DIFFERENCES IN ACCUMULATION OF BLUNT-ENDED AND COHESIVE-ENDED DOUBLE-STRAND BREAKS GENERATED BY RESTRICTION ENDONUCLEASES IN ELECTROPORATED CHO CELLS

Restriction endonucleases (RE) have been used in cytogenetic studies to mimic the DNA double-strand break (dsb)-inducing action of radiation. In the experiments presented here, we have treated electroporated CHO cells with RE and have measured the resulting dsb using the filter elution technique under non-denaturing conditions (pH 9.6). P-nu-uII, which generates blunt-ended dsb, gave rise to a significant number of measurable dsb. The frequency of the dsb induced by P-nu-uII is shown to increase over a 3-12-h post-treatment incubation period, which implies that the RE is active in the cell for a considerable length of time. We postulate that the accumulation of dsb reflects a competition between enzymatic incision and repair of the DNA. The presence of araA, a known inhibitor of DNA synthesis, did not affect the frequency of P-nu-uII-induced breaks indicating a lack of an inhibitory effect of araA on the repair of RE-induced dsb. Two RE which cause cohesive-ended dsb, namely BamHI and EcoRI, were found to be ineffective in giving rise to measurable dsb. Our interpretation of this is that for cohesive-ended dsb (caused by BamHI and EcoRI) the rate at which these breaks are rejoined matches or exceeds the rate of enzymatic incision and hence no dsb were observed. In the case of P-nu-uII, the possibly slower rate of repair of blunt-ended termini would on this hypothesis result in the observed net accumulation of dsb.