Large T1 oligonucleotides of Moloney leukemia virus missing in an env gene recombinant, HIX, are present on an intracellular 21S Moloney viral RNA species

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

High- and low-leukemogenic variants of the radiation leukemia virus (RadLV): immunogenic, suppressive and genetic properties in relation to leukemogenic activity

C57BL/6 (B6) mice inoculated with the highly leukemogenic variant of the radiation leukemia virus (A-RadLV) develop suppressor cells capable of abrogating potential anti-tumor immunity in vitro and in vivo. Inoculation of B6 animals with the low-leukemogenic D-RadLV variant does not result in suppressor cell generation but induces antitumor reactive lymphocytes. A-RadLV and D-RadLV are not leukemogenic in BALB/c or (B6 X BALB/c)F1(F1) mice, and reactive but not suppressor lymphocytes could be demonstrated in F1 animals inoculated with either virus. Infectivity assays and fingerprint analysis revealed that A-RadLV and D-RadLV contain viruses with N and B tropism. In addition, thymoma cells induced by A-RadLV produced another virus with a fingerprint pattern containing X-MuLV elements. The possible implications of the different virus types on the immunogenic and leukemogenic properties of the RadLV variants are discussed.

Biological, chemical, and immunological studies of Rauscher ecotropic and mink cell focus-forming viruses from JLS-V9 cells

Two murine leukemia viruses were isolated from JLS-V9 cells which had been infected with Rauscher plasma virus. One virus was XC positive and failed to grow on mink or cat cells and thus was an ecotropic virus. The other virus formed cytopathic foci on mink cells, was XC negative, and fell into the mink cell focus-forming (MCF) viral interference group and was thus an MCF virus. The glycoproteins of the two viruses could be distinguished immunologically, by peptide mapping, and by size in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The MCF virus produced gp69, and the ecotropic virus produced gp71, explaining the origin of the heterogeneous glycoprotein (gp69 and gp71) of Rauscher leukemia virus. Amino-terminal sequences of gp69 and gp71 were determined. The MCF sequence was distinct from the ecotropic sequence, but retained partial homology to it. The data show that the glycoproteins are encoded by related yet distinct genes. The protein structural data support the proposal that MCF virus gp70 molecules have nonecotropic sequences at the amino terminus, with ecotropic sequences occurring at the 3' end of the gene. The Rauscher MCF virus glycoprotein lacks a glycosylation site found at position 12 of the ecotropic sequence.

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.

Monoclonal antibody to spleen focus-forming virus-encoded gp52 provides a probe for the amino-terminal region of retroviral envelope proteins that confers dual tropism and xenotropism

Monoclonal antibodies which recognize a region common to Friend spleen focus-forming virus encoded gp52 and Friend mink cell focus-inducing viral gp70 were isolated. One such antibody from hybridoma 7C10 was tested extensively in immune precipitation and was found to react with a determinant on envelope gp70s of all mink cell focus-inducing, xenotropic, and amphotropic mouse retroviruses tested, but not with envelope gp70s of ecotropic viruses, including Friend, Moloney, and AKR murine leukemia viruses. Monoclonal antibody from hybridoma 7C10 precipitated a 23,000-molecular-weight fragment, derived by V8 protease digestion of Friend mink cell focus-inducing gp70. This 23,000-molecular-weight peptide was determined to derive from the amino terminus of the molecule. These results correlate well with other genetic data which indicate that endogenously acquired sequences of mink cell focus-inducing viruses are found at the 5' end of the envelope gene.

Nucleotide sequence of the 5' noncoding region and part of the gag gene of Rous sarcoma virus

Several functions of the retrovirus genome involve structural features in the vicinity of its 5' terminus. In an effort to further elucidate the relationship between structure and function in retrovirus RNA, we have determined the sequence of the first 1,010 nucleotides at the 5' end of the genome of Rous sarcoma virus by using the Maxam-Gilbert method to sequence suitable domains in cloned Rous sarcoma virus DNA. The results (i) locate the initiation codon for the gag gene of Rous sarcoma virus 372 nucleotides from the 5' end of viral RNA; (ii) demonstrate that this codon is preceded by three methionine codons that are apparently not used in translation; (iii) sustain previous conclusions that the principal site to which ribosomes bind on the Rous sarcoma virus genome in vitro does not contain the initiation codon for gag; (iv) permit deduction of the amino acid sequence of a viral structural protein, p19; (v) confirm the amino-terminal sequence of Pr76gag; and (vi) substantiate the identification of a splice donor site described in the accompanying manuscript (Hackett et al., J. Virol., 41:527-534, 1982).

