Electron microscopic evidence for splicing of Moloney murine leukemia virus RNAs

Latent murine leukemia virus infection characterized by the release of non-infectious virions

Clonal cell lines derived from cultures infected with a polytropic MuLV release vastly different levels of infectious virions ranging from undetectable to very high. Low producing clones release an overwhelming proportion of non-infectious virions containing retroviral RNA but deficient in the Env protein. Non-infectious virion production is not due to an inability of the cells to support infectious MuLV production or to an inherent replicative defectiveness of the proviruses. Reinfection of the lowest producing lines with the polytropic or an ecotropic MuLV results in enormous increases in the specific infectivity of the released virions. This indicates a reversible state of retroviral latency characterized by the release of non-infectious virions that is likely the result of insufficient levels of Env protein required for infectivity. The latency state described here may have important roles in in vivo retroviral infections including alterations of the immune response and the production of defective interfering particles.

An overview of retrovirus replication and classification

This introductory chapter has presented an overview of how retroviruses replicate and how they are classified within the family Retroviridae. The genomic structure of retroviruses, so reminiscent of bacterial transposons and other similar genetic elements, and reverse transcriptase, which leads to the reverse flow of genetic information from RNA to DNA, are responsible for many of the properties of these viruses which make them both fascinating and important as causes of cancer and other diseases. The requirement for integration shared by most retroviruses leads directly to most of the phenomena resulting from their interaction with target cells. Certainly latency, at the level of the organism, is one such property relevant to how we think of vaccines and therapeutic reagents. The ability of retroviruses to acquire oncogenes from cellular DNA has greatly facilitated our understanding of the genetics of neoplasia. Additionally, the use of retroviral vectors to introduce new genes into genetically defective animals is a consequence of the genetic organization of retroviruses. Classification of viruses at the species level is difficult for several reasons. In particular, viruses do not sexually reproduce in any conventional sense, and it is difficult to identify a population of virions which make up a genetically distinct pool. Thus, the definition of individual species is often controversial and is not necessarily aided by the criteria used to define larger phylogenetic groups. In the latter case, retroviruses have distinctive morphological and biochemical features which allow their classification at the family, subfamily, genus, and subgenus levels. Additional classification occurs by accounting for factors such as host range, cross neutralization, ability to compete in interspecies radioimmunoassays, and genetic homology detected by hybridization under conditions of relaxed stringency. Direct comparison of nucleotide sequences offers the hope that mathematical criteria will be developed that can define the level of differences characteristic of individual species, genuses, and subfamilies.

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.

Generation of a uniform 3' end RNA of murine leukemia virus

Using the S1 nuclease mapping technique, we demonstrated that the majority of Moloney murine leukemia RNA molecules, isolated either from the nucleus or cytoplasm of infected mouse cells, share a uniform 3' end located at the border of the R and U-5 regions of the long terminal repeat. When the long terminal repeat sequences were inserted in the pSV plasmid downstream of the simian virus 40 late promoter, the 3' end of the viral RNA was also generated close to the R region of the long terminal repeat. These results demonstrate that the long terminal repeat signals the generation of an authentic 3' end when situated downstream of an actively transcribed region.

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).

Metabolism of viral RNA in murine leukemia virus-infected cells; evidence for differential stability of viral message and virion precursor RNA

Molecular hybridization techniques were used to examine the stability of viral message and virion precursor RNA in murine leukemia virus-infected cells treated with actinomycin D. Under the conditions used, viral RNA synthesis was inhibited, but viral protein synthesis continued, and the cells produced noninfectious particles (actinomycin D virions) lacking genomic RNA (J. G. Levin and M. J. Rosenak, Proc. Natl. Acad. Sci. U.S.A. 73:1154-1158, 1976). Analysis of total RNA in virions revealed that the amount of hybridizable viral RNA decreased steadily after the addition of actinomycin D and by 8 h was 10% of the control value. Studies on fractionated viral RNA showed that this low level of hybridization is due to residual 70S RNA in the virion population. The results indicated that viral RNA which is destined to be encapsidated into virions has a half-life of approximately 3 to 4 h. In contrast, other intracellular virus-specific RNA molecules appeared to be quite stable and persisted for a long period of time, with a half-life of at least 12 h. These observations support the idea that two independent functional pools of 35S viral RNA exist within the infected cell: one serving as message and the other as precursor to virion RNA. The existence of two viral RNA pools was further documented by the finding that 12 h after the addition of actinomycin D, when virion precursor RNA was depleted, 35S and 21S viral nRNA species could be identified in polyribosomal RNA as well as in total polyadenylated cell RNA. Surprisingly, 35S and mRNA declined more rapidly than did 21S mRNA, which appeared to be increased in amount.

Sub-genomic RNA in Moloney leukemia virus grown in lymphoid-derived cell lines consists primarily of homologous viral RNA

Moloney murine leukemia virus (MoMuLV) grown in 2 lymphoid-derived murine cell lines (JLS-V9 and TB) contained 2 size classes of RNA subunits: 2.8 +/- 0.4 x 10(6) (n = 30) and 1.6 +/- 0.1 x 10(6) (n = 15) daltons. Detectable levels of low molecular weight viral RNA (LMW vRNA) were not present in MoMuLV grown in mouse embryo fibroblasts, nor rat cells, nor was it produced by them when they were infected with MoMuLV containing both species of vRNA. DNA probes complementary to both subunits were synthesized separately using purified reverse transcriptase (R.T.) and calf thymus DNA as primer. Polyadenylated LMW vRNA was selected and, by molecular hybridization was found to be completely homologous with the large RNA subunit. This was confirmed using cDNA probes representing defined regions of the genome. The LMW vRNA therefore contains multiple subsets of the viral RNA, which could correspond to multiple deletion mutants possibly generated by an efficient RNA splicing mechanism. In addition, low levels of non-homologous virus-like RNA were detected in MoMuLV grown in TB (2%) AND NIH/3T3 cells (0.4%).

Viral genome RNA serves as messenger early in the infectious cycle of murine leukemia virus

When NIH/3T3 mouse fibroblasts were infected with the Moloney strain of murine leukemia virus, part of the viral genome RNA molecules were detected in polyribosomes of the infected cells early in the infectious cycle. The binding appears to be specific, since we could demonstrate the release of viral RNA from polyribosomes with EDTA. Moreover, when infection occurred in the presence of cycloheximide, most viral RNA molecules were detected in the free cytoplasm. Size analysis on polyribosomal viral RNA molecules indicated that two size class molecules, 38S and 23S, are present in polyribosomes at 3 h after infection. Analysis of the polyriboadenylate [poly(rA)] content of viral RNA extracted from infected polyribosomes demonstrated that such molecules bind with greatest abundance at 3 h after infection, as has been detected with total viral RNA. No molecules lacking poly(rA) stretches could be detected in polyribosomes. Furthermore, when a similar analysis was performed on unbound molecules present in the free cytoplasm, identical results were obtained. We conclude that no selection towards poly(rA)-containing viral molecules is evident on binding to polyribosomes. These findings suggest that the incoming viral genome of the Moloney strain of murine leukemia virus may serve as a messenger for the synthesis of one or more virus-specific proteins early after infection of mouse fibroblasts.