| Budget Amount *help |
¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 1994: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1993: ¥1,200,000 (Direct Cost: ¥1,200,000)
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| Research Abstract |
Sunlight causes predominantly G-to-C mutation in lacZd gene of phage M13mp2. This result indicated a specific guanine modification of the DNA produced by sunlight. UVB is believed to be a toxic principle in sunlight. We analyzed the spectra of mutations induced by UVB.When SOS-induced E.coli were used as host cells, UVB induced the mutations of M13mp2 effieciently. In a host lacking the induction of SOS function, the induction of mutations was not very efficient. The induction was, however, still significant. The sequencing analysis showed the induction of transitions at pyrimidine bases on assay using SOS-induced E.coli as host cells. In contrast, mutations at guanine were seen most frequently in the experiments on a SOS-deficient host. These results indicate that UVB and sunlight can produce a damage on guanine of M13mp2DNA.
The modification of guanine might be caused by active oxygens. To know whether sunlight and UVB can generate active oxygens in M13mp2 and DNA itself, we analyzed the formation of 8-oxo-guanine. Upon irradiation by sunlight or UVB,8-oxo-guanine was formed depending on the presence of oxygens. 8-Oxo-guanine itself seems not to cause the mutagenesis, because it is known to induce G-to-T transversions. Some other oxidative modification seems to occur in the DNA and induce these mutations.
To compare with mutations induced by a chemical oxidative reagent, we analyzed spectra of E.coli mutations caused by superoxide generated by menadion. In wild-type cells, all possible base-pair changes were observed. In a mutM strain, however, GC-to-CG transversions were most frequent. This result suggests that damages removable by the action of MutM protein, a ring-opened guanine, for example, might cause this type of mutations both in sunlight-or UVB-treated M13mp2 and superoxide-mutagenesis in E.coli.
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