抄録
DNA in living cells is highly protected from radiation-induced damage, and the major part of this protection is provided by nuclear proteins. We examined whether the local protective effects of the nuclear proteins modulate the distribution of strand breaks. DNA single-strand breaks with 5'-phosphates in specific genomic sequences in X-ray-irradiated human cells were detected by ligation-mediated PCR and visualized on sequencing gels. We observed protection from strand cleavage at some of the transcription factor-binding sites in promoters. The regions occupied by positioned nucleosomes showed repeated protection patterns with a periodicity of about 10 bases, reflecting the wrapping of DNA helixes around histone cores. Unexpectedly, sites of modest enhancement of cleavage were found near the transcription start site of the c-FOS gene. Thus, the distribution of DNA single-strand breaks in irradiated cells is not uniform, and seems to be dependent on chromatin structure.
