The nature of mutants by ionising radiation in cultured hamster cells: I. Isolation and initial characterisation of spontaneous, ionising radiation-induced, and ethyl methanesulphonate-induced mutants resistant to 6-thioguanine

Abstract

A large number of thioguanine (TG)-resistant mutants of V79-4 Chinese hamster cells was isolated from untreated cultures and from cultures exposed to γ-rays, α particles or ethyl methanesulphonate (EMS). Selection condictions were chosen to optimise the survival of all types of TG-resistant mutant, and the isolation procedure ensured that each mutant originated independently of any other. Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) enzyme activity was measured in cell-free extracts of each mutant, and compared with repeat measurements made on the parental V79-4 cells and on a series of non-mutant clones. Ionising radiation-induced mutants were found to be mostly (or perhaps entirely) of the ‘zero HGPRT activity’ type, but about 20% of EMS-induced mutants and 50% of spontaneously occurring mutants had significant HGPRT activity. However, none of the TG-resistant mutants were found to lack activity of another X-chromosome-linked enzyme, glucose-6-phosphate dehdrogenase. Few mutants were found with visible X-chromosome changes, but the incidence of hyperploidy was higher among spontaneous mutants than in the parental line and the induced mutants. Isoelectric focussing of cell extracts from those mutants which retained some HGPRT activity revealed several with shifts in the isoelectric points for HGPRT enzyme activity.