SPECTRA OF SUPEROXIDE-INDUCED MUTATIONS IN THE LACI GENE OF A WILD-TYPE AND A MUTM STRAIN OF ESCHERICHIA-COLI K-12

We have analyzed the spectra of superoxide-induced mutations in the ch romosomal lad gene of a wild-type and a mutM strain of Escherichia col i K-12. The mutM strain is known to be deficient in removing 8-hydroxy guanine from DNA. An intracellular superoxide-generating agent, menadi one, was used to cause the mutation. Analysis of the mutated DNA showe d marked differences between the mutants from the wild type and those from the mutM strain. In the mutants from the wild type, all possible base-pair substitutions were present and their proportions were simila r to each other, whereas in those from the mutM bacteria there was a 9 0% bias in favor of transversion. Furthermore, in the mutM strain GC-t o-CG transversion rather than GC-to-TA was predominantly induced. 64% of the GC-to-CG transversions in the mutM strain occurred at the site of (CT/GC)GGC (mutated base underlined). The favorable mutation site, CTGGC, was the same as that of the UV- and sunlight-induced mutations previously reported: the mutations observed there were also G-to-C tra nsversions: We speculate from these results that the superoxide in the cells may lead to production of a modified guanine that can pair with guanine and is subject to removal by the MutM protein.