Novel nonconditional mutants in the src gene of Rous sarcoma virus: isolation and preliminary characterization

We have isolated a number of nonconditional transformation-defective (td) mutants of Prague strain Rous sarcoma virus, subgroup A (PR-RSV-A). Many of these resembled td mutants reported previously, but 11 isolates from low-passage stocks of PR--A showed unusual properties and were designated partially td (ptd) mutants. In mixed infections with temperature-sensitive (ts) transformation-defective RSV mutants the ptd viruses produced cell transformation at restrictive temperature (41°), probably by genetic recombination to yield wild-type virus. In tests with a panel of 4 ts mutants, we found that different ptd isolates varied in the number and pattern of ts mutants with which they showed this effect. In mixed infections with one another the ptd viruses yielded transforming virus. Again, the pattern shown by different ptd viruses varied, and on the basis of this variation the 11 ptd isolates appear to comprise at least 10 distinct mutants. The possibility of genome deletions in some of the viruses was examined in Southern blots of EcoRI digests of proviral DNA. Two ptd viruses, which recombined with all 4 ts mutants tested, had EcoRI restriction fragments identical to those of wild-type PR-A. Three isolates which recombined with either 3, 2, or none of the ts mutants, showed deletions in the EcoRI fragment containing the src gene. These deletions corresponded to losses of 1.0, 1.5, and 1.6 kilobases, respectively, from the RNA genome. We conclude that these ptd viruses bear either point mutations or deletions of varying size but all retain part of the src gene. These mutants are stable and should be useful for further genetic and physiological studies on the src gene and its product.