Effect of growth rate on phenotypic expression of 6-thioguanine resistance in human diploid fibroblasts.

The influence of growth rate on the kinetics of phenotypic expression of 6-thioguanine resistance has been investigated by establishing 3 protocols for the maintenance of mutagenized stock cultures of human diploid fibroblasts. These protocols differed in the frequency with which subculture was performed during the phenotypic expression period. The resultant growth kinetics varied widely from continual exponential proliferation to density-inhibited growth for much of the expression period. With all of the subculture protocols, a maximum mutation frequency was reached by 9 days after exposure of confluent cultures to 6 J/m2 of 254 nm UV light. The maximum frequency was similar in all experimental groups and remained stable up to 17 days after irradiation, the last time point tested. These results indicate that continual proliferation is not required for optimal expression of 6-thioguanine resistance.