polycomb repressive complex 2 (PRC2) is known to play an important role in the regulation of early embryonic development, differentiation, and cellular proliferation by introducing methyl groups onto lysine 27 of histone H3 (H3K27me3). PRC2 is tightly associated with silencing of Hox gene clusters and their sequential activation, leading to normal development and differentiation. To investigate epigenetic changes induced by PRC2 during differentiation, deposition of PRC2 components and levels of H3K27me3 were extensively examined using mouse F9 cells as a model system. Contrary to positive correlation between PRC2 deposition and H3K27me3 level, down-regulation of PRC2 components by shRNA and inhibition of EZH1/2 resulted in unexpected elevation of H3K27me3 level at the Hox gene cluster despite its global decrease. We found that metal response element binding transcriptional factor 2 (MTF2), one of sub-stoichiometric components of PRC2, was stably bound to Hox genes. Its binding capability was dependent on other core PRC2 components. A high level of H3K27me3 at Hox genes in Suz12-knock out cells was reversed by knockdown of Mtf2.This shows that MTF2 is necessary to consolidate PRC2-mediated histone methylation. Taken together, our results indicate that expression of Hox gene clusters during differentiation is strictly modulated by the activity of PRC2 secured by MTF2.

UTF1, a novel transcriptional coactivator expressed in pluripotent embryonic stem cells and extra-embryonic cells

We have obtained a novel transcriptional cofactor, termed undifferentiated embryonic cell transcription factor 1 (UTF1), from F9 embryonic carcinoma (EC) cells. This protein is expressed in EC and embryonic stem cells, as well as in germ line tissues, but could not be detected in any of the other adult mouse tissues tested. Furthermore, when EC cells are induced to differentiate, UTF1 expression is rapidly extinguished. In normal mouse embryos, UTF1 mRNA is present in the inner cell mass, the primitive ectoderm and the extra-embryonic tissues. During the primitive streak stage, the induction of mesodermal cells is accompanied by the down-regulation of UTF1 in the primitive ectoderm. However, its expression is maintained for up to 13.5 days post-coitum in the extra-embryonic tissue. Functionally, UTF1 boosts the level of transcription of the adenovirus E2A promoter. However, unlike the pluripotent cell-specific E1A-like activity, which requires the E2F sites of the E2A promoter for increased transcriptional activation, UTF1-mediated activation is dependent on the upstream ATF site of this promoter. This result indicates that UTF1 is not a major component of the E1A-like activity present in pluripotent embryonic cells. Further analyses revealed that UTF1 interacts not only with the activation domain of ATF-2, but also with the TFIID complex in vivo. Thus, UTF1 displays many of the hallmark characteristics expected for a tissue-specific transcriptional coactivator that works in early embryogenesis.

Cell-type specific activation of the polyomavirus F9-1 regulatory region in transgenic mice

To characterize the activity of the polyomavirus regulatory region, two hybrid marker genes were constructed. In the first construct, the early promoter regulates expression of the CAT gene and the late promoter regulates expression of the lacZ gene. In the second construct, the lacZ gene was placed under the control of the early promoter. The fusion constructs were introduced into the mouse germline. Gene expression was analysed in the generated transgenic mice. A pronounced cell-type specific activation of the transcriptional control region was found in different tissues of the developing embryo and in the adult animal. The control region is recognized and activated in early preimplantation embryos. Around the time of implantation, sequential activation of the Py regulatory region was first observed in differentiating cells. Stage- and tissue-specific expression were noted later in embryonic development. Comparing reporter gene expression on the single-cell level, the different viral promoters display identical expression patterns throughout ontogenesis. Quantitative analysis revealed that marker gene expression from the late promoter was significantly higher than from the early promoter. Furthermore, the cell-type specificity of the control region is not altered in the presence of its regulatory protein, the LT.

