Hpt-13 is a Chinese hamster cell line deficient in hypoxanthine phosphoribosyltransferase (EC 188.8.131.52) and sensitive to a medium containing 10(-4) M hypoxanthine, 5.5 X 10(-6) M aminopterin, and 10(-4) M thymidine. In this cell line there is a high incidence of cells resistant to this selective medium after an incubation with either ethyl methane sulfonate or adenovirus type 2 complete virions or their incomplete particles. The rate of reversion in the presence of these agents was 34-fold higher with ethyl methane sulfonate and 2.5- to 5.6-fold higher with adenovirus particles than the spontaneous rate of reversion. The revertant phenotypes were stable for many generations without selective pressure. All of the revertants tested recovered the hypoxanthine phosphoribosyltransferase activity. Most of them, however, carried an enzyme of lower activity and faster electrophoretic mobility than that of the wild type. The preferential reversion to this type of enzyme was observed among spontaneous revertants as well as among those induced by mutagenesis with ethyl methane sulfonate or exposure to viral particles. Our results suggest that adenovirus particles and ethyl methane sulfonate induce mutations at the hpt locus of Hpt-13 cells through similar mechanisms
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