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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1167-1180
* Institut Jacques Monod, UMR 7592, Paris, France; and During the early development of Xenopus
laevis, we followed in individual nuclei the formation of
a nucleolus by examining simultaneously its structural
organization and its transcriptional competence. Three
distinct situations were encountered with different frequencies during development. During the first period of
general transcriptional quiescence, the transcription factor UBF of maternal origin, was present in most nuclei
at the ribosomal gene loci. In contrast, fibrillarin, a major protein of the processing machinery, was found in
multiple prenucleolar bodies (PNBs) whereas nucleolin was dispersed largely in the nucleoplasm. During the
second period, for most nuclei these PNBs had fused
into two domains where nucleolin concentrated, generating a structure with most features expected from a
transcriptionally competent nucleolus. However, RNA
polymerase I-dependent transcription was not detected
using run-on in situ assays whereas unprocessed ribosomal RNAs were observed. These RNAs were found to
derive from a maternal pool. Later, during a third period, an increasing fraction of the nuclei presented RNA
polymerase I-dependent transcription. Thus, the structural organization of the nucleolus preceded its transcriptional competence. We conclude that during the
early development of X. laevis, the organization of a defined nucleolar structure, is not associated with the transcription process per se but rather with the presence of
unprocessed ribosomal RNAs.
Institut Curie/Section de Recherche, Unité Mixte de Recherche 144, Paris, France
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