Alternative pathways of methyl methanesulfonate-induced mutagenesis in Escherichia coli

Summary

Methyl methanesulfonate (MMS) induced mutagenesis is known to be largely dependent on functional umuCD and recA genes. By phenotypic analysis of Arg⁺ (argE3, ochre) revertants according to their reversion of the mutations his-4 (ochre) and thr-1 (amber), we attempted to deduce the specificity and/or sites of MMS-induced mutations. It is shown that: (1) MMS-induced, umuC-dependent Arg⁺ revertants (which prevail in bacteria proficient in mismatch repair) result from a different mutational pathway from umuC-independent ones. UmuC-dependent Arg⁺ revertants belong to class 2 (Arg⁺His⁺Thr⁻), and umuC-independent ones to class 1 (Arg⁺His⁻Thr⁻). (2) The mismatch repair system very efficiently prevents mutations induced by MMS. We found that in the mutS strain, deficient in mismatch repair, class 1 Arg⁺ revertants are the most numerous, whereas class 2 Arg⁺ revertants occur at similar levels in MMS-treated mutS and mutS ⁺ strains. Therefore the mismatch repair system very efficiently prevents formation of umuC-independent Arg⁺ revertants, but exerts negligible or no effect on umuC-dependent Arg⁺ revertants. (iii) Both mutS umuC and mutS recA strains, are highly mutable by MMS.