RecA-independent resistance to irradiation with u.v. light in acid-habituated Escherichia coli

Growth of Escherichia coli 1829 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. Other E. coli K12 derivatives became more u.v.-resistant at pH 5.0 including uvrA, recA and polA1 mutants. 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.