A DNA element that regulates expression of an endogenous retrovirus during F9 cell differentiation is E1A dependent

The retinoic acid-induced differentiation of F9 cells into parietal endoderm-like cells activates transcription of the endogenous mouse retrovirus, the intracisternal A-particle (IAP). To investigate the elements that control IAP gene differentiation-specific expression, we used methylation interference, Southwestern (DNA-protein), and transient-transfection assays and identified the IAP-proximal enhancer (IPE) element that directs differentiation-specific expression. We find that the IPE is inactive in undifferentiated F9 cells and active in differentiated parietal endoderm-like PYS-2 cells. Three proteins of 40, 60, and 68 kDa bind to the sequence GAGTAGAC located between nucleotides -53 and -47 within the IPE. The 40- and 68-kDa proteins from both the undifferentiated and differentiated cells exhibit similar DNA-binding activities. However, the 60-kDa protein from differentiated cells has greater binding activity than that from undifferentiated cells, suggesting a role for this protein in F9 differentiation-specific expression of the IAP gene. The IAP gene is negatively regulated by the adenovirus E1A proteins, and the E1A sequence responsible for repression is located at the N terminus, between amino acids 2 and 67. The DNA sequence that is the target of E1A repression also maps to the IPE element. Colocalization of the differentiation-specific and E1A-sensitive elements to the same protein-binding site within the IPE suggests that the E1A-like activity functions in F9 cells to repress IAP gene expression. Activation of the IAP gene may result when the E1A-like activity is lost or inactivated during F9 cell differentiation, followed by binding of the 60-kDa positive regulatory protein to the enhancer element.

A DNA element that regulates expression of an endogenous retrovirus during F9 cell differentiation is E1A dependent

The retinoic acid-induced differentiation of F9 cells into parietal endoderm-like cells activates transcription of the endogenous mouse retrovirus, the intracisternal A-particle (IAP). To investigate the elements that control IAP gene differentiation-specific expression, we used methylation interference, Southwestern (DNA-protein), and transient-transfection assays and identified the IAP-proximal enhancer (IPE) element that directs differentiation-specific expression. We find that the IPE is inactive in undifferentiated F9 cells and active in differentiated parietal endoderm-like PYS-2 cells. Three proteins of 40, 60, and 68 kDa bind to the sequence GAGTAGAC located between nucleotides -53 and -47 within the IPE. The 40- and 68-kDa proteins from both the undifferentiated and differentiated cells exhibit similar DNA-binding activities. However, the 60-kDa protein from differentiated cells has greater binding activity than that from undifferentiated cells, suggesting a role for this protein in F9 differentiation-specific expression of the IAP gene. The IAP gene is negatively regulated by the adenovirus E1A proteins, and the E1A sequence responsible for repression is located at the N terminus, between amino acids 2 and 67. The DNA sequence that is the target of E1A repression also maps to the IPE element. Colocalization of the differentiation-specific and E1A-sensitive elements to the same protein-binding site within the IPE suggests that the E1A-like activity functions in F9 cells to repress IAP gene expression. Activation of the IAP gene may result when the E1A-like activity is lost or inactivated during F9 cell differentiation, followed by binding of the 60-kDa positive regulatory protein to the enhancer element.

Identification of a novel murine IAP-promoted placenta-expressed gene

We have cloned and characterized a novel cellular gene that is promoted by an intracisternal A-particle (IAP) LTR and expressed in the mouse placenta (mouse IAP promoted placental gene, MIPP). A 1067bp cDNA clone containing an IAP LTR U5 region duplicated in its 5' terminus and an ORF coding for a potential 202 amino acids protein was isolated from an 8.5 day old mouse embryo cDNA library. Sequence analysis of the 5' region of a genomic clone revealed the presence of a solo IAP LTR with the same U5 duplication, and primer extension analysis confirmed that transcription of the MIPP gene is under the control of the IAP LTR. Expression of the MIPP gene parallels that of IAP genes in normal mouse tissues with abundant transcripts present in the placenta and also in the myeloma MOPC-315. The MIPP-encoded protein is composed of four 48-amino acid repeat units and shares homology with a vaccinia virus gene product. MIPP-related sequences were also detected in higher eukaryotic genomes including human.

