Characterization of a conditional mutant of Rous sarcoma virus with alterations in early and late functions of cell transformation

• Published in: Virology (New York, N.Y.) Vol. 67; no. 2; pp. 474 - 486
• Main Authors: Bookout, J.B., Sigel, M.M.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.01.1975
• ISSN: 0042-6822; 1096-0341
• DOI: 10.1016/0042-6822(75)90448-1

Mutants were isolated from γ-irradiated stocks of Schmidt-Ruppin Rous sarcoma virus (SR-RSV-2). One isolate, MI-100, displayed unusual properties in its temperature sensitivity of cell-transforming capabilities. Focus formation, colony formation in soft agar, and increased [ 3H]deoxyglucose uptake by infected cells were all rendered temperature sensitive ( ts); 37° was permissive for expression of these functions while 41° was nonpermissive. Tumorigenicity in chickens was greatly diminished. Virus replication was not defective since virus yields exceeded those of wild-type RSV at 37 and 41°. Yields of infectious progeny may have been higher than shown by titrations, as virions of MI-100 were more heat labile than wild type. Group-specific antigen concentrations in MI-100 infected were almost equivalent at 37 and 41° and greater at both temperatures than those of wild-type-infected cells. Through genetic recombination with Rous-associated virus (RAV-1), subgroup specificity of MI-100 was altered from B to A without correcting the temperature sensitivity of transformation. However, temperature-shift experiments demonstrated a major difference in the ts properties of the mutant and its recombinant. MI-100 was unable to transform cells at 37° if initial incubation after infection was at 41° (irreversible inhibition); also, shifts of infected cells, at any time, from 37 to 41° resulted in loss of the transformed phenotype (reversible inhibition). Cells infected with the recombinant still required permissive temperature in order to express the transformed phenotype, but initial incubation at 41° after infection did not affect the ability to transform cells at 37°. Thus there appeared to be ts lesions in MI-100 affecting transformation, one in an early function and one related to a late function. Recombination with RAV-1 restored the early but not the late functions, suggesting that leukosis viruses possess a gene coding for some initial event in fibroblast transformation but lack the gene(s) required for full expression (maintenance) of the transformed state. Moreover, as shown by the replicative properties of the mutant, the early gene of transformation initiation is apparently not involved in virus replication.