RESUMO
The nucleolus is the site of ribosome biogenesis and forms around the actively transcribed ribosomal RNA (rRNA) genes. However, the nucleolus is also implicated in cell cycle regulation, tumour suppression and chromosome segregation and nucleolar disfunction is linked to a wide range of human diseases. Interestingly, the nucleolus is also required for genome reprogramming and the establishment of heterochromatin in the mammalian embryo. Mammalian oocytes contain a subnuclear structure that is believed to be the precursor of the functional nucleolus, the Nucleolar Precursor Body (NPB). But the NPB is also required for the organisation of the zygotic heterochromatin and the establishment of pluripotency. We found that disruption of the mouse Upstream Binding Factor (UBF (UBTF)) gene caused disassembly of somatic nucleoli and the accumulation of the key rRNA gene transcription factors into dense subnuclear foci resembling NPBs. Here we show that UBF deletion causes the rRNA genes to collapse onto their centromere-proximal chromosomal sites spatially distinct from NPB-like structures, and that these structures contain rRNA gene transcription factors but not all nucleolar proteins. We further find that embryonic NPBs and their surrounding heterochromatin are both disrupted in UBF-null mouse embryos. These embryos also display subnuclear foci containing the rRNA gene transcription factors and arrest development before completing the forth cleavage division. The data suggest that the rRNA gene transcription factors have an intrinsic ability to interact and form a discrete nuclear compartment even in the absence of any rRNA gene activity and that the formation or maintenance of the zygotic NPB and surrounding heterochromatin requires UBF.