Purification and translation of murine mammary tumor virus mRNA's

We have studied the functions of the intracellular RNAs of mouse mammary tumor virus (MMTV) by purification and translation in vitro. Two major size classes of MMTV RNA, 35S and 24S RNA, were isolated from MMTV-infected rat (XC) cells and cultured mammary tumor cells by preparative hybridization of whole cell or polyadenylated RNA to cloned MMTV DNA covalently bound to chemically activated paper disks (diazobenzyloxymethyl paper). Genomic-length (35S) RNA was prepared free of 24S RNA by rate zonal sedimentation in sucrose gradients. Experiments using [3H]uridine-labeled cellular RNA indicated that the preparative annealing method was highly specific and capable of effecting a 300-fold enrichment for viral RNA; the recovered RNA appeared to be intact under denaturing conditions and directed synthesis of full-length gag and env polypeptides in vitro. The products of in vitro translation were identified by gel mobility, immunoprecipitation tests with antisera against gag and env products, and partial digestion with Staphylococcus V8 protease. The 35S RNA species directed synthesis of several gag-related polypeptides, including three previously reported in extracts of infected cells; 24S RNA directed synthesis of two polypeptides closely related to env proteins from infected cells. Therefore, 35S RNA includes mRNA's for gag and gag-pol, whereas 24S RNA is the mRNA for env. These results help establish the position of env on the physical map of the MMTV genome and bear upon the coding potential of the genome.

Genomic masking and rescue of dual-tropic murine leukemia viruses: role of pseudotype virions in viral lymphomagenesis

The kinetics of genomic masking of nondefective dual-tropic murine leukemia viruses (MuLV) by ecotropic MuLV in mixedly infected mouse cells was studied. The ratio of virus infection (ecotropic to dual-tropic) determined the kinetics of genomic masking. Some dual-tropic virus isolates could be masked routinely (i.e., converted to virions containing a dual-tropic genome and possessing the ecotropic host range) at all ratios of initial infection of mixedly infected mouse cells. The masked genomes could be rescued as infectious viruses of dual-tropic genotype and host range by an infectious center assay of the infected mouse cells on mink lung cells. Infectious center rescue of masked dual-tropic MuLV took place readily, even from cells that had been kept in continuous culture for many months after the onset of genomic masking. Some dual-tropic virus clones did not undergo genomic masking at any infection ratio with ecotropic virus. Nevertheless, such mixedly infected cultures also gave rise to phenotypically mixed virions, which contained a dual-tropic genome and had an ecotropic host range. (The phenotypically mixed virions found among the progeny of mixedly infected mouse cells were not pseudotypes, as both types of viruses were genetically nondefective, nor was the process leading to their generation a bona fide phenotypic mixing [Fischinger et al., Science 201:457-459, 1978]. Nevertheless, in this paper we use the terms pseudotype and phenotypic mixing because of the lack of a better description.) The lymphomagenic potential of dual-tropic lymphomagenic MuLV was compared with that of phenotypically mixed virions possessing an ecotropic host range and with that of a simple mixture of dual-tropic and ecotropic viruses. The phenotypically mixed pseudotype virions were more potent lymphoma inducers than were those of dual-tropic, cloned genotype. Inoculation of a simple mixture of the viruses did not increase dual-tropic virus tumorigenicity. The reason for this was probably the highly efficient inactivation of dual-tropic virus by oncovirus-inactivating factor, which is present in normal mouse serum and did not inactivate the phenotypically mixed virions. Simple mixtures of dual-tropic lymphomagenic and ecotropic virus preparations behaved like the cloned, dual-tropic virus in vivo and were equally sensitive to oncovirus-inactivating factor in vitro. Thus, phenotypic mixing of dual-tropic and ecotropic MuLV with or without concomitant genomic masking may be a highly significant phenomenon in naturally occurring lymphomagenesis. It may also be important to use phenotypically mixed viruses in the procedures used for in vivo testing of lymphomagenic dual-tropic MuLV isolates.

