Abstract
The ion beam is expected to increase the mutation frequency and wide spectrum, since it has the high linear energy transfer (LET), and the breeding technology using the mutations induced and cancer therapy by the ion beam has been greatly developed. However, the detailed molecular mechanism has not been proven considerably.
S. cerevisiae strains used in this study are S288c (RAD+), ogg1, msh2. OGG1p is a DNA glycosylase / AP lyase that excises guanine lesions such as 8-oxoguanine (8-oxoG). MSH2p is the mismatch repair (MMR) protein that mediates DNA repair through the recognition of 1 and 2-bp mismatches. The yeast strains were irradiated with carbon ions (12C5+; 220 MeV) with the dose 10 to 100 Gy, and LET is 107 keV/µm. Carbon ion beams were generated from AVF cyclotron in JAEA. The mutation sites of ura3 mutants were determined by DNA sequencing.
Our results show that the types of base changes in the carbon-ion induced mutants in wild type cells included GC to TA transversions (41 %), the other type of base substitution (41 %) and deletions/insertions (18 %). In the case of ogg1, GC to TA transversions were largely observed (70 %), but the deletion or insertion mutations were not obtained. GC to TA transversions are caused by misincorporation of 8-oxodG to DNA. These results in S. cerevisiae strongly suggest that the endogenous oxidative stress, due to the carbon-ion irradiation, induces the formation of 8-oxodG in DNA. In the msh2 mutant strain, the mutational spectrum exhibited a predominance of small deletions, with the mutations unequally distributed over the URA3 gene region.
