Molecular cloning of unintegrated and a portion of integrated moloney murine leukemia viral DNA in bacteriophage lambda

Replication of Mus dunni endogenous retrovirus depends on promoter activation followed by enhancer multimerization

Mus dunni endogenous virus (MDEV) is an apparently intact retrovirus that normally lies transcriptionally silent in cultured M. dunni cells, but the provirus can be activated by treatment of the cells with hydrocortisone or 5-iodo-2'-deoxyuridine. Sequence analysis of a molecular clone of the replicating virus revealed a simple retrovirus with a chimeric VL30/GALV-like structure. Interestingly, in the region of the long terminal repeat (LTR) that typically contains the retroviral transcription enhancers, we found over six 80-bp repeats with only a single mismatch, indicating that acquisition of the repeats was a recent event. Here we provide evidence for the following model of MDEV activation and replication. The MDEV provirus in M. dunni cells has a chimeric structure similar to that of the molecular clone but has only 1.15 copies of the 80-bp repeat sequence found in the molecular clone. Activating chemicals directly stimulate transcription from the LTR, allowing a low level of virus replication. Copying errors made during reverse transcription allow multimerization of the 80-bp enhancer region, resulting in viruses with higher transcriptional rates and improved fitness, but increased enhancer copy number is likely balanced by the natural instability of retroviral repeats and constraints imposed by virion packaging limits. The resultant population of replicating MDEV is widely heterogeneous, having from 2.15 to 13.15 enhancer repeats in the LTR. These results reveal a novel mechanism for regulation of transcription and replication of an endogenous retrovirus, in terms of both activation of the virus by the steroid hydrocortisone and the large number and variation in enhancer repeats observed.

A new method of gene transfer into hematopoietic progenitors using liquid culture with interleukin-3 and interleukin-6

The technique of gene transfer into hematopoietic cells is expected to offer a new form of therapeutics. As a result of studies on a gene-delivery system using granulocyte-macrophage colony-forming units (CFU-GM), a type of hematopoietic progenitor, we have established a technique for efficient gene transfer into CFU-GM. DGL, a retrovirus vector containing the SV40 promoter and the neomycin resistance gene, was constructed and found to transfer genes effectively into murine CFU-GM, which subsequently expressed the neomycin resistance gene. After gene transfer of murine non-adherent bone marrow cells precultured in liquid culture with recombinant murine IL-3 (rmIL-3) and recombinant human IL-6 (rhIL-6) for 6 days, gene transferred CFU-GM in bone marrow cells were able to proliferate 5-10-fold and the ratio of gene transferred CFU-GM to total CFU-GM reached 70-100% from less than 1% in the liquid culture with rmIL-3, rhIL-6 and neomycin for 6 days. Using this protocol, we have been able to obtain large amounts of highly concentrated gene-transferred CFU-GM for fundamental research on CFU-GM gene-delivery systems.

Construction and characterization of the recombinant Moloney murine leukemia viruses bearing the mouse Fv-4 env gene

A nucleotide sequence of the mouse Fv-4 env gene was completed. Structural comparison revealed a close relationship of Fv-4 to the ecotropic Cas-Br-E murine leukemia virus isolated from a wild mouse in southern California. Various portions of the env gene of Moloney murine leukemia virus were replaced by the corresponding Fv-4 env sequence to construct recombinant murine leukemia virus clones. Infectivity of these recombinants was checked by the S+L- cell focus induction assay and the XC cell syncytium formation assay. Recombinants bearing the following Fv-4 env sequence retained ecotropic infectivity; the AccI-BamHI and BamHI-BalI regions coding for the N- and C-terminal halves of Fv-4 gp70SU, respectively; and the BalI-NcoI region encoding the cleavage site between gp70SU and p15(E)TM of the Fv-4 env. However, when the Fv-4 sequence was substituted for the p15(E)TM-coding NcoI-EcoRV region or the AccI-EcoRV region covering almost the entire env gene, infectivity was undetectable in our assays. The recombinant clone containing the Fv-4 AccI-EcoRV region, i.e., almost the entire Fv-4 env sequence, was introduced with pSV2neo into NIH 3T3 cells, and a G418r cell line named NIH(Fv4)-2 was isolated. The NIH(Fv4)-2 cell released viral particles that contained reverse transcriptase, Fv-4 env molecules as well as the other viral proteins, and viral genomic RNA. However, proviral DNA synthesis was not detected upon inoculation of this virus in NIH 3T3 cells. The loss of infectivity of the recombinant virus bearing the Fv-4 AccI-EcoRV region appeared to be caused by failure in an early step of replication.

Very high frequency of lymphoma induction by a chemical carcinogen in pim-1 transgenic mice

Infection of mice with Moloney murine leukaemia virus (MuLV) induces T-cell lymphomas after an average latency period of 150 days. In these lymphomas the MuLV DNA is frequently integrated into the mouse chromosomal DNA in the vicinity of the pim-1 oncogene. Transgenic mice overexpressing the pim-1 oncogene are predisposed to develop T-cell lymphomas, but only to the extent that approximately 10% of the mice develop a lymphoma within 240 days. When these mice are infected with MuLV, lymphomas develop in all mice in only 50-60 days. In these lymphomas MuLV DNA is integrated near either the c-myc or N-myc gene, suggesting that pim-1 and myc synergize in lymphomagenesis. To determine whether this system has a more general application, we have now tested the susceptibility of pim-1 transgenic mice to N-ethyl-N-nitrosourea (ENU), a chemical carcinogen. With a single low dose of ENU, nearly all pim-1 transgenic mice, but only 15% of non-transgenic mice, develop T-cell lymphomas within 200 days. All ENU-induced lymphomas in both pim-1 transgenic and non-transgenic mice express high levels of c-myc messenger RNA, supporting the notion that pim-1 and c-myc synergize in lymphoma induction. We propose that pim-1 transgenic mice could be used to test the oncogenic potential of other chemical compounds.

Chromatin structure of recombinant Moloney murine leukemia virus proviral DNAs that contain tax-responsive sequences from human T-cell lymphotropic virus type II in the presence and absence of tax

Human T-cell lymphotropic virus types I and II (HTLV-I and HTLV-II) are replication-competent retroviruses which contain two additional regulatory proteins, tax and rex. tax is a transcriptional transactivator of the HTLV-I or HTLV-II long terminal repeat (LTR) and also of some heterologous promoters. To investigate the mechanism of tax transactivation, we used chimeric Moloney murine leukemia viruses (M-MuLVs) with LTRs containing tax-responsive sequences from the HTLV-II LTR (nucleotides -273 to -32). Mo+HTLV-II+ M-MuLV contained the HTLV II sequences inserted into the wild-type M-MuLV LTR at nucleotide -150, whereas delta Mo+HTLV-II+ M-MuLV contained the same sequences inserted into an M-MuLV LTR lacking its own enhancer region. HTLV-II tax (tax II)-positive mouse cells (15S-5a) infected with Mo+HTLV-II+ M-MuLV or delta Mo+HTLV-II+ M-MuLV showed higher rates of viral transcription in nuclear run-on assays than did infected tax-negative NIH 3T3 cells. The chromatin structure of these viruses was investigated by high-resolution mapping of DNase I-hypersensitive (HS) sites. Three prominent HS sites were associated with HTLV-II sequences in proviral chromatin both in tax-positive and in tax-negative cells. The spacing resembled that of the 21-base-pair (bp) repeats, but the HS sites were displaced approximately 50 bp upstream of the 21-bp repeats. This suggested that cellular proteins bound to the HTLV-II sequences in the presence or absence of tax. No direct effect of tax on chromatin structure was found. These in vivo results were consistent with results of in vitro DNase footprinting studies performed by other investigators.