Negative regulation of the major histocompatibility complex class I promoter in embryonal carcinoma cells

Transcription of major histocompatibility complex (MHC) class I genes is negatively regulated in undifferentiated F9 mouse embryonal carcinoma cells via the conserved upstream regulatory region. This region contains constitutive enhancers and an inducible enhancer, the interferon consensus sequence (ICS), that is responsible for interferon-induced transcription. A series of mutations in the ICS, but not in the enhancer elements, resulted in an increase in expression of the MHC class I promoter in F9 cells. However, these ICS mutants did not increase promoter activity in F9 cells differentiated after retinoic acid treatment. Results of mobility-shift DNA-binding assays and methylation interference experiments showed that undifferentiated F9 cells contained a factor(s) that bound to a sequence within the 5' and central part of the ICS. This binding site, termed the MHC negative regulatory element (NRE), coincided with the site of mutations that increased promoter activity in F9 cells and was distinct from the element to which interferon-response factors bind. The factor(s) that binds to the MHC NRE was not detected in differentiated F9 cells treated with retinoic acid or in other cells expressing MHC class I genes. Finally, introduction of concatenated, double-stranded NRE oligomers, but not oligomers of unrelated sequences, into F9 cells abolished negative regulation of the MHC class I promoter activity, providing evidence that the NRE binding factor is responsible for repression of the MHC class I genes in F9 cells.

Control of beta-interferon expression in murine embryonal carcinoma F9 cells

Murine embryonal carcinoma F9 cells, a tissue culture model for early embryonic development, do not produce interferon (IFN) in response to poly(I-C), as determined by an antiviral assay. RNase protection analyses were used to examine total RNA extracted from the cells for the presence of beta-IFN RNA. Whereas F9 cells differentiated in vitro with retinoic acid produced a biologically active protein as well as beta-IFN RNA in response to poly(I-C), undifferentiated F9 cells produced no detectable beta-IFN RNA even in the presence of cycloheximide, an IFN-superinducing agent. These results show that undifferentiated embryonal carcinoma cells do not accumulate beta-IFN RNA in response to an IFN-inducing agent, suggesting a transcriptional regulatory mechanism. However, this control mechanism is altered upon differentiation, since the gene can be transcriptionally activated in retinoic acid-differentiated cells.

Polyomavirus genome and polyomavirus enhancer-driven gene expression during myogenesis

The mRNAs for myogenic functions are coordinately transcribed with polyomavirus (Py) early mRNA during in vitro differentiation of mouse C2 myoblast cells. Sequence analysis shows that the A domain of the Py enhancer includes an E1A-like consensus sequence that is also found in the 5' upstream region of two genes expressed during myoblast differentiation: alpha-actin and myosin light chain. Therefore, the coordinate expression of such genes with Py early mRNA may be activated by a common cellular regulatory factor. In the present work, we report that C2 cells surviving Py infection are unable to differentiate and do not express alpha-actin and myosin light-chain mRNAs. Hybrids between such Py-resistant myoblast cells and the parental cells exhibited dominance of the permissibility to Py growth and of the expression of myogenic mRNAs. In C2 cells transiently transfected with a chimeric plasmid (pSVPy12CAT) harboring the bacterial chloramphenicol acetyltransferase (CAT) gene driven by the Py enhancer-promoter region, the CAT gene was expressed irrespective of their stage of differentiation. Moreover, undifferentiated stably transfected cells expressing the CAT gene restricted viral growth. Py-resistant C2 myoblasts transiently transfected with pSVPy12CAT also expressed the CAT gene driven by the Py enhancer. This contradictory finding is similar to results previously obtained by other investigators with cloned genes specific for myogenic functions, and it may be explained by a structural difference between the pSVPy12CAT and the Py genomic organizations in which the viral enhancer operates.

