Characteristics of revertants induced by EMS and u.v. light from a 6-thioguanine resistant HGPRT deficient V79 Chinese hamster cell line.

A spontaneous TG(R) HAT(s) V79 mutant had an HGPRT specific activity of less than 5% that of the wild type. Only two spontaneous revertants were recovered from more than 10(8) mutant cells screened. However, HAT resistant clones were recovered, after EMS mutagenesis at a frequency of 4.5 x 10(-6) and at a lower frequency from u.v. irradiated mutant cells. Of more than 400 clones tested, over 90% had regained a HAT(R) TG(s) HGPRT(+) revertant phenotype. Nine of these revertants that were further characterised had HGPRT specific activities within the wild type range and showed wild type electrophoretic mobility. These data suggest that the original HGPRT(-) phenotype was of a mutational rather than an adaptational origin.