UBF an essential player in maintenance of active NORs and nucleolar formation

Nucleolar organizer regions (NORs) are comprised of tandem arrays of ribosomal gene repeats. During interphase, ribosomal genes are transcribed by RNA polymerase I resulting in the formation of a nucleolus. Within nucleoli, an intricate and highly coordinated assembly pathway is responsible for the production of biology's most complex machine, the ribosome. Upstream binding factor (UBF) binds extensively and plays a key role in organizing ribosomal gene chromatin throughout the cell cycle. It is responsible for the appearance of active NORs as secondary constrictions on metaphase chromosomes and its levels determine the proportion of ribosomal gene repeats that are active in a given cell type. Extensive UBF binding to NORs directs recruitment of many factors required in the early steps of ribosome biogenesis, thus enabling efficient nucleolar reformation. Finally, we reveal that UBF, once thought to be restricted to vertebrates, is present in many animal phyla.