Isolation and characterization of a family of rat endogenous retroviral sequences

The complete nucleotide sequence of one representative rat genomic unit flanked on both sides with RAL elements, which have structural features specific to retroviral LTRs (1), was determined. The total unit was about 7.5 kbp long, and there was a partial homology to known retroviral sequences in gag, pol, and env regions. The sequence also contained minus- and plus-strand primer binding sites, thereby indicating a retroviral nature in replication. Transcription of the sequence was extensive in tumor cells and was strongly correlated with the state of methylation within 5' LTRs, which were highly methylated in the normal but not in the tumor state. In functional assays with bacterial chloramphenicol acetyltransferase constructs containing a series of deleted LTRs, there seemed to be both positive and negative cis-acting effector sequences.

Control of beta-interferon expression in murine embryonal carcinoma F9 cells

Murine embryonal carcinoma F9 cells, a tissue culture model for early embryonic development, do not produce interferon (IFN) in response to poly(I-C), as determined by an antiviral assay. RNase protection analyses were used to examine total RNA extracted from the cells for the presence of beta-IFN RNA. Whereas F9 cells differentiated in vitro with retinoic acid produced a biologically active protein as well as beta-IFN RNA in response to poly(I-C), undifferentiated F9 cells produced no detectable beta-IFN RNA even in the presence of cycloheximide, an IFN-superinducing agent. These results show that undifferentiated embryonal carcinoma cells do not accumulate beta-IFN RNA in response to an IFN-inducing agent, suggesting a transcriptional regulatory mechanism. However, this control mechanism is altered upon differentiation, since the gene can be transcriptionally activated in retinoic acid-differentiated cells.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.

Pleiotropic derepression of developmentally regulated cellular and viral genes by c-myc protooncogene products in undifferentiated embryonal carcinoma cells

We show here in mouse embryonal carcinoma (EC) cells that the endo A gene is negatively regulated and shares negative transacting factors with the Py and SV40 viruses. The products of the proto-oncogene c-myc derepress at the transcriptional level the appropriately initiated expression of the endo A gene and activate the Py early promoter in EC stem cells. C-myc products also activate the endo A and the Py early promoters in TDM epithelial cells, and the Py early promoter in 3T6 cells in which the two genes are already expressed or can be expressed. Furthermore we show that the myc exon 1 is essential for activation and that this activation might be mediated by AP1 family factors.

Functional analysis of the long terminal repeats of intracisternal A-particle genes: sequences within the U3 region determine both the efficiency and direction of promoter activity

The transcriptional activity of five intracisternal A-particle (IAP) long terminal repeats (LTRs) in mouse embryonal carcinoma PCC3-A/1 cells and in Ltk- cells was determined. We tested the promoter activity of the LTRs by coupling them to the reporter gene chloramphenicol acetyltransferase (CAT) or guanosine-xanthine phosphoribosyltransferase (gpt). Each LTR was tested for promoter function in both the sense (5' to 3') and antisense (3' to 5') orientation preceding the reporter gene. The transcriptional activity of individual IAP gene LTRs varied considerably, and the LTR from IAP81 possessed promoter activity in both directions. The bidirectional activity of the IAP81 LTR confirmed by monitoring Ecogpt expression in stably transfected Ltk- cells, with the initiation sites for sense and antisense transcription being localized to within the IAP81 LTR by S1 nuclease mapping. Deletions of LTR81 show that for normal 5'-to-3' gene transcription (sense direction), the 3'U3/R region determines the basal level of transcription, whereas sequences within the 5'U3 region enhance transcription four- to fivefold. Deletion mapping for antisense transcription indicates that a 64-base-pair region (nucleotides 47 to 110) within the U3 region is essential for activity. These data indicate that the U3 region contains all the regulatory elements for bidirectional transcription in IAP LTRs.

