Five temperature-sensitive mutants (ts 1 to 5) were isolated from a stock of the Moloney strain of murine sarcoma leukaemia virus complex which had been mutagenized by ultraviolet irradiation or -methyl-′-nitro--nitrosoguanidine. In mouse cells at the non-permissive temperature the mutants formed fewer foci of transformed cells than at the permissive temperature.

The ts mutants were characterized by testing: (1) murine leukaemia virus (MuLV) clones from the ts complex, (2) the effect of additional wild type MuLV on focus formation, (3) focus formation in rat cells and (4) focus formation with pseudotypes rescued from non-producer cells. Two mutants (ts 1 and ts 3) were found to be ts MuLVs which did not possess heat labile virion proteins and were not ts in post-penetration helper functions necessary for the fixation of sarcoma virus transformation. The remaining three mutants (ts 2, ts 4 and ts 5) were ts murine sarcoma viruses which, however, showed no temperature-sensitive effect on the maintenance of transformed cell morphology nor on colony forming efficiency in soft agar.


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