Loss of an Apurinic/Apyrimidinic Site Endonuclease Increases the Mutagenicity of N-methyl-N′-nitro-N-nitrosoguanidine to Escherichia coli

xthA-Escherichia coli, which are missing a major cellular apurinic/apyrimidinic (AP) endonuclease, are 5- to 10-fold more sensitive than xthA+bacteria to mutagenesis by N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) under conditions that induce the ``adaptive response.'' The xthA--dependent mutations are also dependent on SOS mutagenic processing and consist of both transversion and transition base substitutions. When MNNG-adapted xthA-bacteria are challenged with a high dose of MNNG, more xthA--dependent SOS-dependent mutations are induced, and transversions are enhanced relative to transitions. The mutations induced by challenge are eliminated in xthA-alkA-bacteria, which are also deficient for 3-methyladenine glycosylase II activity. These data are consistent with the hypothesis that AP sites, at least some of which are produced by glycosylase activity, are mutagenic intermediates following cellular DNA alkylation.