A model for the topology of active ribosomal RNA genes.

The Christmas tree view of active ribosomal RNA (rRNA) genes suggests a gene topology in which a large number of nascent rRNA transcripts are prevented from intertwining. The way in which this is achieved has remained unclear. By using a combination of chromatin immunoprecipitation and chromosome conformation capture techniques, we show that the promoter, upstream region and terminator R3 of active rRNA genes are held together spatially throughout the cell cycle, forming a stable core around which the transcribed region is organized. We suggest a new core-helix model for the topology of rRNA genes, that provides a structural basis for the productive synthesis or rRNA.