DNA Degradation by Decay from Incorporated Tritium Compounds in Escherichia coli

Radioactive decays from each of three intracellularly incorporated tritium$\text{compounds}-\text{thymine-methyl-}{}^{3}{\rm H}$ (a DNA precursor),$\text{uracil-}5{}^{3}{\rm H}$ (a RNA precursor), and$\text{histidine-}{}^{3}{\rm H}$ (a protein precursor)-caused degradation of DNA in Escherichia coli as measured by loss of$\text{thymine-}{}^{14}{\rm C}$ label previously incorporated into DNA. Degradation is specific for DNA, as cells labeled with$\text{uracil-}5{}^{3}{\rm H}$ or$\text{histidine-}{}^{3}{\rm H}$ show little loss of these RNA and protein specific precursors, even after extensive loss of$\text{thymine-}{}^{14}{\rm C}$ label. As much as 80% of the cellular DNA was degraded as a result of decays from each of the tritiated compounds; but the rates of degradation varied, being greater for$\text{thymine-methyl-}{}^{3}{\rm H}$ than for$\text{uracil-}5{}^{3}{\rm H}$ and$\text{histidine-}{}^{3}{\rm H}$ decays. The kinetics of DNA degradation for each of these incorporated compounds bear a relationship to each other that is similar to that previously reported for kinetics of inactivation produced by the same compounds in the same cell line.