Expression of leukemogenic recombinant viruses associated with a recessive gene in HRS/J mice

HRS/J inbred mice carry a mutant autosomal recessive gene (hr), which in homozygotes coincides with susceptibility to spontaneous thymic leukemia. Unlike their heterozygote (hr/+) littermates, hr/hr homozygotes express high levels of xenotropic virus during the preleukemic period, and viruses with a broadened host range (termed polytropic viruses) can be isolated from their preleukemic and leukemic tissues. Because hr/hr and hr/+ mice are otherwise genetically identical, the virological differences between them support the role of polytropic viruses in the generation of thymic leukemia. In the present report we show that the HRS/J polytropic viruses are env gene recombinants with unique oligonucleotide and peptide maps. These polytropic viruses appear to arise by recombination between ecotropic virus and an unidentified genome related, but not identical to, the endogenous xenotropic viruses. Moreover, polytropic viruses not only accelerate leukemogenesis in HRS/J mice, but also induce thymic leukemia in the low leukemia strain CBA/J. By contrast, cloned ecotropic and xenotropic viruses have no leukemogenic action.

Molecular dissection of Rauscher virus gp70 by using monoclonal antibodies: localization of acquired sequences of related envelope gene recombinants

Using hybridoma-specific immune precipitations of fragments derived from Rauscher virus gp70, coupled with peptide patterns (fingerprinting) and partial amino acid sequence analyses, we have generated a linear map of Rauscher gp70. We used a panel of 56 hybridomas derived from the fusion of the drug-selected SP-2 myeloma line with spleen cells from either a 129 GIX+ or a GIX- mouse immunized with purified Rauscher virus gp70. The results showed that by "natural" breakdown, gp70 splits into predominant fragments, with Mr 45,000 (P45), 34,000 (P34), and 32,000 (P32). Peptide fingerprinting of these as well as overlapping fragments coupled with partial amino acid sequence analyses allowed us to align the fragments into the linear arrangement NH2-P45-P32-COOH, with P34 being an NH2-terminal degradation product of P45. Of the 56 hybridomas, 20 immunoprecipitated both P45 and P34; 18 immunoprecipitated only P45; and 18 immunoprecipitated only P32. The hybridomas thus define three domains of the molecule as NH2-P45/P34, P45 only, and P32-COOH. Allowing these hybridomas to react with two Rauscher-derived envelope gene recombinant viruses yielded the following results: (i) all 20 P45/34 reactors bound to the two Rauscher recombinants; (ii) of 18 P45-only hybridomas, 10 reacted; and (iii) only 1 of 18 P32 reactors bound to the Rauscher recombinants. This last hybridoma reacted with various murine retroviruses, indicating that it was directed at conserved determinants of gp70. Peptide fingerprinting of R-gp70, the recombinant gp70s, and their respective breakdown products confirmed the homologies and nonhomologies defined by the hybridomas. Furthermore, peptide patterns showed that these acquired sequences on the COOH-terminal portion of the recombinant gp70s are related to xenotropic virus gp70s.

Nucleotide sequences associated with differences in electrophoretic mobility of envelope glycoprotein gp70 and with GIX antigen phenotype of certain murine leukemia viruses