Harvey murine sarcoma virus: influences of coding and noncoding sequences on cell transformation in vitro and oncogenicity in vivo

The rat-derived Harvey murine sarcoma virus (Ha-MuSV) contains a transduced ras oncogene activated by two missense mutations and flanked by rat retroviruslike VL30 sequences. Ha-MuSV induces focal transformation of mouse NIH 3T3 cells in vitro and tumors (fibrosarcomas and splenic erythroleukemias) in newborn mice. We have used these two assays to study the contribution of coding and noncoding viral sequences to the biological activity of Ha-MuSV. A good correlation was found between the in vitro and in vivo assays. In several different isogenic Ha-MuSV variants, those with a rasH gene that had one or both of the Ha-MuSV missense mutations were much more active biologically than the corresponding proto-oncogene. A Ha-MuSV variant that encoded the proto-oncogene protein induced lymphoid leukemias (with thymomas), with a relatively long latent period, rather than the fibrosarcomas and erythroleukemias characteristic of Ha-MuSV with one or both missense mutations. A VL30-derived segment with enhancer activity was identified downstream from v-rasH. A mutant Ha-MuSV from which this 3' noncoding segment was deleted expressed lower levels of the wild-type viral protein, displayed impaired transforming activity in vitro, and induced lymphoid leukemias (with thymomas). 5' noncoding rat c-rasH sequences were found to increase the biological activity of the virus when substituted for the corresponding segment of v-rasH. We conclude that (i) the biological activity of Ha-MuSV can be influence significantly by noncoding sequences located outside the long terminal repeat as well as by coding sequences, (ii) VL30 sequences positively regulate the expression of v-rasH, (iii) relatively low biological levels of ras, whether resulting from low-level expression of wild type v-rasH or high-levels of ras proto-oncogene protein, induce a type of tumor that differs from tumors induced by high biological levels of ras, and (iv) the in vivo pathogenicity of the Ha-MuSV variants correlated with their transforming activity on NIH 3T3 cells.

Northern blot mapping: a procedure for mapping mRNA immobilized on nitrocellulose by probing with end-labeled DNA fragments

A simple method for mapping RNA on a Northern blot with a mixture of end-labeled DNA fragments is described. The DNA fragments are labeled either in 5' or in 3' directly after digestion by restriction enzyme(s) and used without any further purification step as probe to hybridize a Northern blot. After autoradiography, the DNA fragments hybridized to each mRNA species are recovered by heating the nitrocellulose and analyzed on denaturing polyacrylamide or agarose gels. This method indicates which DNA fragment hybridizes with which mRNA species and requires far fewer different manipulations than successive hybridization of a Northern blot with several nick-translated purified DNA fragments.

Slipped DNA structures within the enhancer region of the Moloney murine leukemia virus

We have examined the S1 nuclease sensitivity of supercoiled plasmids harboring the Moloney Murine Leukemia Virus (MoMuLV) long terminal repeat (LTR). S1 sensitivity was found within the LTR enhancer direct repeats. Transformation of E. coli DH5 cells with a construct containing most of the MoMuLV LTR yielded the precise deletion of one direct repeat and loss of S1 sensitivity. The dependence of S1 sensitivity on the presence of both direct repeats, together with the exact excision of one direct repeat by E. coli, suggests the presence of slipped DNA within the enhancer. Such structures may represent targets for effector proteins which mediate vital functions during viral propagation.

Chromatin structure of hormone-responsive Moloney murine leukemia virus proviruses that contain sequences from mouse mammary tumor virus

The chromatin structure of chimeric Moloney murine leukemia viruses (M-MuLVs) containing a glucocorticoid response element (GRE) from mouse mammary tumor virus (MMTV) inserted into the long terminal repeat (LTR) was investigated. Nuclear run-on assays indicated that transcription from the chimeric proviruses was induced 2- to 4-fold by dexamethasone. The wild-type M-MuLV 5' LTR contained a DNase I hypersensitive (HS) site at the TATA sequences, as well as four sites in the enhancer region. The chimeric LTRs contained these sites, as well as three additional sites in the MMTV sequences. Two of the MMTV sites were present in the absence of hormone, while one was hormone-induced. In addition, internal MMTV sequences appeared protected from DNase I digestion in the absence of hormone, suggesting bound protein. Hormone treatment resulted in loss of the DNase I protection.

Splice acceptor site for the env message of Moloney murine leukemia virus

We report the isolation and sequence of a cDNA clone containing part of the env message of the Moloney murine leukemia virus (MoMuLV). This clone was derived from a rat thymic lymphoma induced by MoMuLV. The AG acceptor site employed in this message is located at position 5490 in the MoMuLV genome. This splice site is detectable at the cDNA level by the creation of a novel SacI restriction site not present in the viral genome. In the -1 to -40 region, this AG acceptor site is preceded by four conserved heptanucleotides (PyXPyTPuAPy) that may function as acceptors for removal of the 5' end of the intron.

Rearrangements and insertions in the Moloney murine leukemia virus long terminal repeat alter biological properties in vivo and in vitro

The effects of rearrangement and insertion of sequences in the Moloney murine leukemia virus (M-MuLV) long terminal repeat (LTR) were investigated. The alterations were made by recombinant DNA manipulations on a plasmid subclone containing an M-MuLV LTR. Promoter activity of altered LTRs was measured by fusion to the bacterial chloramphenicol acetyltransferase gene, followed by transient expression assay in NIH 3T3 cells. M-MuLV proviral organizations containing the altered LTRs were also generated, and infectious virus was recovered by transfection. Infectivity of the resulting virus was quantified by XC plaque assay, and pathogenicity was determined by inoculating neonatal NIH Swiss mice. Inversion of sequences in the U3 region containing the tandemly repeated enhancer sequences (-150 to -353 base pairs [bp]) reduced promoter activity approximately fivefold in the transient-expression assays. Infectious virus containing the inverted sequences (Mo- M-MuLV) showed a 20-fold reduction in relative infectivity compared with wild-type M-MuLV, but the virus still induced thymus-derived lymphoblastic lymphoma or leukemia in mice, with essentially the same kinetics as for wild-type M-MuLV. We previously derived an M-MuLV which carried inserted enhancer sequences from the F101 strain of polyomavirus (Mo + PyF101 M-MuLV) and showed that this virus is nonleukemogenic. In Mo + PyF101 M-MuLV, the PyF101 sequences were inserted between the M-MuLV promoter and the M-MuLV enhancers (at -150 bp). A new LTR was generated in which the PyF101 sequences were inserted to the 5' side of the M-MuLV enhancers (at -353 bp, PyF101 + Mo M-MuLV). The PyF101 + Mo LTR exhibited promoter activity similar (40 to 50%) to that of wild-type M-MuLV, and infectious PyF101 + Mo M-MuLV had high infectivity on NIH 3T3 cells (50% of wild type). In contrast to the nonleukemogenic Mo + PyF101 M-MuLV, PyF101 + Mo M-MuLV induced leukemia with kinetics similar to that of wild-type M-MuLV. Thus, the position of the PyF101 sequences relative to the M-MuLV LTR affected the biological behavior of the molecular construct. Furthermore, PyF101 + Mo M-MuLV induced a different spectrum of neoplastic disease. In comparison with wild-type M-MuLV, which induces a characteristic thymus-derived lymphoblastic lymphoma with extremely high frequency, PyF101 + Mo M-MuLV was capable of inducing both acute myeloid leukemia or thymus-derived lymphoblastic lymphoma, or both. Tumor DNA from both the PyF101 + Mo- and Mo- M-MuLV-inoculated animals contained recombinant proviruses with LTRs that differed from the initially inoculated virus.

