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 ofEscherichia coli in (i) media containing the natural content of40K, (it) media containing potassium from which essentially all of the 40K had been removed by isotope separation, and (ii) media highly enriched in 40K. Growth rates (doubling times) were identical in the presence 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.The biological effects of low levels of ionizing radiation have received much attention in recent years (1, 2). One of the major uncertainties in such studies results from the almost total lack of direct experimental information on the genetic effects of natural background radiation. In the absence of such information, the shape of the dose-response curve at radiation levels below natural background becomes a matter ofopinion and this has generated much controversy. We have therefore undertaken an experimental approach to this important problem by a measurement of the contribution made by 4K to the spontaneous mutation rate in various strains of Escherichia coli. Potassium, the principal intracellular cation, is essential to life. Natural potassium is a mixture of isotopes, and one of these, 40K, present at an isotopic abundance of 0.012%, is radioactive with a half-life of 1.28 x 109 years. 40K decays principally by emission of (3-rays (3), but about 10% of the radioactive decay is by orbital electron capture, which produces very-low-energy Auger electrons. As NaH2PO4, 20 mM; MgSO4'7H20, 0.4 mM; CaCl2-2H2O, 6.8 jiM; FeSO4-7H20, 0.9 jiM. The pH of the medium was adjusted to 7.3-7.5 with concentrated hydrochloric acid. This medium was supplemented with potassium of the desired isotopic composition and concentration. Vogel-Bonner medium (9) and M9 medium (10) were also used as standard defined media. On occasion, we used a medium similar to M9, denoted M8, in which equimolar concentrations of sodium salts replaced the potassium salts in M9 medium.
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