The Function Of LexA Gene And LexA Protein Of Deinococcus Radiodurans

Deinococcus radiodurans is remarkable for its extraordinary resistance to ionizing radiation, UV irradiation and many other agents that damage DNA. This organism can repair at least 150 double strand breaks of each chromosome induced by ionizing radiation without lethality or mutagenesis within several hours . This resistance is known to be due to Deinococcus radiodurans' extremely proficient DNA repair pathway employed by this organism. Also, the exsistence of SOS response and characterization of Deinococcus radiodurans LexA protein remains unclear.In this study, lexA mutant of Deinococcus radiodurans XE has been successfully constructed . The survival curve of this strain was measured using colony formation assay after treatment with different doses of radiation and different times of MMC. When compared with KDS301(lexA+), XE(lexA-)cells showed quite similar levels of survival curve to Y -irradiation and MMC, indicating lexA gene had no effect on radiation resistance of Deinococcus radiodurans . Deinococcus radiodurans' lexA had no revelance with its extremely radioresistance .The recombinant plasmid pZA172 was constructed by cloning the lexA gene of Deinococcus radiodurans into the vector plasmid pUC19 in the downstream of lacZ promoter . The transfected Escherichia coli strain JM109(pZA172) was obtained through transfection of JM109 strain with pZA172 by electroporation. The expression of LexA protein in the IPTG-induced JM109(pZA172) was checked by SDS-PAGE, and the survival curve of this strain was measured using colony formation assay after treatment with different doses of radiation and different concentrations of MMC. After IPTG induction, the LexA protein was significantly expressed in the strain JM109(pZA172), and the survival rate of this strain was lower than that of the control strain JM109. The expression of Deinococcus radiodurans lexA gene in Escherichia coli resulted in the increase of sensitivity of the latter to DNA toxins Y -rays and MMC, although the mechanism of this effect is not clear .In this study, DNA strand breaks in JM109 cells caused by -rays and the protective effect of LexA protein of Deinococcus radiodurans have been studied byfluorometric assay of DNA unwinding., The results indicated that the rate of DNA strand breaks increased with the increase of γ-rays dose. However, when the concentration of LexA protein from 0.1 g/ml to 10 g/ml, it still showed no protective effect on DNA strand breaks, then LexA protein of Deinococcus radiodurans wasn't radioprotector .