Generation and characterization of a recombinant Moloney murine leukemia virus containing the v-myc oncogene of avian MC29 virus: in vitro transformation and in vivo pathogenesis

A new retrovirus consisting of the v-myc oncogene sequences of avian MC29 virus inserted into the genome of Moloney murine leukemia virus (M-MuLV) was generated. This was accomplished by constructing a recombinant DNA clone containing the desired organization, introducing the recombinant DNA into mouse NIH 3T3 cells, and superinfecting the cells with replication-competent M-MuLV. The construction was designed so that an M-MuLV gag-myc fusion protein would be produced. The resulting virus, M-MuLV(myc), morphologically transformed uninfected NIH 3T3 cells. Stocks of M-MuLV(myc)-M-MuLV were infected into secondary mouse embryo cultures. M-MuLV(myc) induced striking growth and proliferation of hematopoietic cells. These cells were of the myeloid lineage by morphology, phagocytic properties, and surface staining with Mac-1 and Mac-2 monoclonal antibodies. They resembled mature macrophages, although they displayed minor properties of immaturity. The myeloid cells were transformed in comparison with uninfected myeloid cells since they were less adherent and had unlimited proliferative capacity and reduced growth factor requirements. The transformed myeloid cells with proliferative potential were actually myeloid progenitors which apparently underwent terminal differentiation to macrophages. It was possible to derive a permanent line of factor-independent macrophages from M-MuLV(myc)-transformed myeloid cells. M-MuLV(myc) also immortalized and morphologically transformed mouse embryo fibroblasts. These in vitro properties closely resembled the biological activity of MC29 virus in avian cells and suggested that the nature of the v-myc oncogene was an important determinant in transformation specificity. Neonatal NIH Swiss mice inoculated intraperitoneally with M-MuLV(myc)-M-MuLV only developed lymphoblastic lymphoma characteristic of the M-MuLV helper alone, and no acute fibrosarcomas or myeloid tumors resulted. In light of the strong myeloid transformation observed in vitro, the absence of acute in vivo myeloid disease was noteworthy. Interestingly, when a derivative of M-MuLV(myc) carried by a nonpathogenic amphotropic MuLV helper was inoculated, T lymphomas developed with long latency. Molecular hybridization confirmed that these tumors contained M-MuLV(myc).

A Thy-1- mutant defining a gene acting in trans position to regulate cell-surface Thy-1 glycoprotein expression and Thy-1 messenger RNA content

The Thy-1 glycoprotein is a differentiation antigen which exhibits tissue-specific regulation. A mutant of a Thy-1.1+ T-cell lymphoma has been isolated which does not express Thy-1 glycoprotein on the cell surface and does not accumulate Thy-1 mRNA in the cytoplasm. Hybrids between the mutant and a Thy-1.2+ T-cell lymphoma express 20-30-fold lower levels of Thy-1 glycoprotein on their cell surface compared to wild-type T-cell lymphomas, and they have correspondingly low levels of cytoplasmic Thy-1 mRNA. A revertant of one hybrid was isolated which expressed wild-type levels of both Thy-1 alleles on its surface and contained correspondingly increased levels of Thy-1 mRNA. A Thy-1+ revertant of the Thy-1- mutant was isolated by cell sorting. A second generation Thy-1- mutant could be isolated from this revertant which also did not accumulate Thy-1 mRNA and which behaved in a way similar to the first generation mutant when hybridized to a Thy-1.2+ lymphoma. No changes in the structure or copy number of the Thy-1 structural gene could be detected in this series of mutants and revertants. These properties are consistent with a mutation in one (or more) gene(s) which acts in trans position to regulate Thy-1 glycoprotein expression.

Tissue tropism of a leukemogenic murine retrovirus is determined by sequences outside of the long terminal repeats

Although it has been previously determined that the long terminal repeat (LTR) sequences of several murine retroviruses specify the major tissue tropism of leukemias they induce, data reported here show that the LTR is not responsible for tissue tropism in the case of all leukemogenic viruses. In an effort to determine whether LTR sequences of the acute erythroleukemia-inducing spleen focus-forming virus (SFFV), like those of the other murine leukemia viruses, are uniquely required to confer tissue specificity to the virus, we prepared recombinant SFFVs in which the LTR region containing promoter and enhancer functions was replaced with analogous LTR regions from Friend and Moloney ecotropic and mink cell focus-inducing viruses. It was found that all of the SFFV constructs, even those with a LTR derived from lymphoma-inducing viruses such as Moloney murine leukemia virus, transformed erythroid cells in vitro and induced exclusively an erythroid disease. These results demonstrate that sequences in SFFV that determine the tissue-specific nature of the disease reside outside the LTR.

Distribution of type D retrovirus sequences in tissues of macaques with simian acquired immune deficiency and retroperitoneal fibromatosis

Horizontally acquired SAIDS retrovirus type 2 (SRV-2), a type D retrovirus related to the Mason-Pfizer monkey virus, has been associated with the simian acquired immunodeficiency syndrome (SAIDS) including retroperitoneal fibromatosis (RF) in several macaque species at two primate research centers. Virus specific gene sequences are present in lymphoid and RF tissues but not in muscle tissue of diseased macaques or in any tissues of uninfected normal monkeys. Serologic and restriction endonuclease mapping techniques have defined unique SRV-2 strains in the Celebes (SRV-2C) and rhesus (SRV-2R) macaques at the Oregon Regional Primate Center, SRV-2 is related to both MPMV and SAIDS type 1 retroviruses and it has no detectable molecular homology with the human AIDS retroviruses.

Normal expression of polymorphic endogenous retroviral RNA containing segments identical to mink cell focus-forming virus

In the absence of infectious virus, strains of mice express polyadenylated RNA transcripts homologous to the genome of murine leukemia virus. In addition to transcripts consistent with full-length and spliced env retroviral RNAs, several unique RNA species which lack the env sequence accumulate in a tissue-specific manner. These RNA species are presumed to be transcribed from endogenous retroviral sequences that constitute the bulk of the murine leukemia virus-related sequences in the murine genome. To determine the relationship of these RNA transcripts to infectious murine leukemia virus and the precise structural basis of the heterogeneity observed for the env-lacking transcripts, we isolated and sequenced cDNA recombinants representing the RNAs expressed in strain 129 GIX+ mice. Comparisons of the nucleotide sequences demonstrated that the endogenous retroviral transcripts differed in pol, p15E, and R-peptide regions by single nucleotide changes. In contrast, the gp70-coding regions of two cDNA clones derived from epididymis and liver were completely homologous over a 599-nucleotide overlapping sequence. The structures of env-lacking transcripts were examined in two independent cDNA clones, and each was found to contain a different deletion that was potentially mediated by seven-base pair direct repeats in the intact sequence. The extensive sequence homology between cDNAs allowed construction of a cumulative sequence map of the 3' end of an intact endogenous retroviral transcript. A comparison of this sequence with infectious ecotropic and mink cell focus-forming viruses revealed that the endogenous transcripts are highly homologous with the substituted portions of leukemogenic mink cell focus-forming viruses and therefore further define the boundaries of recombination required to generate these viruses.

