The role of RNA polymerase I transcription and embryonic genome activation in nucleolar development in bovine preimplantation embryos.

The aim of the present study was to investigate the role of RNA polymerase I (RPI) transcription in nucleolar development during major transcriptional activation (MTA) in cattle. Late eight-cell embryos were cultured in the absence (control group) or presence of actinomycin D (AD) (RPI inhibition, AD 0.2 microg/ml; total transcriptional inhibition, AD 2.0 microg/ml). Late four-cell embryos were cultured to late eight-cell stage in 0.2 microg/ml AD (MTA prevention, ADLT (long-term total transcriptional inhibition group). Embryos were processed for autoradiography, transmission electron microscopy, fluorescent in situ hybridization (ribosomal RNA, rRNA), silver staining (nucleolar proteins), and immunofluorescence (RPI). Control embryos displayed extranucleolar and nucleolar transcription, functional nucleoli, and distinct RPI localization. Nuclei (97%) showed large rRNA clusters, in 94.1% co-localized with nucleolar proteins deposits. In AD 0.2 group, only extranucleolar transcription was detected. Segregated dense-fibrillar and granular components, but no fibrillar centers, were observed. RPI was dispersed. Nuclei (55%) presented rRNA clusters, in 38.8% co-localized with silver-stained deposits. AD 2.0 and ADLT groups displayed no transcription and disintegrating nucleolar precursors. AD 2.0 (34%) and 14% (ADLT) of nuclei presented clusters of maternally inherited rRNA. In AD 2.0 group, RPI was dispersed, but 17.2% of nuclei showed colocalization of rRNA with nucleolar proteins. In ADLT group, RPI was lacking and clustering of nucleolar proteins was hampered. In conclusion, rDNA transcription is not required for targeting of rRNA processing proteins, rRNA is maternally inherited and target to rDNA independent of transcription, and de novo transcription is required for proper nucleologenesis in cattle.