DNA base sequence changes induced by ultraviolet light mutagenesis of a gene on a chromosome in Chinese hamster ovary cells

The DNA base sequence changes induced by mutagenesis with ultraviolet light have been determined in a gene on a chromosome of cultured Chinese hamster ovary (CHO) cells. The gene was the Escherichia coli gpt gene, of which a single copy was stably incorporated and expressed in the CHO cell genome. The cells were irradiated with ultraviolet light and gpt- colonies were selected by resistance to 6-thioguanine. The gpt gene was amplified from chromosomal DNA by use of the polymerase chain reaction (PCR), and the amplified DNA sequenced directly by the dideoxy method. Of the 58 sequenced mutants of independent origin 53 were base change mutations. Forty-one base substitutions were single base changes, ten had two adjacent (or tandem) base changes, and one had two base changes separated by a single base-pair. Only one mutant had a multiple base change mutation with two or more well separated base changes. In contrast much higher levels of such mutations were reported in ultraviolet mutagenesis of genes on a shuttle vector in primate cells. Two deletions of a single base-pair were observed and three deletions ranging from 6 to 37 base-pairs. The mutation spectrum in the gpt gene had similarities to the ultraviolet mutation spectra for several genes in prokaryotes, which suggests similarities in mutational mechanisms in prokaryotes and eukaryotes.