Thymotropic envelope gene recombinants of Moloney leukemia virus have highly conserved envelope structures

Envelope gp70s were isolated from the thymotropic recombinant viruses related to Moloney murine leukemia virus (RM-M-MuLVs) which were generated by the inoculation of two strains of ecotropic M-MuLV (strain 1869 and temperature-sensitive mutant-1) into BALB/c or CFW/D mice. Chymotrypsin oligopeptide maps of parental ecotropic MuLV, RM-M-MuLV, and inducible xenotropic MuLV showed each of the above virus types had a distinctly characteristic peptide map. The majority of RM-M-MuLV gp70 molecules examined showed a high degree of peptide homology. Data from restriction endonuclease mapping demonstrated that the newly acquired sequences in each of the RM-M-MuLVs were very related and encompassed both the polymerase and the envelope genes. The source of the sequences acquired by the RM-M-MuLV was from endogenous nonecotropic and nonxenotropic proviruses. This suggested that the family of endogenous proviruses which combined with the parental ecotropic virus was either specifically selected or was much more available than other endogenous proviruses. Although slight variations of envelope-specific sequences and peptides existed among various RM-M-MuLV isolates; within a single thymoma, individual clones of tumor cells yielded RM-M-MuLV gp70s which were identical to each other. These findings are discussed within the context of the leukemogenic potential of RM-MuLVs.

Suppression of leukaemia virus pathogenicity by polyoma virus enhancers

The long terminal repeats (LTRs) of retroviruses contain sequences necessary for the initiation and termination of retroviral transcription. These sequences include promoter elements, transcriptional termination signals and transcriptional enhancer elements. The enhancer elements of Moloney murine leukaemia virus (M-MuLV) are localized in a tandemly repeated region (approximately 75 base pairs (bp) long), which lies 5' to the CAT and TATA promoter elements in the U3 region of the LTR (see Fig. 1). We have shown that the tandem repeats are required both for LTR promoter activity, as measured by transient expression assays, and for biological activity, as measured by production of infectious virus. Furthermore, they can be replaced by transcriptional enhancers from the F101 host-range mutant of polyoma virus without loss of function. We report here that the addition of the polyoma (PyF101) enhancers to the M-MuLV LTRs (either with or without the M-MuLV tandem repeats) results in complete loss of viral leukaemogenicity, even though the virus can replicate to high titres in tissue culture fibroblasts and can establish infection in animals.

Molecular cloning of two paralytogenic, temperature-sensitive mutants, ts1 and ts7, and the parental wild-type Moloney murine leukemia virus

ts1 and ts7, the paralytogenic, temperature-sensitive mutants of Moloney murine leukemia virus (MoMuLV), together with their wild-type parent, MoMuLV-TB, were molecularly cloned. ts1-19, ts7-22, and wt-25, the infectious viruses obtained on transfection to NIH/3T3 cells of the lambda Charon 21A recombinants of ts1, ts7, and wt, were found to have retained the characteristics of their non-molecularly cloned parents. In contrast to the wt virus, ts1-19 and ts7-22 are temperature-sensitive, inefficient in the intracellular processing of Pr80env at the restrictive temperature, and able to induce paralysis in CFW/D mice. Like the non-molecularly cloned ts7, the ts7-22 virion was also shown to be heat labile. The heat lability of the ts7 virion distinguishes it from ts1. Endonuclease restriction mapping with 11 endonucleases demonstrated that the base composition of MoMuLV-TB differs from that of the standard MoMuLV, but no difference was detected between the molecularly cloned ts1 and ts7 genomes. However, ts1 and ts7 differ from MoMuLV in the loss or acquisition of four different restriction sites, whereas they differ from MoMuLV-TB in the loss or acquisition of three different restriction sites.

Generation of glucocorticoid-responsive Moloney murine leukemia virus by insertion of regulatory sequences from murine mammary tumor virus into the long terminal repeat

The glucocorticoid-regulatory sequences from the murine mammary tumor virus long terminal repeat (MMTV LTR) were introduced into the LTR of Moloney murine leukemia virus (M-MuLV) by recombinant DNA techniques. The site of insertion was in the M-MuLV LTR U3 region at -150 base pairs with respect to the RNA cap site. Infectious M-MuLVs carrying the altered LTRs (Mo + MMTV M-MuLVs) were recovered by transfection of proviral clones into NIH-3T3 cells. The Mo + MMTV M-MuLVs were hormonally responsive in that infection was 3 logs more efficient when performed in the presence of dexamethasone, irrespective of the orientation of the inserted MMTV sequences. However, even in the presence of hormone, the Mo + MMTV M-MuLVs were less infectious than wild-type M-MuLV. In contrast to the large effect on infectivity, dexamethasone induced virus-specific RNA levels in chronically Mo + MMTV M-MuLV-infected cells only two- to fourfold. Fusion plasmids between the altered LTRs and the bacterial chloramphenicol acetyltransferase gene allowed the investigation of LTR promoter strength by the transient chloramphenicol acetyltransferase expression assay. The chloramphenicol acetyltransferase assays indicated that the insertion of MMTV sequences into the M-MuLV LTR reduced promoter activity in the absence of glucocorticoids but that promoter activity could be induced two- to fivefold by dexamethasone. The Mo + MMTV M-MuLVs were also tested for the possibility that viral DNA synthesis or integration during initial infection was enhanced by dexamethasone. However, no significant difference was detected between cultures infected in the presence or absence of hormone. The insertion of MMTV sequences into an M-MuLV LTR deleted of its enhancer sequences did not yield infectious virus or active promoters, even in the presence of dexamethasone.

The involvement of a type-B retrovirus in the induction of thymic lymphomas

A highly leukemogenic virus (DMBA-LV) (in vivo leukemogenic titer 1-5 X 10(6) IU/ml, and 35-40 days to thymic lymphoma detection) is produced by a chemical carcinogen-induced transplanted thymic lymphoma. The virus preparation is a mixture of a type-B retrovirus highly related to exogenous type-B retroviral isolates and a biologically defective type-C retrovirus. The DNA of DMBA-LV-induced-tumors contains new type-B proviruses but no additional type-C proviruses could be detected. The leukemogenicity of DMBA-LV was completely neutralized by a monoclonal antibody against MMTV envelope glycoprotein, but was not affected by a broadly reacting Friend MuLV anti-gp70 serum which effectively neutralizes type-C ecotropic, xenotropic, and recombinant retroviruses and which completely abolishes the leukemogenic activity of Moloney leukemia virus. Three type-B mammary tumor-inducing retroviral isolates, while containing type-C retroviral sequences, were not leukemogenic. A further characterization of the type-C retroviral sequences present in DMBA-LV indicated that sequences characteristic of endogenous, nonxenotropic proviruses are present. In addition, using a variety of type-C-specific retroviral DNA probes, no evidence was obtained for the presence of a type-B-C-recombinant genome in DMBA-LV. Leukemogenesis was absolutely dependent upon the presence of a functional type-B retroviral envelope gp 52 and DMBA-LV does not appear to contain a leukemogenic retroviral type-C genome.

Presence of markers for virulence in the unique short region or repeat region or both of pseudorabies hybrid viruses

The unique short region and part of the repeat region of virulent pseudorabies virus strain NIA-3 was replaced by the corresponding region of the avirulent NIA-4 strain by transfection with subgenomic DNA fragments. The resulting hybrid virus showed a reduced virulence in both mice and pigs. Therefore, important markers for virulence are located in the unique short or repeat region or both of pseudorabies virus. We provide evidence that the terminally located repeat is not required for the generation of progeny with intact pseudorabies virus genomes. Apparently, the terminal repeat is regenerated from the internal repeat.

