DELETIONS AT SHORT DIRECT REPEATS AND BASE SUBSTITUTIONS ARE CHARACTERISTIC MUTATIONS FOR BLEOMYCIN-INDUCED DOUBLE-STRAND AND SINGLE-STRANDBREAKS, RESPECTIVELY, IN A HUMAN SHUTTLE VECTOR SYSTEM

Using the radiomimetic drug, bleomycin, we have determined the mutagen ic potential of DNA strand breaks in the shuttle vector pZ189 in human fibroblasts, The bleomycin treatment conditions used produce strand b reaks with 3'-phosphoglycolate termini as >95% of the detectable dose- dependent lesions, Breaks with this end group represent 50% of the str and break damage produced by ionizing radiation. We report that such s trand breaks are mutagenic lesions. The type of mutation produced is l argely determined by the type of strand break on the plasmid (i.e. sin gle versus double), Mutagenesis studies with purified DNA forms showed that nicked plasmids (i.e. those containing single-strand breaks) pre dominantly produce base substitutions, the majority of which are multi ples, which presumably originate from error prone polymerase activity at strand break sites, In contrast, repair of linear plasmids (i.e. th ose containing double-strand breaks) mainly results in deletions at sh ort direct repeat sequences, indicating the involvement of illegitimat e recombination. The data characterize the nature of mutations produce d by single- and double-strand breaks in human cells, and suggests tha t deletions at direct repeats may be a 'signature' mutation for the pr ocessing of DNA double-strand breaks.