γ-ray-induced Chain Breaks and Alkali-labile Bonds in λdvDNA ofEscherichia ColiMinicells Irradiated under Aerobic and Anoxic Conditions

SummaryChromosomeless minicells are formed by misplaced cell fissions near the polar extremities of an Escherichia coli K12 mutant strain. Superhelical λdv DNA can be introduced into minicells by segregation from a λdv-containing derivative of the original mutant. One single-strand break changes the sedimentation constant of such a DNA and hence sedimentation of minicell lysates in neutral and alkaline sucrose gradients enables one accurately to quantitate the numbers of ‘clean’ chain breaks and alkali-labile bonds produced in DNA by γ-irradiation in vivo. Although the total number of single-strand breaks measured in alkali is enhanced three- to fourfold when 60Co γ-rays are delivered to an oxygenated minicell suspension, the fraction which requires an alkaline environment for expression is almost twice as great in minicell samples irradiated in the absence (∼ 35 per cent) as in the presence (∼ 21 per cent) of oxygen.In contrast to chain breaks, alkali-labile bonds do not disappear from DNA of minicells during post-radiation incubation and, therefore, are presumably unsuitable substrates for certain DNA repair enzymes operative in minicells.