Previous genetic and biochemical studies led to the identification of two large RNase T1-resistant oligonucleotides, designated the G(IX) (+) and G(IX) (-) oligonucleotides, whose presence in the genomes of closely related murine leukemia viruses is mutually exclusive and predictive of two properties of the viral envelope glycoprotein gp70. Viruses harboring the G(IX) (+) oligonucleotide induce expression of the gp70-associated antigen G(IX) and possess gp70s with more rapid electrophoretic mobility on sodium dodecyl sulfate/polyacrylamide gels than viruses that possess the G(IX) (-) oligonucleotide. The latter viruses fail to induce G(IX) on infected fibroblasts. The G(IX) (+) and G(IX) (-) oligonucleotides lie in corresponding positions in the 3' third of the oligonucleotide maps of their respective viruses. We have determined the nucleotide sequences of the G(IX) (+) and G(IX) (-) oligonucleotides. The sequence of the G(IX) (-) oligonucleotide is U-A-U-C-U-C-A-A-C-C-A-C-C-A-U-A-C-U-U-A-A-C-C-U-C-A-C-C-A-C-[unk]-G, and the sequence of the G(IX) (+) oligonucleotide is U-A-U-C-U-C-A-A-C-C-A-C-C-A-U-A-C-U-U-G. Thus, a single base change could result in the interconversion of the two oligonucleotides. Consideration of the amino acids specified by the two oligonucleotides suggests that this single base difference may result in the presence of an additional oligosaccharide chain in the gp70s of the G(IX) (-) viruses. Evidence supporting this prediction has been obtained by M. R. Rosner, J.-S. Tung, E. Fleissner, and P. W. Robbins (personal communication). It is entirely possible that the single nucleotide change that apparently results in a different electrophoretic mobility of the gp70s of the G(IX) (+) and G(IX) (-) viruses is also responsible for the presence or absence of the G(IX) antigenic determinant; however, the validity of this possibility awaits further investigation.

Molecular epidemiology of DNA viruses: applications of restriction endonuclease cleavage site analysis

Restriction endonucleases which cleave DNA at specific nucleotide sequences can be used to produce a set of DNA fragments of a viral genome which, when separated by gel electrophoresis, gives a characteristic "fingerprint" for that virus genome. This simple technique has been used to identify and classify DNA viruses of the herpes, adeno, and papova virus groups. Small variants within a given type (e.g., herpes simplex type I) are genetically stable and permit study and identification of individual strains of viruses. Such analyses have recently been applied to study the epidemiology of some DNA virus outbreaks. Restriction endonuclease fingerprinting provides a useful addition to methods for virus identification and classification.

Mutant and recombinant avian retroviruses with extended host range

Avian retroviruses of subgroups B and D efficiently infect chicken (C/E) but not turkey (T/BD) cells. We describe here three variants of subgroup B and D viruses that infect both cell types equally well. One of these viruses, NTRE-4, was a recombinant between transformation-defective Prague (Pr) strain Rous sarcoma virus (RSV) subgroup B and the endogenous virus RAV-0; the second, SR-DE-1, was a recombinant between Schmidt-Ruppin RSV subgroup D and defective endogenous virus information. T1 oligonucleotide fingerprint analysis of the genomes of these two viruses showed only a small alteration in the portion of the env gene responsible for subgroup specificity, as indicated by the presence of a single subgroup E oligonucleotide in an otherwise purely subgroup B or D gene. The third virus, hrBO1Pr-B, was a variant of Pr-RSV-B that did not appear to be a recombinant and whose altered host range we attribute to mutation. Analysis of the host range of all three viruses by infection of selectively resistant cells and by interference testing indicates that all use the subgroup B receptor on chicken cells and the subgroup E receptor on turkey cells. These viruses may be analogous to the polytropic recombinant viruses recently found to be associated with leukemia in some strains of mice.

Genome organization of retroviruses. VI. Heteroduplex analysis of ecotropic and xenotropic sequences of moloney mink cell focus-inducing viral RNA obtained from either a cloned isolate or a thymoma cell line

