The formation of variants with a reversion of properties of transformed cells. vi. stability of the reverted state

Abstract The stability of the reverted state has been analysed in revertants from polyomavirus‐transformed cells in which reversion was not associated with a loss of the virus genome. The regaining of two transformed properties, the ability to form colonies at high temperature (41° C) and in soft agar, were used as markers for re‐reversion. The frequency of re‐reversion was studied in 12 cloned revertants after different periods of in vitro cultivation. When tested at 45 days after isolation, there were no temperature re‐revertants and 2 to 184 agar re‐revertants per 5 × 104 cells. At 90 days after isolation, there were 14 to 784 temperature re‐revertants, and 242 to 1160 agar re‐revertants per 5 × 104 cells. The stability of the reverted state was thus decreased by in vitro cultivation. The study of isolated re‐revertants has indicated that, once a transformed property has been regained during re‐reversion, the immediate progeny of the re‐revertant may contain a high frequency of segregants which had lost this regained property. The re‐reverted state was more stable in re‐revertants from revertants that were 100 days after isolation, than in re‐revertants from revertants that were 55 days after isolation. Re‐reversion for one transformed property was not necessarily associated with re‐reversion for the other transformed property. The different degrees of stability are explained by differences in the balance between chromosomally located factors that determine the expression and suppression of transformed properties.