Escherichia coli and bacteriophage T4 DNA containing [2-3H]adenine accumulated crosslinks between the complementary strands. For T4 DNA stored in frozen solution there were 0.41 to 0.54 crosslinks formed per tritium decay. The crosslinks were demonstrated both by an increased DNA sedimentation rate in alkaline sucrose gradients and by an increasing amount of DNA that renatured quickly after denaturation by heat or alkali. Single-strand breaks were also formed with an efficiency of 0.08 to 0.50 breaks per tritium decay. DNA containing both [8-3H]adenine and [8-3H]guanine showed no crosslinking but did undergo single-strand breaks at a rate of 0.08 per tritium decay. T4 bacteriophage containing [2-3H]adenine lost plaque-forming ability when stored at 4 °C, with 0.34 lethal hits per tritium decay, whereas the same phage labeled with a mixture of [8-3H]adenine and [8-3H]guanine sustained only 0.12 lethal hits per tritium decay. The loss of plaque-forming ability in the latter case is probably due to a radiation effect from the emitted beta particle; the high lethal efficiency for tritium decay at 2-adenine is probably caused either by crosslinks between complementary strands or from some undetected lesion produced in the DNA.
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