Escherichia coli mutants, all lacking DNA polymerase I and having single temperature-sensitive mutations in the five known dna genes, have been tested for their ability to support M13 phage synthesis at the restrictive temperature. The dnaA. product is required early after infection. Neither dnaA nor dnaB product is required late after infection. The products of dnaC (D), dnaE and dnaG genes are all required continuously for phage production. Unlike the effect of dnaE mutation, the effects of dnaC (D) and dnaG mutations on phage production, as well as on host DNA synthesis, are reversed by temperature shift-down. Unlike the wild-type strain, an endonuclease I- and DNA polymerase I-deficient strain with no temperature-sensitive lesion in DNA synthesis used as the control in the present studies is abortively infected at the restrictive temperature (44 °), but behaves like the wild type when the temperature is shifted up to 44 ° early after infection at permissive temperature.
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