PRESENCE OF PRE-RIBOSOMAL-RNAS BEFORE ACTIVATION OF POLYMERASE-I TRANSCRIPTION IN THE BUILDING PROCESS OF NUCLEOLI DURING EARLY DEVELOPMENTOF XENOPUS-LAEVIS

During the early development of Xenopus laevis, we followed in individ ual nuclei the formation of a nucleolus by examining simultaneously it s structural organization and its transcriptional competence. Three di stinct situations were encountered with different frequencies during d evelopment. During the first period of general transcriptional quiesce nce, the transcription factor UBF of maternal origin, was present in m ost nuclei at the ribosomal gene loci. In contrast, fibrillarin, a maj or protein of the processing machinery, was found in multiple prenucle olar bodies (PNBs) whereas nucleolin was dispersed largely in the nucl eoplasm, During the second period, for most nuclei these PNBs had fuse d into two domains where nucleolin concentrated, generating a structur e with most features expected from a transcriptionally competent nucle olus. However, RNA polymerase I-dependent transcription was not detect ed 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 presente d RNA polymerase I-dependent transcription. Thus, the structural organ ization of the nucleolus preceded its transcriptional competence. We c onclude that during the early development of X. laevis, the organizati on of a defined nucleolar structure, is not associated with the transc ription process per se but rather with the presence of unprocessed rib osomal RNAs.