UV-induced allevation of λ restriction in Escherichia coli K-12: Kinetics of induction and specificity of this SOS function

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Abstract
In UV-irradiated cells of Escherichia coli K-12 a partial release of the restriction of non-modified phage λ is observed when the cells are recA + lexA +. We show here that the induction of this restriction allevation (RA) also depends on the recBC enzyme and that the expression of RA requires protein synthesis. Maximum expression was reached within 60 to 90 min after irradiation. Experiments are presented which show that upon UV-irradiation a signal is created which triggers the development of RA when protein synthesis is allowed. This signal decayed with a half-life of only a few minutes in cells treated with chloramphenicol. The decay kinetics were similar in uvr + and uvrA mutants. RA appeared to be specific for EcoK insofar as no allevation of λ restriction by EcoRI, EcoRII and EcoP1 occurred. During maximum expression of RA no gross reduction of the activities of the recBC enzyme (exonuclease V) and the restriction endonuclease EcoK was observed and no new DNA modifying activity appeared in the cells. Since, in fully expressed cells, up to 75% of the infecting λ DNA was converted to acid-soluble material within 20 min after infection we suggest that only a small specific fraction of λ infections may undergo RA.