Octamer displacement and redistribution in transcription of single nucleosomes.

Single nucleosomes were assembled on a 357bp DNA fragment containing a 5S RNA gene from sea urchin and a promoter for SP6 RNA polymerase, and were fractionated as a function of their positions by gel electrophoresis. Transcribed nucleosome positions were detected by observing band disappearance in gels, which in turn provided evidence for the displacement of the histone octamer upon transcription. Differential band disappearance showed that nucleosomes closer to the promoter were harder to transcribe, and transcription was blocked when the nucleosome proximal boundary was at the start site. Nucleosomes located at discrete positions were also eluted from the gel bands and transcribed. In this case, new bands appeared as a consequence of octamer redistribution. Such redistribution occurred over all untranscribed positions, as well as over transcribed positions close enough to the promoter. Similar conclusions were derived from another previously investigated fragment containing a Xenopus 5S RNA gene

Nucleolin is a histone chaperone with FACT-like activity and assists remodeling of nucleosomes

Remodeling machines play an essential role in the control of gene expression, but how their activity is regulated is not known. Here we report that the nuclear protein nucleolin possesses a histone chaperone activity and that this factor greatly enhances the activity of the chromatin remodeling machineries SWI/SNF and ACF. Interestingly, nucleolin is able to induce the remodeling by SWI/SNF of macroH2A, but not of H2ABbd nucleosomes, which are otherwise resistant to remodeling. This new histone chaperone promotes the destabilization of the histone octamer, helping the dissociation of a H2A–H2B dimer, and stimulates the SWI/SNF-mediated transfer of H2A–H2B dimers. Furthermore, nucleolin facilitates transcription through the nucleosome, which is reminiscent of the activity of the FACT complex. This work defines new functions for histone chaperones in chromatin remodeling and regulation of transcription and explains how nucleolin could act on transcription