Generation of AKR mink cell focus-forming virus: nucleotide sequence of the 3' end of a somatically acquired AKR-MCF

The 3' end of an AKR-MCF provirus (MCFr35) was cloned and found to be biologically active. Comparison of the nucleotide sequence of MCFr35 with the sequence of other MuLVs revealed that the MCFr35 was most likely derived from the same xenotropic and ecotropic parents, which were involved in the generation of AKR-MCF247. Ecotropic sequences are present around the XbaI site at position 7.9 on the genomic map, and in the long terminal repeat. Most of the T1 oligonucleotide sequences, characteristic for the leukemogenic "class I" MCFs, are also present in MCFr35, with the exception of T1 oligonucleotides 108 and 18. The MCFr35 LTR contains a duplicated enhancer sequence from a xenotropic-like provirus, which is present only once per haploid genome equivalent. The 3' end of MCFr35 consist predominantly of nonecotropic sequences, thereby delimiting the positions of recombination in various MCF viruses.

Molecular cloning and characterization of a leukemia-inducing myeloproliferative sarcoma virus and two of its temperature-sensitive mutants

The myeloproliferative sarcoma virus (MPSV) induces extensive hematopoietic changes, including spleen foci in adult mice, and transforms fibroblasts in vitro. NRK nonproducer cell lines of MPSV and ts temperature-sensitive mutants were analyzed by restriction enzyme digestion and Southern blotting. EcoRI fragments containing the proviral DNAs of MPSV and two temperature-sensitive mutants and rat cellular sequences homologous to c-mos were molecularly cloned. By comparing restriction enzyme cleavage sites, it was shown that the MPSV genome consists only of sequences related either to Moloney murine leukemia virus or to the c-mos mouse oncogenic sequences. Two regions of fragment heterogeneity were observed: (i) in the defective pol gene, where MPSV and the two cloned temperature-sensitive mutants were different from Moloney murine sarcoma virus and from each other, although MPSV wild-type retained more of the pol gene than any of the Moloney murine sarcoma virus isolates; (ii) in the area 3' to the mos gene, which was identical in MPSV and its temperature-sensitive mutants but different from other Moloney murine sarcoma virus variants. Transfection of cloned MPSV DNA in RAT4 cells and virus rescue on infection with Friend murine leukemia virus yielded MPSV which transformed fibroblasts in vitro and also induced spleen foci in adult mice, thus proving that both properties are coded by the same viral genome.

Generation of AKR mink cell focus-forming viruses: a conserved single-copy xenotrope-like provirus provides recombinant long terminal repeat sequences

AKV and AKR mink cell focus-forming virus-specific probes from the envelope and long terminal repeat (LTR) regions were prepared for study of the structure of recombinant proviruses in tumor tissues of AKR mice. The results showed that (i) all somatically acquired proviruses possessed, besides a recombinant gp70 gene, an altered U3 LTR; (ii) in a substantial portion of the somatically acquired AKR mink cell focus-forming proviruses, the LTR comprised sequences derived from the same xenotropic-like provirus; (iii) this U3 LTR donating parental provirus (Xeno-dL) was present only once per genome equivalent in several mouse strains; (iv) in the strains containing the Xeno-dL provirus, the provirus was present in the same chromosomal site; (v) restriction analysis of the Xeno-dL revealed that the mink cell focus-forming gp70 sequences were derived from a parental provirus, different from Xeno-dL. Therefore, at least two non-ecotropic parents participate in the generation of leukemogenic AKR mink cell focus-forming viruses: a xenotropic-like virus, Xeno-dL, donating U3 LTR sequences, and another xenotropic-like virus or viruses providing gp70 sequences.

Non-function of a Moloney murine leukaemia virus regulatory sequence in F9 embryonal carcinoma cells

Moloney murine leukaemia virus (M-MuLV) infection of embryonal carcinoma (EC) cells results in the integration of proviral DNA into the host cell genome, but not in virus production. One suggested explanation for the lack of viral gene expression in EC cells has been methylation of the integrated viral DNA. However, subsequent reports indicated that integration of the M-MuLV DNA occurs soon after infection, but that viral DNA methylation occurs considerably later. Nevertheless, viral gene expression is not observed even at early times. One possible explanation is that certain M-MuLV regulatory sequences do not function in EC cells. We now present evidence which supports this hypothesis.

Regulation of Thy-1 and Pgp-1 glycoproteins in hybrids between T-cell lymphomas and Abelson-leukemia-virus-induced lymphomas

Most cells in the normal adult mouse thymus express Thy-1 glycoprotein but do not express Pgp-1 glycoprotein. In contrast, cells of the mouse B-cell lineage are Thy-1 negative and Pgp-1 positive. Somatic cell hybrids between pseudodiploid Thy-1+, Pgp-1- T-cell lymphomas and pseudotetraploid Thy-1-, Pgp-1+ Abelson-leukemia-virus-induced cell lines do not express detectable cell-surface Thy-1 but show activation of the T-cell Pgp-1 glycoprotein. Hybrids between pseudodiploid lines, in contrast, show extinction of Pgp-1. Thy-1+ or Pgp-1+ revertants were isolated by cell sorting from hybrids in which extinction occurred, demonstrating that all genes required for expression of these cell-surface antigens were present in antigen-negative hybrids. Thy-1- hybrids did not contain detectable cytoplasmic Thy-1 messenger RNA, while Thy-1 message could be detected in parental lines and Thy-1+ revertants. No obvious rearrangements of the Thy-1 structural genes could be demonstrated in Thy-1- hybrids and their Thy-1+ revertants, nor could rearrangements be demonstrated when parental cells and Thy-1- hybrids were compared. These results are consistent with the idea that diffusible gene products regulate both Thy-1 and Pgp-1 expression in these hybrids. These products act in a gene dosage-dependent manner in somatic cell hybrids. Regulation of Thy-1 is at the level of either messenger RNA transcription or processing.

Two base changes restore infectivity to a noninfectious molecular clone of Moloney murine leukemia virus (pMLV-1)

The complete nucleotide sequence of a molecular clone of Moloney murine leukemia virus (pMLV-1) has previously been reported (Shinnick et al., Nature [London] 293:543-548, 1981). However, pMLV-1 does not generate infectious virus after transfection into cells (Berns et al., J. Virol. 36:254-263, 1980). The lesion in pMLV-1 has been localized by determining the biological activity of recombinants containing DNA from an infectious clone of Moloney murine leukemia virus (pMLV-48) and pMLV-1. Replacement of a 1.0-kilobase pair region which spans the gag-pol junction of pMLV-1 with the corresponding DNA fragment from the infectious clone restores its infectivity. Nucleotide sequence analysis of this fragment obtained from the infectious clone (pMLV-48) and pMLV-1 reveals two single base pair changes, one in the p30gag gene and the other in the 5' end of the pol gene. The mutation in the pol gene does not affect the production of infectious virus but renders them XC negative, whereas the mutation in the gag gene appears to be lethal. The complete nucleotide sequence of an infectious clone of Moloney murine leukemia virus can now be deduced.

Unusual properties of AKR MCF-247 murine leukemia virus unintegrated proviral DNA

Analyses of Hirt extracts from endogeneous murine leukemia virus (MuLV)-infected cells revealed the presence of 9-kbp linear DNA and two superhelical forms with one or two tandem copies of the long terminal repeat (LTR). In contrast, cells that were infected with AKR MCF247 MuLV yielded two major linear forms of 9.0 and 8.4 kbp and one discrete superhelical DNA. In addition, there was a heterogeneous population of superhelical DNAs that were larger and smaller than the major superhelical DNA species. Restriction endonuclease treatment of purified linear and superhelical DNAs have revealed that MCF247 MuLV unintegrated viral DNA is very heterogeneous. Evidence is presented that there are at least two linear DNAs; one is 9-kbp full-length linear DNA, whereas the other major form contains a 0.6 to 0.7-kbp deletion in the envelope gene adjacent to the right LTR. In addition, there are two size classes of the LTR in at least the full-length linear DNA. The major superhelical DNA species is a 8.4-kbp form which contains one copy of the LTR. Other heterogeneous superhelical DNAs appear to contain env-gene deletions or partially deleted copies of a tandem LTR region.

