PROCESSING OF TOPOISOMERASE-I CLEAVABLE COMPLEXES INTO DNA-DAMAGE BY TRANSCRIPTION

Topoisomerase I (TOP1)-mediated DNA damage induced by camptothecin (CP T) in the presence of active transcription has been studied using puri fied calf thymus TOP1 and T7 RNA polymerase, OPT-stabilized TOP1 cleav able complexes located on the template strand within the transcribed r egion were found to be converted into irreversible strand breaks by th e elongating RNA polymerase, By contrast, CPT-stabilized TOP1 cleavabl e complexes located on the non-template strand within the transcribed region was unaffected by the elongating RNA polymerase. Previous studi es have demonstrated that the elongating T7 RNA polymerase is arrested by TOP1 cleavable complexes located on the template but not the non-t emplate strand [Bendixen at al., (1990) Biochemistry, 29, 5613-5619], Together, these results suggest a model in which collision between the TOP1-cleavable complexes located on the template strand and the elong ating RNA polymerase results in transcription arrest and conversion of TOP1 cleavable complexes into 'irreversible' strand breaks, The impli cation of the transcription collision model in DNA damage and repair, as well as cell killing, Is discussed.