Induction and Repair of Double- and Single-strand DNA Breaks in Bacteriophage Lambda Superinfecting Escherichia Coli

Summary

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 λ 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 λ and suspended in growth medium. Irradiation induced 42 single-strand breaks per double-strand break. The rates of break induction were 6·75 × 10⁻¹⁴ (double-strand breaks) and 2·82 × 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 λ DNA are repaired to a very small extent, if at all.