CONTRIBUTION OF TOPOISOMERASE-I TO CONVERSION OF SINGLE-STRAND INTO DOUBLE-STRAND DNA BREAKS

An in vitro system composed of nicked pBR322 DNA and purified topoisom erase I was employed to study the efficiency of the topoisomerase I-dr iven single-strand to double-strand DNA breaks conversion. At 1.4 x 10 (5) topoisomerase I activity units per mg DNA about 20% single-strand nicks were converted into double-strand breaks during 30 min due to to poisomerase I action. Camptothecin inhibited the conversion. The conve rsion was also inhibited when the relaxing activity of the used topois omerase I was increased by phosphorylation of the enzyme with casein k inase 2. The presented data suggest that topoisomerase I may be involv ed in production of double-stranded breaks in irradiated cells and tha t this process positively depends on the amount of topoisomerase I but not on its phosphorylation state.