Functional analysis of the long terminal repeats of intracisternal A-particle genes: sequences within the U3 region determine both the efficiency and direction of promoter activity

The transcriptional activity of five intracisternal A-particle (IAP) long terminal repeats (LTRs) in mouse embryonal carcinoma PCC3-A/1 cells and in Ltk- cells was determined. We tested the promoter activity of the LTRs by coupling them to the reporter gene chloramphenicol acetyltransferase (CAT) or guanosine-xanthine phosphoribosyltransferase (gpt). Each LTR was tested for promoter function in both the sense (5' to 3') and antisense (3' to 5') orientation preceding the reporter gene. The transcriptional activity of individual IAP gene LTRs varied considerably, and the LTR from IAP81 possessed promoter activity in both directions. The bidirectional activity of the IAP81 LTR confirmed by monitoring Ecogpt expression in stably transfected Ltk- cells, with the initiation sites for sense and antisense transcription being localized to within the IAP81 LTR by S1 nuclease mapping. Deletions of LTR81 show that for normal 5'-to-3' gene transcription (sense direction), the 3'U3/R region determines the basal level of transcription, whereas sequences within the 5'U3 region enhance transcription four- to fivefold. Deletion mapping for antisense transcription indicates that a 64-base-pair region (nucleotides 47 to 110) within the U3 region is essential for activity. These data indicate that the U3 region contains all the regulatory elements for bidirectional transcription in IAP LTRs.

A labile inhibitor blocks endo A gene transcription in murine undifferentiated embryonal carcinoma cells

The endo A gene encoding for an intermediate filament protein, a cytokeratin is usually expressed in epithelial cells. The regulation of this gene, probed by using cycloheximide, an inhibitor of protein synthesis was studied in various cell lines. The lines explored were undifferentiated embryonal carcinoma PCC4 cells which normally do not express endo A gene, PCC4 cells cultivated permanently at 31 degrees C (PCC4-31), which are epithelial-like cells derived by differentiation from PCC4 cells, but which do express endo A gene, TDM1 cells, an epithelial teratocarcinoma cell line, and 3T6 mouse fibroblasts. Treatment of undifferentiated PCC4 cells by cycloheximide led to transcriptional induction of the endo A gene, and the same effect was observed after this treatment in PCC4-31 cells. By contrast, cycloheximide did not induce endo A gene expression in 3T6 cells, and reduced the transcriptional activity of this gene in TDM1 cells. We conclude that a labile inhibitor (or several) blocks endo A gene expression in undifferentiated PCC4 cells. We suggest that in these cells, the expression of the endo A gene is regulated both positively and negatively, possibly by a cellular E1A-like activity, as we previously demonstrated it for Py virus (C. Crémisi and C. Babinet, 1986 J. Virol. 59; 761-763). We further suggest that negative regulatory factors involved in this regulation are absent in TDM cells and reduced in PCC4-31 cells.

Sequence-dependent DNA replication in preimplantation mouse embryos

Circular, double-stranded DNA molecules were injected into nuclei of mouse oocytes and one- or two-cell embryos to determine whether specific sequences were required to replicate DNA during mouse development. Although all of the injected DNAs were stable, replication of plasmid pML-1 DNA was not detected unless it contained either polyomavirus (PyV) or simian virus 40 (SV40) DNA sequences. Replication occurred in embryos, but not in oocytes. PyV DNA, either alone or recombined with pML-1, underwent multiple rounds of replication to produce superhelical and relaxed circular monomers after injection into one- or two-cell embryos. SV40 DNA also replicated, but only 3% as well as PyV DNA. Coinjection of PyV DNA with either pML-1 or SV40 had no effect on the replicating properties of the three DNAs. These results are consistent with a requirement for specific cis-acting sequences to replicate DNA in mammalian embryos, in contrast to sequence-independent replication of DNA injected into Xenopus eggs. Furthermore, PyV DNA replication in mouse embryos required PyV large T-antigen and either the alpha-beta-core or beta-core configuration of the PyV origin of replication. Although the alpha-core configuration replicated in differentiated mouse cells, it failed to replicate in mouse embryos, demonstrating cell-specific activation of an origin of replication. Replication or expression of PyV DNA interfered with normal embryonic development. These results reveal that mouse embryos are permissive for PyV DNA replication, in contrast to the absence of PyV DNA replication and gene expression in mouse embryonal carcinoma cells.

