ИСТИНА

Single-strand breaks (SSB) are the most frequently occurring DNA damages. SSBs in template strand induce arrest of RNA polymerase II (PolII) in vitro and in vivo. Break in non-template strand have no effect on transcription of histone-free DNA in vitro, but in eukaryotic nuclei DNA is organized in chromatin, that could affect the efficiency of Pol II elongation on SSB-containing template. We have studied the effect of SSBs present in the non-template strand of nucleosomal DNA on transcription by yeast Pol II and E.coli RNA polymerase utilizing very similar mechanisms of transcription through chromatin. Our analysis revealed two distinct areas of nucleosomal DNA where SSB have different effects on the rate of transcription. SSBs localized in promoter-proximal region of nucleosomal DNA induce stalling of RNA polymerase while more promoter-distal SSBs facilitate transcription. These effects were observed only on chromatin-organized DNA. In living cells arrested RNA polymerase can initiate DNA repair. Therefore the data suggest that the nucleosome-specific arrest of Pol II on SSBs likely contributes to transcription-coupled DNA repair.