It was subsequently digested with SmaI and then ligated to a 1-kb HincII fragment harboring the aph cassette from pKatAPH3. This resultant plasmid was transformed into D. Fulvestrant radiodurans cells, and the transformants were screened on TGY-kanamycin agar plates. Gene replacement was confirmed by diagnostic PCR using the primers dr0053�C3F and dr0053�C3R, which bind outside the mutant cassette on the genomic D. radiodurans DNA. The polyextremophilic D. radiodurans encodes 13 DinB/YfiT homologues, which is the highest number identified in any bacteria to date. Such particular expansions of certain protein families are favored during evolution to aid in organism survival. Considering the extreme multiple stress tolerance of this bacterium, genes belonging to such protein families may hold essential information to help elucidate its resistance mechanisms. The prototype of Deinococcal DinB family proteins is B. subtilis DinB whose expression is controlled by the SOS system. In D. radiodurans, RecA positively regulated the expression of dr0053, one of the DinB family proteins, as it does in B. subtilis; however, LexA was not involved in this regulation. This result is Reversine consistent with the finding that neither of the Deinococcal LexA homologues repressed recA expression although they are cleavable by RecA. In addition to its participation in the SOS response, B. subtilis RecA is also responsible for DNA damage-dependent alterations in gene expression for nearly 600 genes, most of which are not repressed directly by LexA. These observations indicate the presence of another transcriptional repressor, substituting for LexA, which suppresses dr0053 expression under non-irradiated conditions. Deletion analysis of the dr0053 promoter demonstrates the possibility that unidentified repressor binding sites are present in the 133-bp region upstream of the transcriptional start site of Pdr0053-1. Some regulators are involved in the repression of radiation-inducible genes under non-irradiated conditions. The deletion of pprM, which encodes a modulator of the PprI-dependent DNA damage response, and recX results in constitutive production of PprA and RecA, respectively, regardless of ��-radiation treatment. The TCS, which is composed of an HK and an RR, is one of the most ubiquitous means by which bacteria sense, respond, and adapt to environmental changes. The HK perceives the environmental signal and transduces the signal to its cognate RR which, in turn, activates the specific response to adapt the cell to its new surroundings. Until now, three RRs, DrRRA, RadR, and DrtR, have been shown to be necessary for radiation resistance in D. radiodurans. Deletion of drRRA downregulates the transcriptional levels of numerous genes related to stress response and DNA repair, such as kat, sod, recA and pprA. Microarray analysis demonstrated that the drRRA mutation slightly reduced dr0053 expression under both normal and irradiation stress conditions. Taking the effect of RecA on dr0053 expression into consideration, DrRRA is likely to have a positive effect on dr0053 expression via RecA.