Biological effects of background radiation: mutagenicity of 40K.

The naturally occurring radioactive isotope 40K is the single largest contributor to the internal background radiation dose in living organisms. We examined cell growth and mutation rate or frequency in several strains of Escherichia coli in (i) media containing the natural content of 40K, (ii) media containing potassium from which essentially all of the 40K had been removed by isotope separation, and (iii) media highly enriched in 40K. Growth rates (doubling times) were identical in the present or absence of 40K. In more than 40 chemostat experiments, we were unable to detect any significant differences in mutation rate to bacteriophage T5 resistance or in mutation frequency to valine resistance or tryptophan prototrophy attributable to 40K. We conclude that, in the bacterial systems we have studied, 40K does not make a significant contribution to spontaneous mutation.