RecA‐independent resistance to irradiation with u.v. light in acid‐habituatedEscherichia coli

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Abstract
Growth ofEscherichia coli1829 ColV, I‐K94 at pH 5.0 led to an increase in u.v. resistance compared with cells grown at pH 7.0. This was due to a phenotypic change, since organisms grown at pH 7.0 showed increased resistance after only 2.5–5.0 min incubation at the mildly acid pH. OtherE. coliK12 derivatives became more u.v.‐resistant at pH 5.0 includinguvrA, recAandpolA1mutants. Organisms grown at pH 5.0 also showed increased Weigle reactivation of u.v.‐irradiated Λ phage and this applied to the repair‐deficient mutants as well as the parent strains. Both the increased u.v. resistance of acid‐habituated cells and their increased ability to bring about Weigle reactivation appear to involve RecA‐independent processes and are presumably, therefore, independent of the SOS response.