ИСТИНА

Chromatin Thousands of eukaryotic genes are regulated at the level of transcript elongation and nucleosomes are likely targets for this regulation. Previously we have identified +45 region of strong, ubiquitous, nucleosome-specific pausing/arrest of RNA polymerase II (Pol II) in vitro, but the mechanism of arrest remains poorly studied. Here we show that the strong nucleosomal arrest occurs when Pol II active center is 48 bp in the nucleosome (+48), 1 bp before formation of the intranucleosomal DNA loop allowing further transcription through the nucleosome (Ø-loop). Footprinting of the arrested complexes indicates that Pol II backtracks 6 bp and displaces upstream nucleosomal DNA from the octamer to form elongation complex EC+42. After the arrest, EC+42 remains functional, indicating that downstream DNA-histone interactions form a physical barrier to transcription. Structural analysis of EC+42 using ECM suggests the mechanism involved in formation and overcoming of the nucleosomal barrier to transcription.