Chromosome distribution of intracisternal A-particle sequences in the Syrian hamster and mouse

Metaphase chromosomes of Syrian hamster and BALB/c mice were hybridized in situ with radiolabeled probes derived from cloned intracisternal A-particle (IAP) genes of the corresponding species. The DNAs of these species are known to contain about 900 and 1,000 copies, respectively, of the retrovirus-like IAP sequence elements per haploid genome. Multiple IAP sequences were found on all chromosomes of both hamster and mouse. In the hamster, more than half of the IAP sequences were located in regions of non-centromeric constitutive heterochromatin, at an average concentration per unit chromosome length 5 times greater than in the euchromatic regions. The other dispersed sequences showed marked local variations in concentration along the chromosome lengths; both discrete foci and large grain clusters were observed as well as regions apparently lacking IAP sequences. Within the resolution of the techniques, IAP sequences appeared to be more evenly distributed over the mouse chromosomes; however, some prominent variations in concentration were seen. The number of potentially active IAP genes in the Syrian hamster, and by extension in the mouse, may be restricted by the preferential location of IAP sequences in genetically inert regions of the genome.

Sequence-dependent DNA replication in preimplantation mouse embryos

Circular, double-stranded DNA molecules were injected into nuclei of mouse oocytes and one- or two-cell embryos to determine whether specific sequences were required to replicate DNA during mouse development. Although all of the injected DNAs were stable, replication of plasmid pML-1 DNA was not detected unless it contained either polyomavirus (PyV) or simian virus 40 (SV40) DNA sequences. Replication occurred in embryos, but not in oocytes. PyV DNA, either alone or recombined with pML-1, underwent multiple rounds of replication to produce superhelical and relaxed circular monomers after injection into one- or two-cell embryos. SV40 DNA also replicated, but only 3% as well as PyV DNA. Coinjection of PyV DNA with either pML-1 or SV40 had no effect on the replicating properties of the three DNAs. These results are consistent with a requirement for specific cis-acting sequences to replicate DNA in mammalian embryos, in contrast to sequence-independent replication of DNA injected into Xenopus eggs. Furthermore, PyV DNA replication in mouse embryos required PyV large T-antigen and either the alpha-beta-core or beta-core configuration of the PyV origin of replication. Although the alpha-core configuration replicated in differentiated mouse cells, it failed to replicate in mouse embryos, demonstrating cell-specific activation of an origin of replication. Replication or expression of PyV DNA interfered with normal embryonic development. These results reveal that mouse embryos are permissive for PyV DNA replication, in contrast to the absence of PyV DNA replication and gene expression in mouse embryonal carcinoma cells.

Intracisternal A-particle gene expression in normal mouse thymus tissue: gene products and strain-related variability