The genome of a recombinant murine leukemia virus capable of inducing focal areas of morphological alteration in mink lung fibroblasts was studied by heteroduplex analysis. The dual-tropic recombinant virus was isolated from a thymoma cell line (Th16.3) and is referred to as BALB/Moloney mink cell focus-inducing virus (BALB/Mo-MCF virus). The nucleic acid sequences of RNA from virions obtained from either a thymoma cell line (Th16.3) or a clonal isolate (BALB/Mo-MCF81) were compared with the genomes of ecotropic and xenotropic viruses. The following inferences were drawn (i) A single nonhomologous region (substitution loop alpha) of about 0.7 kilobase was observed in a heteroduplex formed between Moloney murine leukemia virus complementary DNA (cDNA) and BALB/MoMCF81 RNA. This nonhomology region was mapped between 1.71 and 2.40 kilobases from the 3' end of the genome. (ii) The predominant class of heteroduplexes formed between virion RNA obtained from the thymoma cell line (Th16.3) and Moloney murine leukemia virus cDNA showed a substitution loop similar to that observed with the RNA obtained from a cloned isolate, BALB/Mo-MCF81. However, there were other molecules with additional regions of nonhomology. (iii) Heteroduplexes formed between NZB xenotropic RNA and ecotropic Moloney murine leukemia virus cDNA exhibited four major nonhomology regions extending 0.75 to 1.46, 2.0 to 2.8, 3.6 to 4.3, and 7.4 to 7.9 kilobases from the 3' end of the genome. (iv) The MCF-specific substitution loop alpha (1.71 to 2.40 kilobases) appeared as a duplex region when NZB xenotropic RNA was hybridized to cDNA transcripts synthesized by virions obtained from thymoma cell line Th16.3. The position of the other substitution loops observed in a heteroduplex formed between NZB xenotropic RNA and Moloney murine leukemia virus cDNA was not affected. (v) Heteroduplexes formed between xenotropic BALB virus 2 cDNA and NZB xenotropic RNA demonstrated a large degree of nucleic acid sequence homology. Of the 29 heteroduplexes examined, 24 appeared to be homoduplexes, and in the remaining 5 heteroduplexes only one region of nonhomology located between 3.2 and 3.8 kilobases from the 3' end of the genome could be identified. Hybridization of BALB virus 2 xenotropic RNA to NZB xenotropic cDNA followed by digestion with single-strand-specific nuclease S1 showed an 80% sequence homology.

Glycoprotein encoded by the Friend spleen focus-forming virus

The Friend spleen focus-forming virus (F-SFFV) released from cultured erythroleukemia cells (cell line F4-6/K) was cloned free of its helper lymphatic leukemia virus (F-MuLV). After allowing adsorption to Sc-1 fibroblasts at a low multiplicity of infection, the cells were seeded individually into wells of a microtitier test plate and the resulting colonies were grown into large cultures. Among 14 of these cell cultures that have been analyzed thoroughly, 6 contained F-SFFV alone, 1 contained F-MuLV plus F-SFFV, and 7 were uninfected. Each of the Sc-1 cell lines which had been infected with cloned F-SFFV contained a glycoprotein with an apparent molecular weight of 55,000 (gp55) that was absent from the cell lines that lacked F-SFFV. gp55 was also present in Friend erythroleukemia cells and in fibroblasts infected with an F-SFFV that had been doubly cloned in another laboratory. These results indicate that gp55 is encoded by the F-SFFV genome. gp55 has the following additional properties. It can be immunoprecipitated with antiserum made to the F-MuLV virion envelope glycoprotein (gp75). Its unglycosylated polypeptide, formed in cells treated with 2-deoxy-D-glucose, has a molecular weight of approximately 45,000. Its tryptic peptide map contains peptides in common with F-MuLV gp75 but it also contains unique peptides. It appears to be absent or present in only low concentrations in erythroleukemia cell plasma membranes as determined by lactoperoxidase-catalyzed iodination, and it accumulates intracellularly in large amounts. In addition, it is absent from released virions. The majority of the cellular gp55 has an isoelectric point of 8.5 to 9.0. These results are consistent with the idea that an env gene recombination event was involved in the origin of F-SFFV.

Isolation and characterization of a mouse cell line containing a defective Moloney murine leukemia virus genome

A culture of mouse cells containing a 1,000-nucleotide deletion mutant of Moloney murine leukemia virus has been isolated. The deletion did not affect the size or function of the 21S mRNA that encodes the env gene products. Both the deleted RNA and the 21S mRNA were recovered in polyribosomes. Cells containing the deleted virus made no detectable Pr180gag-pol. Pr65gag synthesis with also absent, but a 45,000-molecular-weight gag gene product was found that might be encoded by the deleted genome. Biosynthesis of Pr80env proceeded normally in these cells; the intracellular precursor was cleaved and migrated to the cell surface as gp70. The cells could not be superinfected by homologous Moloney murine leukemia virus presumably because of surface restriction due to the gp70. Although the cells express the Moloney murine leukemia virus gp70 on their surface, they will not make pseudotypes after infection with vesicular stomatitis virus implying that Pr65gag may play a critical role in pseudotype formation. Induction of endogenous virus expression in the cells carrying the deletion mutant generated an N-tropic murine leukemia virus that can fuse XC cells. This may represent a recombinant between the deletion mutant and an endogenous virus.

