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, an d were fractionated as a function of their positions by gel electropho resis (1,2). Transcribed nucleosome positions were detected by observi ng 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 nucleoso me proximal boundary was at the start site. Nucleosomes located at dis crete positions were also eluted from the gel bands and transcribed. I n this case, new bands appeared as a consequence of octamer redistribu tion. Such redistribution occurred over all untranscribed positions, a s well as over transcribed positions close enough to the promoter. Sim ilar conclusions were derived from another previously investigated fra gment containing a Xenopus 5S RNA gene (3,4).