RecA-Mediated Asymmetric Repair of Lethal DNA Lesions in E. coli

Abstract

Mechanisms of action of DNA lesions produced by decay of genetically incorporated ¹²⁵I or by 313-nm photolysis of incorporated 5-bromouracil (5BU) in exponential phase cultures of Escherichia coli were studied by inducing lesions after subsequent periods of DNA replication. The analogues were incorporated during brief labeling periods to provide prelesional substrates for later induction of the lesions, and label concentrations were chosen to give cell survivals of about 10% when lesions were induced immediately after labeling. When the same number of lesions were induced at intervals during the first generation after labeling survival levels decreased. After replication of the labeled regions, however, survival levels increased sharply in the wildtype strains. Three very different patterns of recovery were observed.

• 1. 5BU photolysis, wildtype strains. Recovery was essentially complete.

• 2. ¹²⁵I decay, wildtype strain. Approximately half of the cells recovered at the end of the first round of DNA replication.

• 3. ¹²⁵I decay, recA strain. No significant increase in recovery occurred at the first generation, but recovery increased gradually during the second generation, as expected for production of irreparable damage in labeled regions.

The results are consistent with recA-mediated asymmetric repair of ¹²⁵I decay-induced lesions.

This work was supported by the U.S. Department of Energy under contract No. W-31-109-ENG-38.