Induction of transversion mutations in Escherichia coli by N-methyl-N'-nitro-N-nitrosoguanidine is SOS dependent

Abstract

Escherichia coli alkA mutants, which are deficient for an inducible DNA glycosylase, 3-methyladenine-DNA glycosylase II, are sensitive to mutagenesis by low doses of the alkylating agent N-methyl-N'-nitro-Nnitrosoguanidine (MNNG). As many as 90% of the alkA-dependent mutations induced by MNNG are also umuC+ dependent and thus are due to DNA lesions that are substrates for the mutagenic functions of the SOS response. A great number of these mutations are base substitutions at A T sites, particularly A. T transversions. We discuss which DNA lesions may be responsible for these mutations. Our results show that the induction of 3-methyladenine-DNA glycosylase II, which occurs as part of the adaptive response to alkylating agents such as MNNG, significantly reduces the mutagenicity as well as the lethality of alkylation damage. The biological consequences of exposing bacteria to DNA-alkylating agents depend on the chemical nature of the DNA damage induced. For example, alkylation of the 06 position of guanine creates a modified base with the coding properties of adenine (45). Thus, this lesion is highly and directly mutagenic, giving rise to G * C-to-A. T transitions (41). In contrast, alkylation of the N3 position of adenine creates a modified base that blocks the replication fork (8;

Overexpression of endonuclease III protects Escherichia coli mutants defective in alkylation repair against lethal effects of methylmethanesulphonate

“…A deficiency of AlkA caused an increase in the mutation frequency induced by MMS (Fig. 2A), similarly as has previously been observed for the alkylating agent N ‐methyl‐ N ′‐nitro‐ N ‐nitrosoguanidine [19]. H 2 O 2 was found to induce mutations more frequently in nth strains than in wild‐type cells.…”

In vitro mispairing specificity of O2-ethylthymidine

“…DNA lesions that impede the progress of in vivo DNA replication require the "error-prone" functions of the SOS response to produce mutations in bacteria (Witkin, 1976;Little & Mount, 1982;Walker, 1984). A significant component of the SOS-dependent mutational spectra induced by alkylating agents in bacteria includes events at A-T base pairs (Foster & Eisenstadt, 1985;Zielenska et al, 1988;Cuoto et al, 1989;Eckert et al, 1989). In vivo, SOS-induced proteins may facilitate incorporation of dT opposite C^-Et-dT and subsequent extension of the C^-Et-dT-dT base pair, as mediated by Mn2+ in vitro, generating A*T -* T-A transversion mutations.…”

“…Transversion mutations at A-T base pairs form an important component of alkylating agent-induced SOS-dependent mutagenesis in E. coli (Foster & Eisenstadt, 1985; Eckert & Drinkwater, 1987;Cuoto et al, 1989;Eckert et al, 1989).…”

Mutational specificity of alkylating agents and the influence of DNA repair

“…The biological significance of such a mechanism in vivo is, however, still uncertain [Snow and Mitra, 19871. In addition to direct mispairing, some A : T = > G : C transitions may arise via SOS processing [Foster and Eisenstadt, 1985;Couto et al, 19891 of either persistent alkylation damage or their abasic repair intermediates, which raises the possibility that an alkyl lesion(s) other than 04-alT may direct these mutations but only under conditions of reduced polymerase fidelity. The in vivo role of 04-etT in directing A : T = > G : C transition is suggested by the correlation of the ratio of A : T = > G : C to G : C = > A : T transitions (0.21-0.36) induced by ENU [Richardson et al, 1987b;Burns et al, 1988b;Eckert et al, 19881 with the ratio of 04-etT to 06-etG (0.28-0.32) expected [Richardson et al, 1987b;Singer and Grunberger, 1983;Hu and Gutenplan, 1985;Zielenska et al, 19881. This assumes, however, that these lesions possess roughly equivalent in vivo mutagenic potencies.…”