A transmissible retrovirus expressing human hypoxanthine phosphoribosyltransferase (HPRT): gene transfer into cells obtained from humans deficient in HPRT

A cDNA corresponding to the human gene for hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) has been ligated into murine retroviral vectors such that it is under the transcriptional control of viral long terminal repeats. Transfection of HPRT- cells followed by superinfection with various helper viruses has led to the rescue of chimeric virus capable of transmitting the HPRT+ phenotype to HPRT- rodent or human cells. These genetically transformed cells contain authentic human HPRT at levels similar to normal HPRT+ cells.

Elements in the long terminal repeat of murine retroviruses enhance stable transformation by thymidine kinase gene

We have investigated the effects of long terminal repeats (LTRs) of murine retroviruses on the frequency of obtaining stable transfectants by the herpes virus thymidine kinase (TK) gene. The results indicate that addition of LTRs enhances the number of TK+ transformants by 10-20 fold. A 5-12 fold enhancement was also observed when chromosomal DNA from either human or hamster cells was mixed with a plasmid containing LTR sequences and transfected onto LTR- cells. The LTR sequences involved in the enhancement were localized in the region which contains tandem repeats. All other regions of the LTR did not show any enhancement of stable TK+ transfectants. The location or the orientation of the enhancer sequences with respect to the TK gene did not exert any influence on the frequency of transformation. The enhancement effect does not appear to be linked to either increased numbers of chromosomal integrations or elevated levels of transcription of the TK gene.

Molecular cloning of viral DNA from leukemogenic Gross passage A murine leukemia virus and nucleotide sequence of its long terminal repeat

The viral DNA genome of the leukemogenic Gross passage A virus was cloned in phage Charon 21A as an infectious molecule. The virus recovered by transfection with this infectious DNA was ecotropic, N-tropic, fibrotropic, and XC+. It was leukemogenic when reinjected into newborn SIM mice, indicating that ecotropic murine leukemia virus (MuLV) from an AKR mouse thymoma can harbor leukemogenic sequences. Its restriction map was similar to that of nonleukemogenic AKR MuLV, its putative parent, but differed at the 3' end and in the long terminal repeat (LTR). The nucleotide sequence of the Gross A virus LTR was identical to the AKR MuLV LTR sequence (Van Beveren et al., J. Virol. 41:542-556, 1982) in U5, R, and part of U3. All differences between both LTRs were found in U3. Only one copy of the U3 tandem direct repeat was conserved in the Gross A virus LTR, and it was rearranged by the insertion of a 36-base-pair sequence and by five point mutations. Only one additional point mutation common to several oncogenic MuLVs was present in U3. These structural changes in the U3 LTR and at the 3' end of the genome may be related to the leukemogenicity of this virus.

Analysis of recombinant DNA clones of the endogenous BALB/c murine leukemia virus WN1802N: variation in long terminal repeat length

We analyzed 15 recombinant DNA clones of the unintegrated closed circular DNA intermediate of the BALB/c endogenous ecotropic murine leukemia virus WN1802N. Thirteen of these clones had an insert which corresponded to the complete murine leukemia virus genome. Of these, six contained a single long terminal repeat (LTR) and seven contained two LTRs. The viral genomes in nine clones had an LTR of 520 base pairs (bp), one had an LTR of 570 bp, three had an LTR of 600 bp, and one had an LTR of 670 bp. Restriction endonuclease analysis demonstrated that the size variability resides in the U3 region. Seven of eight clones which yielded infectious virus by DNA transfection had the 520-bp LTR, and the other had a 600-bp LTR. More detailed examination of plasmid subclones of three isolates with different-sized LTRs revealed that the approximate position which varies in the U3 region corresponds to the 72-bp repeat region of Moloney sarcoma virus. Possible consequences of these variations are discussed.

Cloning of endogenous murine leukemia virus-related sequences from chromosomal DNA of BALB/c and AKR/J mice: identification of an env progenitor of AKR-247 mink cell focus-forming proviral DNA

Recombinant phages containing murine leukemia virus (MuLV)-reactive DNA sequences were isolated after screening of a BALB/c mouse embryo DNA library and from shotgun cloning of EcoRI-restricted AKR/J mouse liver DNA. Twelve different clones were isolated which contained incomplete MuLV proviral DNA sequences extending various distances from either the 5' or 3' long terminal repeat (LTR) into the viral genome. Restriction maps indicated that the endogenous MuLV DNAs were related to xenotropic MuLVs, but they shared several unique restriction sites among themselves which were not present in known MuLV proviral DNAs. Analyses of internal restriction fragments of the endogenous LTRs suggested the existence of at least two size classes, both of which were larger than the LTRs of known ecotropic, xenotropic, or mink cell focus-forming (MCF) MuLV proviruses. Five of the six cloned endogenous MuLV proviral DNAs which contained envelope (env) DNA sequences annealed to a xenotropic MuLV env-specific DNA probe; in addition, four of these five also hybridized to an ecotropic MuLV-specific env DNA probe. Cloned MCF 247 proviral DNA also contained such dual-reactive env sequences. One of the dual-reactive cloned endogenous MuLV DNAs contained an env region that was indistinguishable by AluI and HpaII digestion from the analogous segment in MCF 247 proviral DNA and may therefore represent a progenitor for the env gene of this recombinant MuLV. In addition, the endogenous MuLV DNAs were highly related by AluI cleavage to the Moloney MuLV provirus in the gag and pol regions.

A genetic locus regulates the expression of tissue-specific mRNAs from multiple transcription units

129 GIX- mice, unlike animals of the congeneic partner strain GIX+, do not express significant amounts of the retroviral antigens gp70 and p30. Evidence is presented indicating that the GIX phenotype is specified by a distinct regulatory gene acting on multiple transcription units to control the levels of accumulation of specific mRNA species. The steady-state levels of retroviral-homologous mRNA from the tissues of GIX+ and GIX- mice were examined by blot hybridization using as probes DNA fragments from cloned murine leukemia viruses. RNA potentially encoding viral antigens was reduced or absent in GIX- mice, even though no differences in integrated viral genomes were detected between these congeneic strains by DNA blotting. Tissue-specific patterns of accumulation of these RNA species were detected in brain, epididymis, liver, spleen, and thymus, and several distinct RNA species were found to be coordinately regulated with the GIX phenotype. Measurements of RNA synthesis suggest a major role for transcriptional control in the regulation of some retroviral messages.

DNA methylation affecting the expression of murine leukemia proviruses

The endogenous, vertically transmitted proviral DNAs of the ecotropic murine leukemia virus in AKR embryo fibroblasts were found to be hypermethylated relative to exogenous AKR murine leukemia virus proviral DNAs acquired by infection of the same cells. The hypermethylated state of the endogenous AKR murine leukemia virus proviruses in these cells correlated with the failure to express AKR murine leukemia virus and the lack of infectivity of cellular DNA. Induction of the endogenous AKR murine leukemia virus proviruses with the methylation antagonist 5-azacytidine suggested a causal connection between DNA methylation and provirus expression. Also found to be relatively hypermethylated and noninfectious were three of six Moloney murine leukemia virus proviral DNAs in an unusual clone of infected rat cells. Recombinant DNA clones which derived from a methylated, noninfectious Moloney provirus of this cell line were found to be highly active upon transfection, suggesting that a potentially active proviral genome can be rendered inactive by cellular DNA methylation. In contrast, in vitro methylation with the bacterial methylases MHpaII and MHhaI only slightly reduced the infectivity of the biologically active cloned proviral DNA. Recombinant DNA clones which derived from a second Moloney provirus of this cell line were noninfectious. An in vitro recombination method was utilized in mapping studies to show that this lack of infectivity was governed by mechanisms other than methylation.

