SUMMARY: Two temperature-sensitive mutants of defective in DNA synthesis, which were very resistant to the lethal effect of ultraviolet and ionizing radiation at permissive temperatures, became sensitive to radiation and also to the action of -methyl--nitro--nitrosoguanidine when held at the restrictive temperature of 39 °C. With ts1 the sensitization began soon after transfer to 39 °C and reached a maximum 4 h later. During this period there was no loss of viability. After 4 h the shoulders of the ultraviolet and ionizing radiation survival curves had almost completely disappeared and the exponential part of the curves had doubled in slope. The size of the shoulder fell exponentially with the time the bacteria were held at 39 °C. Sensitization occurred in the presence of chloramphenicol. During the period the bacteria were held at 39 °C their ability to effect recombination as measured by transformation fell exponentially and was correlated with the rate of loss of the shoulder. This suggests that the repair which gives rise to the large shoulders of the radiation survival curves is of the post replication recombination type.

The recovery of radiation resistance at 30 °C in bacteria which had been exposed to 39 °C for 75 min did not begin immediately. For 55 min there was no measurable increase in resistance but after 75 min substantial recovery had occurred and by 105 min was complete. Recovery of resistance did not occur in the presence of chloramphenicol even when the chloramphenicol was added 30 min after the bacteria had been at 30 °C.

The sensitization to radiation was not a general property of temperature-sensitive (ts) mutants.


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