Virus-like 30S RNA in mouse cells

Uninfected JLS-V9 mouse cells are known to express high levels of viral sequences that hybridize to complementary DNA made by the BrdU-induced virus of JLS-V9 cells. The genome in the BrdU-induced virus has been found to consist mainly of an RNA species that migrates as 30S RNA material during electrophoresis through agarose gels. This virus-like 30S RNA, designated VL30 RNA, apparently represents a new class of endogenous defective retroviruses that are not generally evident because of their defectiveness and lack of biological function. Fingerprint analysis and hybridization studies show that VL30 RNA does not have homology with the standard nondefective murine leukemia viruses. Upon superinfection with a nondefective murine leukemia virus, or upon induction of endogenous virus with BrdU, VL30 RNA is rescued into virions by phenotypic mixing. When VL30 RNA is rescued by BrdU induction, the VL30 RNA is mainly organized as a 50S complex, but when VL30 is rescued by superinfection, VL30 is also found in 70S RNA. Rescued VL30 RNA sequences can be reverse transcribed by the virion-associated DNA polymerase in an endogenous reaction. Many mouse cells express the sequences, whereas heterologous cells such as rat or rabbit cells do not contain them. By using hybridization of a complementary DNA probe to cellular RNA immobilized on paper, no subgenomic RNA related to the VL30 RNA could be found in cells expressing the VL30 sequences. From 20 to 50 copies of these sequences were found to be contained in the mouse genome. VL30 RNA is probably present in most stocks of leukemia and sarcoma viruses made in mouse cells.

Structure of the murine leukemia virus envelope glycoprotein precursor

The glycosylated env gene precurosr (Pr80env) of Moloney murine leukemia virus has been isolated by selective immunoprecipitation. Use of the drug tunicamycin to inhibit nascent glycosylation or specific cleavage with endoglycosidase H demonstrated that the precursor contained an apoprotein with a molecular weight of 60,000. The finished virion glycoprotein (gp70) was largely resistant to the action of endoglycosidase H. Chromatography of the glycopeptides of Pr80env in conjunction with endoglycosidase H digestion studies suggested that the precursor contained two distinct major glycosylation sites. Analysis of partial proteolytic cleavage fragments of Pr80env before and after endoglycosidase H treatment placed the two glycosylation sites within a 30,000-dalton region of the apoprotein sequence. Kinetic experiments showed that carbohydrate processing as well as proteolytic cleavage are late steps in the maturation of Pr80env.

Heteroduplex analysis of the nonhomology region between Moloney MuLV and the dual host range derivative HIX virus

The dual host range virus HIX has been previously characterized as an envelope gene recombinant between Moloney murine leukemia virus (Mo-MuLV) and an unidentified xenotropic murine leukemia virus. Using long reverse transcripts of Mo-MuLV, a region of nonhomology has been mapped by electron microscopic analysis of heteroduplexes formed with HIX 35S virion RNA. In this nonhomology region, the Mo-MuLV cDNA strand measured approximately 900 nucleotides, mapping between 1.6 and 2.5 kilobases from the 3' end. In a previous study, hybridization of Mo-MuLV 21S RNA with Mo-MuLV cDNA resulted in the formation of different heteroduplex structures diagnostic of a noncontiguously coded leader sequence at the 5' end of the 21S RNA. Following hybridization of poly(A)+ HIX 21S RAN with 8.2 kb Mo-MuLV cDNA, analogous heteroduplex structures were observed exhibiting the Mo-MuLV:HIX substitution loop in the DNA:RNA segment of the molecules. This analysis permitted more precise mapping of the nonhomology region with respect to the splice point in the 21S presumptive glycoprotein mRNA. The mapping of this nonhomology region in HIX virus provides an internal visual marker for the 3' end of the genome which may prove useful in future analyses of other deletion or substitution derivatives of Mo-MuLV.