The antimutator phenotype of E. coli mud is only apparent and results from delayed appearance of mutants

Antimutator strains are strains that have a lower mutation rate than the wild-type strain. We have reexamined the properties of one reported antimutator strain of Escherichia coli, termed mud [Mol. Gen. Genet. 153 (1977) 87]. This strain contains a temperature-sensitive mutation in the purB gene, leading to adenine-dependent growth at higher temperature. When grown at permissive or semi-permissive temperature in the absence of adenine it displays large reductions in the number of both spontaneous and mutagen-induced mutants (e.g. several hundred-fold for valine-resistant mutants). However, our studies show that strains containing the purB allele generate mutations at the same level as the wild-type strain, and that the apparent antimutator effect is the consequence of the delayed appearance of mutants on the selective plates. This delay likely results from the combined stress exerted by the adenine deficiency and the presence of the selective agent (i.e. valine).