MUTATIONS IN radA ARE RESPONSIBLE FOR ACQUIRED RADIO-RESISTANCE IN AN Escherichia coli STRAIN

When bacteria are exposed to chronic or cyclic irradiation with ultraviolet (UV) light, it is observed that their resistance to this agent is increased by the selection of advantageous mutations under those conditions. UV light produces different damages in DNA, the repair of which is necessary to maintain the integrity of the genome. However, some damages can lead to such mutations when they are not properly repaired. In an earlier work, five subcultures of a wild-type Escherichia coli strain (PQ30) were cyclically irradiated with UV and different strains resistant to UV light and gamma radiation were obtained. In a preliminary mapping, different genes involved in their resistance to radiation were identified. In one of these strains, designated as IN801, the radA gene, the product of which is involved in recombinational DNA repair, was identified. In this work, cells from another wild-type strain (AB1157) were transformed with a plasmid (pUC19) that carries the radA gene from either PQ30 or IN801, in order to establish whether the radio-resistant phenotype can be transferred to a normal strain. Only cells that received the IN801 radA gene showed increased resistance to UV and gamma radiation. Further radA sequencing showed that the gene of IN801 acquired two-point mutations that replace two amino acids in the RadA protein, which most likely changed its enzymatic activities. These results confirm that radA participates in the radiation resistance of IN801.