Down-regulation of radioresistance by LexA2 in Deinococcus radiodurans

The extremely radioresistant bacterium Deinococcus radiodurans contains two LexA homologues (LexA1 and LexA2) that are possible transcriptional regulators associated with the DNA damage response. In this study, resequencing revealed that there was an additional cytosine nucleotide (nucleotide position 612) in the D. radiodurans lexA2 gene. Purified LexA2 possessed proteolytic activity that could be stimulated by RecA. In an effort to gain an insight into the role of LexA2 in the radiation response mechanism, recA, lexA1 and lexA2 disruptant strains were generated and investigated. The intracellular level of RecA increased in lexA1 and lexA2 disruptant strains following gamma-irradiation as in the wild-type strain. These results indicated that the two LexA homologues did not possess functional overlap regarding the induction of RecA. The lexA2 disruptant strains exhibited a much higher resistance to gamma-rays than the wild-type strain. Furthermore, a luciferase assay showed that pprA promoter activation was enhanced in the lexA2 disruptant strain following gamma-irradiation. The pprA gene encoding the novel radiation-inducible protein PprA plays a critical role in the radioresistance of D. radiodurans. The increase in radioresistance of the lexA2 disruptant strain is explained in part by the enhancement of pprA promoter activation.