Preliminary Studies on Base Substitutions and Repair of DNA Mismatch Damage Stimulated by Low Energy N^+ Ion Beam Implantation in Escherichia coli

摘要

Ever since the low energy N+ ion beam has been accepted that the mutation effectsof ionizing radiation are attributed mainly to direct or indirect damage to DNA. Evidences basedon naked DNA irradiation in support of a mutation spectrum appears to be consistent, but directproof of such results in vivo are limited. Using mutS, dam and/or dcm defective Eschericha colimutator strains, an preliminary experimental system on induction of in vivo mutation spectra oflow energy N+ ion beam has been established in this study. It was observed that the mutationrates of rifampicin resistance induced by N+ implantation were quite high, ranging from 9.2 ×10-8 to 4.9 × 10-5 at the dosage of 5.2 × 1014 ions/cm2. Strains all had more than 90-fold highermutation rate than its spontaneous mutation rate determined by this method. It reveals thatbase substitutions involve in induction of mutation of low energy nitrogen ion beam implantation.The mutation rates of mutator strains were nearly 500-fold (GM2929), 400-fold (GM5864) and6-fold larger than that of AB1157. The GM2929 and GM5864 both lose the ability of repair DNAmismatch damage by virtue of both dam and dcm pathways defective (GM2929) or failing toassemble the repair complex (GM5864) respectively. It may explain the both strains had a similarhigher mutation rate than GM124 did. It indicated that DNA cytosine methylase might play animportant role in mismatch repair of DNA damage induced by N+ implantation. The furtherrelated research were also discussed.