To identify the specificity of base substitutions, a novel experimental system was established based on rifampicin-resistant (Rif r) mutant screening and sequencing of the defined region of the rpoB gene in E. coli. We focused on comparing mutational spectra of base substitutions induced by either low energy nitrogen ion beam implantation or 60Co-gamma rays. The most significant difference in the frequency of specific kinds of mutations induced by low energy nitrogen ion beam was that CG <FONT FACE=Symbol>®</font>TA transitions were significantly increased from 32 to 46, AT <FONT FACE=Symbol>®</font>TA transversions were doubled from 7 to 15 in 50 mutants, respectively. The preferential base substitutions induced by nitrogen ion beam implantation were CG <FONT FACE=Symbol>®</font>TA transitions, AT <FONT FACE=Symbol>®</font>GC transitions, AT <FONT FACE=Symbol>®</font>TA transversions, which account for 92.13% (82/89) of the total. The mutations induced by 60Co-gamma rays were preferentially GC <FONT FACE=Symbol>®</font>AT and AT <FONT FACE=Symbol>®</font>GC transitions, which totaled 84.31% (43/51).
base substitution; low-energy nitrogen ion beam implantation; 60Co-gamma rays; Escherichia coli
