Concordance of the RNA termini of recombinants from crosses between avian retroviruses with different termini

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.

Participation of subgenomic retroviral mRNAs in recombination

Envelope glycoprotein (env) mRNA from avian retroviruses was injected into cells transformed by env-deleted Bryan Rous sarcoma virus [RSV(-)]. The genetic deficiency of RSV(-) was complemented, and infectious transforming virus was released for many days after these injections. The long-term activity of the injected env mRNA is believed to be due to reverse transcription of the injected RNA after its incorporation into virus particles. The resulting subgenomic provirus, presumed to be integrated into host DNA, is able to direct the continuous synthesis of additional env mRNA. In some of these cultures, replication-competent viruses appeared many days after injection. The analysis by RNase T1 oligonucleotide fingerprinting showed that the RNA of these virus genomes contained oligonucleotides characteristic of both RSV(-) and the env mRNA injected. In all viruses analyzed the 5' two-thirds and the 3' terminus of the genome were derived from RSV(-) and the env gene from the injected mRNA. Our results thus strongly indicate that these viruses were generated via recombination between RSV(-) and env mRNA. The demonstration of involvement of an mRNA sequence in recombination may be of importance in the divergence of retroviruses and in the mechanism of interaction between retroviruses and host nucleotide sequences.

Viral genes involved in leukemogenesis. I. Generation of recombinants between oncogenic and nononcogenic mouse type-C viruses in tissue culture

An approach toward elucidation of the mechanisms of action of mammalian leukemia viruses has been made by the generation in tissue culture of recombinant viruses between a potent murine leukemia virus (MuLV), Rauscher-MuLV, and an endogenous xenotropic mouse type-C virus, BALB:virus-2, without known malignant potential. Using a double selection system devised to select against the temperature-sensitive (ts) lesion associated with a mutant of Rauscher-MuLV and the xenotropic host range of BALB:virus-2, recombinant viruses were obtained at frequencies ranging from 0.01 to 0.1%. Recombinant viruses were identified on the basis of the type specific antigenic determinants in the translational products of gag (p15, p12, p30, and p10 proteins), pol (reverse transcriptase), and env (gp70 glycoprotein) genes. By this approach, the partial genetic maps of a large number of recombinants were obtained. The fact that p10 of Rauscher-MuLV ts 25, the mutant utilized, was the only protein uniformly lacking in recombinant viruses, localized the lesion inhibiting gag precursor cleavage in this mutant at the carboxy terminus of its gag gene. The recombinant viruses demonstrated two host range phenotypes as defined by Fv-1 host cell restriction. In each case, NB-tropic recombinants possessed the p30 of BALB:virus-2 p30. Thus, it was possible to assign the site of Fv-1 action at, or closely linked, to the viral p30. The target within the viral genome of a second host restriction was also mapped. A serum factor, previously shown to specifically inactivate xenotropic virus infectivity, was demonstrated to exert its action on the viral env gene product. The system described here allows the generation of specific recombinant genotypes that should be useful in defining those regions of the viral genome involved in leukemogenesis.