Reversible inactivation of autosomal alleles in Chinese hamster cells.

Evidence is presented which suggests that a CHO-derived thymidine kinase (Tk) heterozygote, ts201, can reversibly inactivate the wild type Tk allele, and that this event may result in simultaneous inactivation of a linked allele, galactokinase (Glk). The clone ts201 was isolated as a revertant of a stable Tk- 5-bromodeoxyuridine (BrdU)-resistant mutant of CHO. The reacquired Tk activity differed from that of the wild type with respect to Km and heat resistance, supporting the contention that ts201 was genetically heterozygous (Tk+/-). At frequencies varying from 0.02-0.5, segregants of ts201 could be isolated by cloning in BrdU at 39 degrees. These derivatives were phenotypically Tk- (by enzymology and by autoradiography of 3H-dT labeled cells), but after removal of BrdU, reacquired the Tk+ phenotype at frequencies varying randomly from clone to clone. From mutagenized populations of ts201 two variants were isolated, 71t and 72c, by selection at 39 degrees in 2-deoxygalactose (2-dgal). Resistance to 2-dgal has been correlated with a mutation in the gene for Glk, which is syntenic with that for Tk and evidence is presented suggesting that 71t and 72c are Glk+/-. Cloning efficiencies in doubly selective media and autoradiographic data showed that at 39 degrees a coordination existed between the levels of Tk and Glk, which in 71t was inverse, and in 72c, direct. These data led to the hypothesis that pairs of linked alleles can be inactivated or re-expressed simultaneously. The inactivating effect was temperature-sensitive, since at 33 degrees, the cloning efficiency in BrdU and in 2-dgal was very low.