Comparison of base substitutions in response to nitrogen ion implantation and 60Co-gamma ray irradiation in Escherichia coli

Abstract

To identify the specificity of base substitutions, a novel experimental system was established based on rifampicinresistant (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 60 Co-gamma rays. The most significant difference in the frequency of specific kinds of mutations induced by low energy nitrogen ion beam was that CG → TA transitions were significantly increased from 32 to 46, AT → TA transversions were doubled from 7 to 15 in 50 mutants, respectively. The preferential base substitutions induced by nitrogen ion beam implantation were CG → TA transitions, AT → GC transitions, AT → TA transversions, which account for 92.13% (82/89) of the total. The mutations induced by 60 Co-gamma rays were preferentially GC → AT and AT → GC transitions, which totaled 84.31% (43/51).

Experimental Evolution of Extreme Resistance to Ionizing Radiation in Escherichia coli after 50 Cycles of Selection

“…GC-to-AT and AT-to-GC transitions were detected in high numbers, agreeing with mutational patterns detected in previously evolved IR-resistant E. coli populations (45,46). Base substitutions are approximately 50% transitions and 50% transversions, whereas E. coli exposed to 60 Co gamma ray irradiation exhibit transversions at about four times the frequency of transitions (45,55). In particular, GC-to-TA transitions appeared at very high levels in each population, indicative of significant templatestrand 8-oxodG-mediated mutagenesis (56).…”

“…While enhanced DNA repair is a hallmark of the previously evolved populations and the populations presented here, it appears as though the enhancements are indeed different. In addition, the frequency of transversion mutations detected in the new populations (ϳ50% of all mutations) is far higher than that observed in the earlier studies (ϳ20% of all mutations) (45,55). This appears to be due to the extreme amount of detected GC-to-TA transitions, specifically.…”