Bacteria are able to adapt to changes in the environment using two-component signal transduction systems (TCSs) composed of a histidine kinase (HK) and a response regulator (RR). Deinococcus radiodurans, one of the most resistant organisms to ionizing radiation, has 20 putative HKs and 25 putative RRs. In this study, we constructed 12 D. radiodurans mutant strains lacking a gene encoding a HK and surveyed their resistance to γ-radiation, UV-B radiation (302 nm), mitomycin C (MMC), and H(2)O(2). Five (dr0860 (-), dr1174 (-), dr1556 (-), dr2244 (-), and dr2419 (-)) of the 12 mutant strains showed at least a one-log cycle reduction in γ-radiation resistance. The mutations (1) dr1174, dr1227, and dr2244 and (2) dr0860, dr2416, and dr2419 caused decreases in resistance to UV radiation and MMC, respectively. Only the dr2416 and dr2419 mutant strains showed higher sensitivity to H(2)O(2) than the wild-type. Reductions in the resistance to γ-radiation and H(2)O(2), but not to UV and MMC, were observed in the absence of DR2415, which seems to be a cognate RR of DR2416. This result suggests that DR2415/DR2416 (DrtR/S: DNA damage response TCS) may be another TCS responsible for the extreme resistance of D. radiodurans to DNA-damaging agents.
This study evaluated the effect of gamma irradiation on the reduction of the toxicity of mistletoe lectin using both in vitro and in vivo models. To extract the lectin from mistletoe, an (NH4)2SO4 precipitation method was employed and the precipitant purified using a Sepharose 4B column to obtain the pure lectin fraction. Purified lectin was then gamma-irradiated at doses of 0, 5, 10, 15, and 20 kGy, or heated at 100 °C for 30 min. Toxic effects of non-irradiated, irradiated, and heat-treated lectins were tested using hemagglutination assays, cytotoxicity assays, hepatotoxicity, and a mouse survival test and immunological response was tested using cytokine production activity. Hemagglutination of lectin was remarkably decreased (P < 0.05) by irradiation at doses exceeding 10 kGy and with heat treatment. However, lectin irradiated with 5 kGy maintained its hemagglutination activity. The cytotoxicity of lectin was decreased by irradiation at doses over 5 kGy and with heat treatment. In experiments using mouse model, glutamate oxaloacetate transaminase (GOT) and glutamic pyruvic transaminase (GPT) levels were decreased in the group treated with the 5 kGy irradiated and heat-treated lectins as compared to the intact lectin, and it was also shown that 5 kGy irradiated and heat-treated lectins did not cause damage in liver tissue or mortality. In the result of immunological response, tumor necrosis factor (TNF-α) and interleukin (IL-6) levels were significantly (P < 0.05) increased in the 5 kGy gamma-irradiated lectin treated group. These results indicate that 5 kGy irradiated lectin still maintained the immunological response with reduction of toxicity. Therefore, gamma-irradiation may be an effective method for reducing the toxicity of lectin maintaining the immune response.
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