The involvement of LexA in induction of RecA was investigated in Deinococcus radiodurans. As in the wild-type strain, an increase in RecA protein synthesis following ␥ irradiation was detected in a lexA disruptant, indicating that LexA is not involved in the induction of RecA in D. radiodurans.Deinococcus radiodurans is characterized by its extraordinary radiation resistance phenotype, which is considered to be due to a highly proficient DNA repair capacity (3,25,34). The most striking feature of D. radiodurans is that it can mend over 100 double-strand breaks (DSBs) of genomic DNA during postirradiation incubation (2, 18). As the rejoining of DSBs can be prevented by adding chloramphenicol to the incubation mixture, proteins induced by irradiation are necessary for the rejoining of DNA breakages (18). Several DNA damage-inducible proteins that may be required for DNA repair have been detected in the cell extract of D. radiodurans by twodimensional polyacrylamide gel electrophoresis (PAGE) (12,35). These observations suggest that D. radiodurans possesses a DNA damage response mechanism. However, little is known about the molecular basis for the control of the inducible proteins.In Escherichia coli, the inducible DNA repair system (the SOS system) is regulated by two key proteins; RecA and LexA (8,38). E. coli RecA is activated by DNA damage to mediate the proteolytic cleavage of the E. coli LexA repressor, resulting in derepression of the SOS regulon. The SOS response in Bacillus subtilis progresses in a similar manner, with B. subtilis RecA having an identical role in controlling the SOS regulon together with a cellular repressor protein that is functionally homologous to the E. coli LexA repressor (42). The B. subtilis repressor (termed DinR) binds the promoter regions of several din genes and B. subtilis recA (20,23,40,41) and undergoes autodigestion under alkaline conditions and RecA-mediated cleavage under more physiological conditions (23, 41). It has also been shown that the intracellular level of intact DinR is significantly reduced following DNA damage (23). Thus, the basic mechanism of the SOS response seems to be conserved between E. coli and B. subtilis. Deinococcus species form a coherent phylogenetic cluster related to the Thermus-Meiothermus lineage (30), indicating that the Deinococcus lineage is distinct from the lineages of both proteobacteria and grampositive bacteria. Although SOS-like processes have been documented in a wide variety of eubacterial species (24, 32), the involvement of RecA and LexA in the SOS response is poorly understood in Deinococcus and closely related bacterial species.As expression of the deinococcal recA gene is enhanced after ␥ irradiation (4), the recA gene seems to be a member of a DNA damage response regulon in D. radiodurans. In the present study, D. radiodurans LexA was purified from E. coli cells and its ability to cleave itself was examined. The changes in intracellular levels of the LexA and RecA proteins following ␥ irradiation were also investigated by usin...
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