title: Induction and repair of double- and single-strand DNA breaks in bacteriophage lambda superinfecting Escherichia coli.
abstract: Induction and repair of double- and single-strand DNA breaks have been measured after decays of 125I and 3H incorporated into the DNA and after external irradiation with 4 MeV electrons. For the decay experiments, cells of wild type Escherichia coli K-12 were superinfected with bacteriophage lambda DNA labelled with 5'-(125I)iodo-2'-deoxyuridine or with (methyl-3H)thymidine and frozen in liquid nitrogen. Aliquots were thawed at intervals and lysed at neutral pH, and the phage DNA was assayed for double- and single-strand breakage by neutral sucrose gradient centrifugation. The gradients used allowed measurements of both kinds of breaks in the same gradient. Decays of 125I induced 0.39 single-strand breaks per double-strand break. No repair of either break type could be detected. Each 3H disintegration caused 0.20 single-strand breaks and very few double-strand breaks. The single-strand breaks were rapidly rejoined after the cells were thawed. For irradiation with 4 MeV electrons, cells of wild type E. coli K-12 were superinfected with phage lambda and suspended in growth medium. Irradiation induced 42 single-strand breaks per double-strand break. The rates of break induction were 6.75 x 10(-14) (double-strand breaks) and 2.82 x 10(-12) (single-strand breaks) per rad and per dalton. The single-strand breaks were rapidly repaired upon incubation whereas the double-strand breaks seemed to remain unrepaired. It is concluded that double-strand breaks in superinfecting bacteriophage lambda DNA are repaired to a very small extent, if at all.
mesh_terms: D010582:Bacteriophage lambda / Q000378:metabolism / Q000528:radiation effects*; D004260:DNA Repair; D004279:DNA, Viral / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D004583:Electrons; D004926:Escherichia coli; D007457:Iodine Radioisotopes; D014316:Tritium
chemical_list: D004279:DNA, Viral; D007457:Iodine Radioisotopes; D014316:Tritium