Intracisternal A-particle (IAP)-specific sequences were 5- to 10-fold enriched in polyadenylated RNA from BALB/cJ thymus as compared with RNAs from liver, spleen, and kidney. The major transcripts of 7.2 and 5.4 kilobases were the same size as those found in an IAP-rich neuroblastoma cell line. The absolute levels and proportions of these transcripts varied in thymuses from mice of different inbred strains. With antiserum prepared against p73, the main IAP structural protein, several size classes of IAP-related proteins were immunoprecipitated from extracts of thymus cells incubated with [35S]methionine; these included p73 itself and a group of polypeptides in the size range of 114 to 120 kilodaltons (p114-p120). The inbred strains showed marked characteristic differences in the electrophoretic patterns of their IAP-related proteins. Earlier studies showed that the 7.2-kilobase RNA from neuroblastoma IAPs coded for p73 in a cell-free translation system. Correlations between the RNA and protein patterns in thymuses of the different inbred strains indicated that 5.4-kilobase RNA gives rise to the p114-p120 polypeptides. Metabolically labeled p120 was found to include methionine-containing tryptic peptides of p73 plus additional peptides consistent with its larger size. In vivo labeling kinetics showed that the p114-p120 polypeptides were not major precursors of p73 in intact neuroblastoma cells. This study shows that IAP gene expression in mouse thymus is genetically determined and that a novel class of IAP-related polypeptides can be expressed independently of the major particle structural protein.

Intracisternal A-particle gene expression in normal mouse thymus tissue: gene products and strain-related variability

Intracisternal A-particle (IAP)-specific sequences were 5- to 10-fold enriched in polyadenylated RNA from BALB/cJ thymus as compared with RNAs from liver, spleen, and kidney. The major transcripts of 7.2 and 5.4 kilobases were the same size as those found in an IAP-rich neuroblastoma cell line. The absolute levels and proportions of these transcripts varied in thymuses from mice of different inbred strains. With antiserum prepared against p73, the main IAP structural protein, several size classes of IAP-related proteins were immunoprecipitated from extracts of thymus cells incubated with [35S]methionine; these included p73 itself and a group of polypeptides in the size range of 114 to 120 kilodaltons (p114-p120). The inbred strains showed marked characteristic differences in the electrophoretic patterns of their IAP-related proteins. Earlier studies showed that the 7.2-kilobase RNA from neuroblastoma IAPs coded for p73 in a cell-free translation system. Correlations between the RNA and protein patterns in thymuses of the different inbred strains indicated that 5.4-kilobase RNA gives rise to the p114-p120 polypeptides. Metabolically labeled p120 was found to include methionine-containing tryptic peptides of p73 plus additional peptides consistent with its larger size. In vivo labeling kinetics showed that the p114-p120 polypeptides were not major precursors of p73 in intact neuroblastoma cells. This study shows that IAP gene expression in mouse thymus is genetically determined and that a novel class of IAP-related polypeptides can be expressed independently of the major particle structural protein.

Isolation of polyomavirus mutants multiadapted to murine embryonal carcinoma cells

Polyomavirus mutants were isolated from PCC4 embryonal carcinoma cells infected with a variant strain of polyomavirus (ev 1001h) and were found to contain a tandem duplication overlapping the enhancers and the origin of replication. These mutants were able to infect several lines of embryonal carcinoma cells, including PCC4, F9, and LT1. The sequence and structure of one of these mutants are presented and compared with those of other PyEC PCC4 mutants previously described.

Nucleotide sequences of murine intracisternal A-particle gene LTRs have extensive variability within the R region

Nucleotide sequences of the long terminal repeats (LTRs) of four murine intracisternal A-particle (IAP) genes IAP62, 19, 81 and 14 were determined. Each IAP LTR contains three sequence domains, 5'-U3-R-U5-3', and each is bound by 4 bp imperfect inverted repeats. The transcriptional regulatory sequences, CAAT and TATA, as well as the enhancer core sequence GTGGTAA are conserved and precisely positioned within the U3 region. In the R region, the sequence AATAAA is located twenty base pairs preceding the dinucleotide CA, the polyadenylation site. In IAP19 and IAP81, the 5' and 3' LTRs are flanked by a six nucleotide direct repeat of cellular sequences representing the possible integration sites for these IAP proviruses. Both the size and sequences of different IAP LTRs vary considerably, with the majority of the variation localized within the R regions. The size of R varies from 66 bp in IAP14 to 222 bp in IAP62; in contrast, the U3 and U5 regions are all similar in size. These extra sequences within the R region of large LTRs consist of several unusual directly repeating sequences which account for this variability.

