Recovery of avian sarcoma virus from tumors induced by transformation-defective mutants.

Transformation-defective (td) mutants of the Schmidt-Ruppin strain of Rous sarcoma virus (RSV), which contains deletions in the gene responsible for transformation (src gene), are unable to transform chicken embryo fibroblasts in vitro. Injection of some of these td mutants into newborn chickens resulted in the formation of sarcomas from which sarcoma virus was unfailingly recovered. The possibility that transforming RSV was present in the td virus preparations was excluded by further purification of the td viruses. Morphology of the foci induced by the newly recovered sarcoma virus was distinct from that of foci induced by the parental Schmidt Ruppin strain of RSV. It is suggested that the new sarcoma virus was generated as a result of the genetic interaction between the genomes of td virus and chicken cells.

Characterization of some isolates of newly recovered avian sarcoma virus.

We previously reported the isolation of a newly recovered avian sarcoma virus (rASV) from tumors of chickens injected with transformation-defective (td) mutants of the Schmidt-Ruppin strain of Rous sarcoma virus (SR-RSV). In this paper, we present further biological and biochemical characterization of the recovered sarcoma viruses. High titers of rASV's were generally obtained by cocultivation of tumor cells with normal chicken embryo fibroblasts or by homogenization of tumor tissues. Most rASV isolates were similar to SR-RSV, subgroup A (SR-RSV-A), in their growth characteristics and were nondefective in replication. The subgroup specificity of rASV's and the electrophoretic mobilities of their structural proteins were the same as those parental td viruses. The nondefectiveness of rASV's was further substantiated by the size of their genomic RNA, which was indistinguishable from that of SR-RSV-A and substantially larger than that of parental td RNA. Molecular hybridization using complementary DNA specific to the src gene of SR-RSV (cDNAsrc) showed that the RNAs of td mutants used in this study contained extensive deletions within the src gene (7 to 30% hybridization with cDNAsrc); the same probe hybridized up to 90% with RNA from two isolates of rASV. These data indicate that rASV has regained genetic information which had been deleted in the td mutants and strongly suggest that the generation of rASV involves a genetic interaction between td virus and host cell genetic information.

Recombination between viral and cellular sequences generates transforming sarcoma virus.

A series of sarcoma viruses has been obtained from tumors induced by transformation-defective (td) mutants of the Schmidt-Ruppin strain of Rous sarcoma virus, subgroup A (SR-A). The RNA sequences of these "recovered avian sarcoma viruses" (rASVs) were compared with those of td mutants and of SR-A by oligonucleotide fingerprinting. Of six sarcoma-specific oligonucleotides present in SR-A RNA, three to six were missing in the RNAs of the four td mutants examined. All six isolates of rASV examined have regained these six oligonucleotides. In addition, most rASV RNAs have three new oligonucleotides not present in the RNA either of td mutants or of SR-A. The newly obtained oligonucleotides are located between 800 and 2600 nucleotides from the 3' end of rASV RNA, which corresponds to the src region of SR-A RNA mapped previously. Furthermore, viral RNAs of two td mutants isolated from a clone of rASV lack most src-specific oligonucleotides, including the three new ones. No differences were found among RNAs of td, SR-A, and rASV in the regions outside of src. Our results indicate that RNA sequences that rASVs have acquired from cells in the process of conversion from td virus to transforming virus are mapped within the src region and segregate with the transforming function. Some of the sequences are new and some are identical with those in SR-A RNA.