Repair of X-ray damage in Micrococcus radiodurans

Micrococcus radiodurans , a red-pigmented bacterium, is very resistant both to ionizing and to ultra-violet radiations. The dose-response curves in both cases are sigmoidal with high extra-polation numbers. Two non-pigmented mutants of M . radiodurans have been isolated. One of these shows the same type of survival curve as the wild type while the other has an exponential survival curve. Energy transfer reactions including the possible role of pigment have been excluded as a mechanism of resistance. Similarly, resistance is not due to unusual amounts or unique properties of the bacterial DNA . Survivors of both sensitive and resistant bacteria show a lag period following irradiation. In the case of sensitive bacteria no change occurs in the radiosensitivity of the cells during this period. On the other hand, M . radiodurans recovers during the lag period, at the end of which it has become as resistant as the unirradiated cells. All available evidence for the loss of viability in bacteria points to DNA as the target. Recovery of M . radiodurans from radiation damage, therefore, is concluded to be due to repair of damaged DNA by an active cellular, probably enzymic process which occurs during the lag period. The large shoulder of the survival curve and the high extrapolation number are a reflexion of the efficiency of the repair system. The significance of the sigmoidal and exponential survival curves of resistant bacteria is discussed and a theory put forward which postulates two types of damage to the genetic structures, each having a specific repair system. Some experimental evidence in support of this theory is produced.