Analysis of the env gene of a molecularly cloned and biologically active Moloney mink cell focus-forming proviral DNA

A biologically active molecular clone of BALB/Moloney mink cell focus-forming (Mo-MCF) proviral DNA has been reconstructed in vitro. It contains the 5' half of BALB/Moloney murine leukemia virus (Mo-MuLV) DNA and the 3' half of BALB/Mo-MCF DNA. The complete nucleotide sequence of the env gene and the 3' long terminal repeat (LTR) of the cloned Mo-MCF DNA has been determined and compared with the sequence of the corresponding region of parental Mo-MuLV DNA. The substitution in the Mo-MCF DNA encompasses 1,159 base pairs, beginning in the carboxyl terminus of the pol gene and extending to the middle of the env gene. The Mo-MCF env gene product is predicted to be 29 amino acids shorter than the parental Mo-MuLV env gene product. The portion of the env gene encoding the p15E peptide is identical in both viral DNAs. There is an additional A residue in the Mo-MCF viral DNA in a region just preceding the 3' LTR. The nucleotide sequence of the 3' LTR of Mo-MCF DNA is similar to that of the 5' LTR of BALB/Mo-MuLV DNA with the exception of two single base substitutions. We conclude that the sequence substitution in the env gene is responsible for the dual-tropic properties of Mo-MCF viruses.

Cloning of two genetically transmitted Moloney leukemia proviral genomes: correlation between biological activity of the cloned DNA and viral genome activation in the animal

The Mov-7 and Mov-9 substrains of mice, carrying Moloney murine leukemia virus (M-MuLV) in their germ line at the Mov-7 locus and Mov-9 locus, respectively, are different with respect to virus activation. Infectious virus appears in all mice carrying the Mov-9 locus but is not activated in animals carrying the Mov-7 locus. Consequently, only Mov-9 mice develop viremia and subsequent leukemia. The endogenous M-MuLV provirus with flanking mouse sequences corresponding to the Mov-7 and Mov-9 loci was molecularly cloned. Detailed restriction maps obtained from the cloned DNAs revealed no detectable differences in the proviral genomes. The flanking mouse sequences, however, were different, confirming that the Mov-7 and Mov-9 loci represent different integration sites of M-MuLV. Both clones induced XC plaques in a transfection assay. The specific infectivity of the clones, however, was different. A total of 10(-5) XC plaques per genome equivalent were induced by the Mov-9 clone, whereas only 10(-9) XC plaques per genome equivalent were induced by the Mov-7 clone. Moreover, NIH 3T3 cells transfected with the Mov-9 clone produced NB-tropic M-MuLV, whereas cells transfected with the Mov-7 clone did not produce infectious virus. The results suggest that M-MuLV integrated at the Mov-7 locus carries a mutation which prevents synthesis of infectious virus but permits XC plaque induction by partial genome expression or synthesis of noninfectious particles. Thus, the pattern of virus expression in Mov-7 and Mov-9 mice correlates with the biological properties of the respective clones. Genomic DNA from Mov-9 mice was not infectious in the transfection assay (specific infectivity < 10(-7) PFU per genome equivalent). As the only difference between the genomic and the cloned Mov-9 DNA appears to be the presence of 5-methylcytosine in CpG sequences, our results suggest that removal of methyl groups by molecular cloning in procaryotes permits genome expression in transfected eucaryotic cells. Our results support the hypothesis that DNA methylation is relevant not only in genome expression in the animal but also in expression of genes transfected into eucaryotic cells.

Infectivity and structure of molecular clones obtained from two genetically transmitted Moloney leukemia proviral genomes

The Mov-2 and Mov-10 substrains of mice, each carrying Moloney leukemia virus (= M-MuLV) in their germ line at the Mov-2 and Mov-10 locus, respectively, do occasionally at a later age (Mov-2) or not at all (Mov-10) activate infectious virus. The M-MuLV proviruses with flanking mouse sequences corresponding to the Mov-2 and Mov-10 locus, respectively, were molecularly cloned. Restriction enzyme analysis revealed no major deletions or insertions in the proviral genomes of the Mov-2 and Mov-10 locus. Both cloned DNAs induced XC plaques in a transfection assay. The specific infectivity, however, was very low and 3T3 cells transfected with the Mov-2 or Mov-10 clone did not produce infectious virus. Removing part of the 5' cellular sequences from the Mov-10 clone did not increase the infectivity. The results suggest that the M-MuLV integrated at the Mov-2 and Mov-10 locus carry a mutation which prevents synthesis of infectious virus but permits XC plaque induction by partial genome expression or synthesis of non-infectious particles.

Molecular cloning of murine endogenous viral sequences and expression of a newly constructed recombinant murine leukemia virus DNA in transfected mink cells

In the process of molecularly cloning unintegrated proviral DNA from NIH-3T3 mouse cells infected with Rauscher murine leukemia virus, we observed the occurrence of clones with inserts smaller than the expected Rauscher murine leukemia virus fragments. The insert of one of these clones, lambda.Xe-1, was characterized in more detail. It had a size of 3.5 kilobases. The restriction map was similar but not identical to that of the envelope regions of Moloney and Rauscher murine leukemia viruses. After ligation to previously cloned Moloney murine leukemia viral sequences and transfection of the ligated DNA into mink lung cells a nondefective xenotropic murine leukemia virus, XH-19, was isolated. Restriction mapping of proviral DNA isolated from mink lungs cells chronically infected with XH-19 showed the presence of Moloney murine leukemia virus-derived sequences coupled to xenotropic viral sequences.

Molecular cloning of AKR xenotropic murine leukemia virus unintegrated proviral DNA

Suprahelical proviral DNA of AKR xenotropic murine leukemia virus was purified from agarose gels and cloned in lambda Charon 28 DNA (BamHI sites). Nine viral DNA recombinants were identified and mapped with 12 restriction endonucleases. Three calsses of cloned viral DNA inserts were found: (1) Six inserts were apparently full-length 9.0-kb DNA with tandem long terminal repeat (LTR) elements; (2) two inserts contained DNAs with deletions in or adjacent to the LTR regions; (3) a single isolate contained an inversion of 2.3 kb around the LTR in the envelope gene.

Long terminal repeat of murine retroviral DNAs: sequence analysis, host-proviral junctions, and preintegration site

The nucleotide sequence of the long terminal repeat (LTR) of three murine retroviral DNAs has been determined. The data indicate that the U5 region (sequences originating from the 5' end of the genome) of various LTRs is more conserved than the U3 region (sequences from the 3' end of the genome). The location and sequence of the control elements such as the 5' cap, "TATA-like" sequences, "CCAAT-box," and presumptive polyadenylic acid addition signal AATAAA in the various LTRs are nearly identical. Some murine retroviral DNAs contain a duplication of sequences within the LTR ranging in size from 58 to 100 base pairs. A variant of molecularly cloned Moloney murine sarcoma virus DNA in which one of the two LTRs integrated into the viral DNA was also analyzed. A 4-base-pair duplication was generated at the site of integration of LTR in the viral DNA. The host-viral junction of two molecularly cloned AKR-murine leukemia virus DNAs (clones 623 and 614) was determined. In the case of AKR-623 DNA, a 3- or 4-base-pair direct repeat of cellular sequences flanking the viral DNA was observed. However, AKR-614 DNA contained a 5-base-pair repeat of cellular sequences. The nucleotide sequence of the preintegration site of AKR-623 DNA revealed that the cellular sequences duplicated during integration are present only once. Finally, a striking homology between the sequences flanking the preintegration site and viral LTRs was observed.

