Nucleotide Sequence Relationships of Avian RNA Tumor Viruses: Measurement of the Deletion in a Transformation-Defective Mutant of Rous Sarcoma Virus

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Stocks of cloned helper-independent Rous sarcoma virus (RSV) spontaneously segregate transformation-defective (td) mutants that appear to have an RNA genome composed of smaller subunits than those of the patent virus. Differential hybridization and competitive hybridization techniques involving reactions between viral RNA and proviral sequences in host cell DNA (under conditions of initial DNA excess) were used to measure the extent of the deletion in a td mutant of Prague strain (Pr) of RSV (Pr RSV-C). Viral 60 to 70S RNA sequences labeled to 1 to 5 × 107 counts per min per μg with 125I were characterized with respect to their properties in hybridization reactions and used to reinforce data obtained with [3H]RNA of lower specific activity. By these techniques, about 13% ± 3% of the sequences Pr RSV-C that formed hybrids with DNA from virus-induced sarcomas appeared to be deleted from the genome of td Pr RSV-C. Studies comparing hybridization of RNA from Pr RSV-C and td Pr RSV-C with RSV-related sequences in normal cells, and competition experiments with RNA from the endogenous chicken oncornavirus Rous-associated virus type 0 (RAV-0) provided evidence that the majority, if not all, of the RNA sequences of Pr RSV-C deleted from its transformation-defective mutant are not represented in normal chicken DNA. Competition studies with a leukosis virus, RAV-7, indicated this virus also lacks a genome segment of about the same size as the deletion in the td mutant. Finally, the genome of all three “exogenous” viruses was found to lack a small segment (about 12%) of sequences present in the endogenous provirus of RAV-O.

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