Transcription of intracisternal A-particle genes in mouse myeloma and Ltk- cells

In vivo- and in vitro-synthesized RNA from the murine plasmacytoma MOPC-315 was found to hybridize to all regions of a 7.2-kilobase-pair intracisternal A-particle (IAP) gene. IAP-specific transcripts were also detected in mouse Ltk- cells, but not in cells derived from normal tissues (kidney, liver, spleen) of 6-week-old BALB/c mice. Three RNA species of 7.2, 5.3, and 3.8 kilobases were identified by Northern blot analysis of MOPC-315 polyadenylated RNA. The 7.2- and 5.3-kilobase transcripts were found in greater levels in nuclear as compared with whole cell RNA, suggesting the involvement of one or more of the following mechanisms: RNA processing, preferential nuclear transport, or differential RNA stability. We show that the primary IAP transcript is initiated within the long terminal repeat by hybridization analysis with restriction digests of cloned IAP DNA and [gamma-S]pppApNp ... RNA synthesized in nuclei with [gamma-S]ATP as the RNA initiating probe. Low concentrations of alpha-amanitin (2 micrograms/ml) inhibited IAP RNA synthesis by greater than 90%, suggesting that RNA polymerase II is responsible for IAP transcription.

Polyomavirus growth and persistence in Friend erythroleukemic cells

Infection of Friend erythroleukemic (FL) cells by polyomavirus (Py) invariably results in the selection of persistently infected FL-Py cell lines and clones. Anti-Py serum treatment of FL-Py lines and clones leads to the loss of Py genome and consequent cell cure. Conversely, cure has not been obtained in FL-PytsA cell lines (isolated after infection by a Py thermosensitive early mutant) and their derivative clones cultivated for a long time at nonpermissive temperature (39 degrees C), where viral large-T protein is inactive. Rescue of viral particles has always been obtained after shifting cells to 32 degrees C. Integrated viral genomes were detected by blot hybridization in an FL-PytsA clone at 39 degrees C. Long-term observation of FL-Py cell lines and their derivative clones reveals a reciprocal selection mechanism (coevolution) between the viral and the cellular populations, resulting in either a completely virus-free Py-resistant FL cell line (cure) or in a continuously Py-shedding line bearing Py genome variants. Structural analysis of these viral populations has been carried out, and some viral variants have been isolated and characterized. On the basis of the results obtained, the possible mechanisms of Py persistence in FL cells will be discussed.

Amounts, synthesis, and some properties of intracisternal A particle-related RNA in early mouse embryos

Early mouse embryos express two morphological subtypes of intracisternal A-type particles, one resembling those occurring in mouse tumors (referred to as IAP) and the other apparently specific for early embryos [referred to as IAP(epsilon)]. Using cloned fragments of IAP genes as labeled probes in dot-hybridization experiments, we detected IAP-related RNA sequences in mouse oocytes and preimplantation embryos. IAP RNA is relatively abundant in ovarian oocytes, is reduced in amount to approximately equal to 1/10th in the ovulated egg, and increases approximately equal to 100 times (from approximately equal to 1.3 X 10(3) to approximately equal to 1.5 X 10(5) molecules per embryo) between the one-cell stage and late blastocyst stage. Most of the IAP RNA consists of a single size class of about 5.4 kilobases, and a major fraction of this RNA is polyadenylylated. Quantitative considerations suggest that only a few percent of the IAP RNA in embryos are associated with particles. In two-cell embryos, the number of IAP RNA molecules is less than 1/10th the number of IAP(epsilon) particles, suggesting that IAP(epsilon) is genetically distinct from IAP and presumably represents a family of as yet unidentified retrovirus-like elements.