Bleomycin causes release of nucleosomes from chromatin and chromosomes

THE antibiotic bleomycin is effective against certain human neoplastic diseases¹. Interaction of purified DNA with bleomycin results in the release of free bases from the DNA backbone^2,3, disruption of phosphodiester linkage, and destruction of the deoxyribose moiety⁴. Bleomycin-induced DNA strand scissions, both single- and double-strand, were found in bacterial as well as in mammalian systems^5,6. But, all previous work on bleomycin has dealt with DNA molecules extracted from treated cells, and analysed on sucrose density gradients. This is not sensitive enough to probe the action of bleomycin with respect to chromatin structure. It is now known that chromatin is composed of a number of repeating subunits connected by a continuing DNA duplex analogous to 'beads on a string'⁷. The bead-like structure, termed nucleosome or nu-body, contains 140 base pairs and eight histone molecules^8,9. The DNA between nucleosomes, about 40-60 base pairs in length, is referred to as the linker DNA. Both DNAs in the nucleosomes and in the linker are susceptible to nuclease digestion, but the latter is more sensitive^10-12. We have analysed the effects of bleomycin on chromatin with special reference to the characterisation of the breakage sites in a number of mammalian cell systems. The results reported here indicate that bleomycin is one of the few non-enzymatic chemicals which can be used to study the chromatin structure, and that the action of bleomycin is not identical to that of nucleases - bleomycin does not digest nucleosome DNA but attacks only the linker DNA.