Characterization of proviruses cloned from mink cell focus-forming virus-infected cellular DNA

Two proviruses were cloned from EcoRI-digested DNA extracted from mink cells chronically infected with AKR mink cell focus-forming (MCF) 247 murine leukemia virus (MuLV), using a lambda phage host vector system. One cloned MuLV DNA fragment (designated MCF 1) contained sequences extending 6.8 kilobases from an EcoRI restriction site in the 5' long terminal repeat (LTR) to an EcoRI site located in the envelope (env) region and was indistinguishable by restriction endonuclease mapping for 5.1 kilobases (except for the EcoRI site in the LTR) from the 5' end of AKR ecotropic proviral DNA. The DNA segment extending from 5.1 to 6.8 kilobases contained several restriction sites that were not present in the AKR ecotropic provirus. A 0.5-kilobase DNA segment located at the 3' end of MCF 1 DNA contained sequences which hybridized to a xenotropic env-specific DNA probe but not to labeled ecotropic env-specific DNA. This dual character of MCF 1 proviral DNA was also confirmed by analyzing heteroduplex molecules by electron microscopy. The second cloned proviral DNA (designated MCF 2) was a 6.9-kilobase EcoRI DNA fragment which contained LTR sequences at each end and a 2.0-kilobase deletion encompassing most of the env region. The MCF 2 proviral DNA proved to be a useful reagent for detecting LTRs electron microscopically due to the presence of nonoverlapping, terminally located LTR sequences which effected its circularization with DNAs containing homologous LTR sequences. Nucleotide sequence analysis demonstrated the presence of a 104-base-pair direct repeat in the LTR of MCF 2 DNA. In contrast, only a single copy of the reiterated component of the direct repeat was present in MCF 1 DNA.

DNA methylation and gene expression: endogenous retroviral genome becomes infectious after molecular cloning

The Mov-3 substrain of mice carries Moloney murine leukemia virus as a Mendelian gene in its germ line. All mice segregating the Mov-3 locus activate virus and develop viremia and leukemia. The integrated provirus (i.e., Mov-3 locus) was molecularly cloned from Mov-3 liver DNA as a 16.8 kilobase long EcoRI fragment. Comparison of the cloned and genomic Mov-3 specific EcoRI fragment by restriction enzyme analysis showed no differences in the size of the fragments, indicating that no major sequence rearrangements occurred during cloning. The genomic and cloned Mov-3 DNAs were compared for methylation and infectivity. Analysis with Hha I showed that the genomic proviral and the flanking mouse sequences were methylated at cytosine residues, in contrast to the cloned Mov-3 locus. The cloned Mov-3 locus, however, was highly infectious in a transfection assay (1 x 10(-3) plaque-forming unit per viral genome) in contrast to the genomic Mov-3 DNA (less than 10(-7) per viral genome). Our results suggest that genes containing 5-methylcytosine are not expressed after transfection into susceptible cells and that removal of the methyl groups by molecular cloning in prokaryotes leads to expression generating infectious proviral DNA. If gene expression of transfected DNA is controlled by mechanisms that are relevant for gene expression in the animal, this suggests that DNA methylation may play a causative role in eukaryotic gene regulation.

Endogenous type C retroviral sequences of mice are organized in a small number of virus-like classes and have been acquired recently

We studied endogenous type C virus-related sequences of mice by annealing Moloney murine leukemia virus DNA to agarose gel blot transfers of uninfected mouse cell DNA which had been cleaved with restriction enzymes. We found that many of the endogenous murine leukemia virus-related sequences in mice consist of two organizational classes that are integrated into many different loci. Both of these classes resemble standard murine leukemia virus proviral DNA in both size and sequence organization. All lines of inbred mice examined contained both organizational classes, as did feral isolates of Mus musculus domesticus. However, a related Asian subspecies, Mus musculus molossinus, contained different organizational classes of endogenous murine leukemia virus-related sequences. Among inbred strains and feral isolates of M. musculus domesticus, the murine leukemia virus-related sequences were present at different loci. This suggested that most of these sequences were acquired relatively recently during subspeciation and inbreeding.

Infectivity and methylation of retroviral genomes is correlated with expression in the animal

We studied mechanisms controlling gene expression during animal development using retroviruses as model genes. For this, substrains of mice have been previously derived carrying the Moloney leukemia virus (M-MuLV) in their germ line. Virus activation occurs in some of these substrains at different stages of development, resulting in two classes of viral genomes. The genetically transmitted (endogenous) copy is present in every cell, whereas somatically acquired ("exogenous") copies are carried only in cells that were superinfected. We compared these two classes of M-MuLV genomes using two parameters. DNA sequences of the endogenous M-MuLV genome in all mouse substrains were highly methylated in GCGC, the recognition sequence of the restriction enzyme Hha I, and were not infectious (specific infectivity less than 10(-7) pfu per proviral genome) in a DNA transfection assay. In contrast, the "exogenous" copies were hypomethylated and infectious. These parameters are strongly correlated to genome activity in the animal: only tissues carrying exogenous copies express virus-specific RNA. With the assumption that gene expression of transfected DNA is controlled by mechanisms that are relevant for gene expression in the animal, our results suggest that DNA methylation plays a causative role in gene regulation during development and differentiation.

Genomes of murine leukemia viruses isolated from wild mice

The genomes of murine leukemia viruses (MuLV) isolated from wild mice have been studied. Detailed restriction endonuclease maps of the 8.8-kilobase (kb) unintegrated linear viral DNAs were derived for five ecotropic and five amphotropic MuLV's from California field mice, for Friend MuLV, and for one ecotropic and one xenotropic MuLV from Mus musculus castaneus. In general, the California MuLV's were similar in their leftward 6 kb (corresponding to the leftward long terminal repeat [LTR], gag, and pol) and rightward 1 kb (7.8 to 8.8 kb, corresponding to p15E and the rightward LTR). For the region spanning 6.0 to 7.7 kb (which includes the sequences that encode gp70) the amphotropic MuLV's shared few enzyme sites with the ecotropic MuLV's, although the California ecotropic MuLV's were highly related to each other in this region, as were the amphotropic MuLV's. Cross-hybridization studies between amphotropic and California ecotropic MuLV DNAs indicated that they were not homologous in the region 6.3 to 7.6 kb; the California ecotropic viral DNAs cross-hybridized in this region to AKR ecotropic MuLV. When the California viral DNAs were compared with AKR ecotropic viral DNA, many differences in enzyme sites were noted throughout the genome. The U3 regions of the wild mouse LTRs showed partial homology to this region in AKR MuLV. The LTR of Moloney MuLV was highly related to that of the California MuLV's, whereas the LTR of Friend MuLV appeared to be a recombinant between the two types of LTRs. The M. musculus castaneus isolates were most closely related to ecotropic and xenotropic MuLV's isolated from inbred mice. One amphotropic MuLV DNA was cloned from supercoiled viral DNA at its unique EcoRI site in pBR322. Viral DNAs with one and two LTRs were isolated. After digestion with EcoRI, DNAs of both types were infectious. It is concluded that ecotropic and amphotropic MuLV's differ primarily in the region which encodes gp70.