γ-Radiation-induced mutation spectrum in the episomal lacI gene of Escherichia coli under oxic conditions

Abstract In this study we have determined the mutation spectrum in the complete episomal lacI gene of Escherichia coli induced by γ-radiation under oxic conditions. Mutants were generated by 60 Co γ-irradiation of an E. coli culture of stationary cells in LB medium, under continuous flushing with oxygen. Oligonucleotide probe analysis showed that 14% of the γ-ray-induced mutations were located at the lacI gene hot spot at position 620–632, which is characterized by a triple repeat of the 5′-TGGC-3′ sequence. Previously it was shown that about 70% of the spontaneous mutations were located at this site due to the loss or the addition of a TGGC sequence. The non-hot spot mutations were further characterized by automated sequence analysis. The results show that base pair (bp) substitutions were the main type of γ-ray-induced mutations. Although all types of bp substitutions were observed, 74% of the bp substitutions involved C G base pairs. C G → T A and C G → A T substitutions were predominant, both accounting for 35% of all bp substitutions, whereas A T → C G substitutions were only seldomly observed (3%). A relatively large amount of −1 bp deletions (15% of all mutations) was detected in the γ-ray-induced mutation spectrum, mainly affecting C G base pairs, and 10% were deletions, ranging in size from 11 to 532 bp. It can be concluded that under oxic conditions γ-radiation induces in E. coli mainly bp substitutions of all types but preferentially at C G base pairs, and that the mutations tend to be randomly distributed within the lacI gene sequence.