Repair of cross-linked DNA and survival of Escherichia coli treated with psoralen and light: effects of mutations influencing genetic recombination and DNA metabolism

Abstract

Repair of cross-linked DNA was studied in Escherichia coli strains carrying mutations affecting DNA metabolism. In wild-type cells, DNA strands cut during cross-link removal were rejoined during a subsequent incubation into high-molecular-weight molecules. This rejoining was dependent on gene products involved in genetic recombination. A close correlation was found relating recombination proficiency, the rate of strand rejoining, and formation of viable progeny after DNA cross-linking by treatment with psoralen and light. Wild-type cells and other mutants which were Rec+ (sbcB, recL, recL, sbcB, recB recC sbcA, recB recC sbcB, xthA1, and xthA11) rejoined cut DNA strands at a rate of 0.8 ± 0.1 min-1 at 37°C and survived 53 to 71 cross-links per chromosome, recB, recC, recB recC, recF, or polA strains showed reduced rates of strand rejoining and survived 4 to 13 cross links per chromosome. Recombination-deficient strains (recA, recB recC sbcB recF, recB recL) and lexA failed to rejoin DNA strands after cross-link removal and were unable to form colonies after treatments producing as few as one to two cross-links per chromosome. Strand rejoining occurred normally in cells with mutations affecting DNA replication (dnaA, dnaB, dnaG, and dnaE) under both permissive and nonpermissive conditions for chromosome replication. In a polA polB dnaE strain strand rejoining occurred at 32°C but not at 42°C, indicating that some DNA synthesis was required for formation of intact recombinant molecules.