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Immunolocalization of Upstream Binding Factor and Pocket Protein p130 During Final Stages of Bovine Oocyte Growth¹

Institute of Animal Physiology,³ Slovak Academy of Sciences, 040 01Kosice, Slovakia Institute of Animal Physiology and Genetics,⁴ Academy of Sciences of the Czech Republic, 277 21 Libechov, Czech Republic Department of Anatomy and Physiology,⁵ The Royal Veterinary and Agricultural University, Copenhagen, Denmark Department of Biotechnology,⁶ Institute for Animal Science (FAL), Mariensee, D-31535 Neustadt, Germany Institute of Experimental Medicine,⁷ Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic

The aim of this study was to describe the dynamic changes in the localization of the key nucleolar protein markers, fibrillarin, B23/nucleophosmin, C23/nucleolin, protein Nopp140, during the final stages of bovine oocyte growth. All these proteins were present in the large reticulated nucleoli of oocytes from the small-size category follicles (<1 mm). The entire nucleolus exhibited strong positivity for UBF (upstream binding factor, RNA polymerase I-specific transcription initiation factor), which displayed a dotted staining pattern. In contrast, protein p130 was diffusely distributed throughout the nucleus and excluded from nucleoli. In oocytes approaching the late period of growth (2–3-mm follicles), UBF localization shifted to the nucleolar periphery. Double staining of UBF-p130 revealed a gradual accumulation of p130 at the periphery shell around the nucleolus. In fully grown oocytes (>3-mm follicles), all studied nucleolar proteins were detected in the small compact nucleoli. The cap structure, attached to the compact nucleolus surface, was positive for UBF and PAF53 (subunit of RNA polymerase I). The UBF-positive cap showed a close structural association with p130. It is concluded that, during the process of oocyte nucleolus compaction, UBF and PAF53, proteins involved in the rDNA transcription, are segregated from fibrillarin and Nopp140, proteins essential for early steps of pre-rRNA processing. The observed changes may reflect the transition from pre-rRNA synthesis to pre-rRNA processing as an analysis of the relative abundance of the developmentally important gene transcripts confirmed. In addition, discovered structural association between UBF and p130 suggests a role for pocket proteins in ribosomal gene silencing in mammalian oocytes.

¹ Supported by EU Grant QLK3-CT1999-00104, by the Ministry of Education, Youth and Sport of the Czech Republic, projects LN00A065 and ME573, and Slovak Academy of Sciences grant VEGA 2/3065/23.

² Correspondence: Jan Motlik, Department of Reproduction, Institute of Animal Physiology and Genetics, Academy of Sciences of the Czech Republic, 277 21 Libechov, Czech Republic. FAX: 420 315 697186; motlik{at}iapg.cas.cz

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