Maintenance of nucleolar machineries and pre-rRNAs in remnant nucleolus oferythrocyte nuclei and remodeling in Xenopus egg extracts

The nuclear functions in erythrocytes are almost completely extinct. There is no RNA polymerase I transcription, although a remnant nucleolar structur e is still present. The remnant nucleolus of Xenopus laevis erythrocytes ma intains a morphologically organized structure, nearly exclusively fibrillar . In this inactive nucleolar remnant, we revealed the presence of a modifie d form of transcription factor UBF. Several proteins of the processing mach inery such as fibrillarin, nucleolin and B23/NO3S, snoRNAs U3 and US, and p artially processed preribosomal RNAs colocalized in these remnant structure s. Attempts to reprogram these erythrocyte nuclei in Xenopus egg extract sh owed that import of several nucleolar proteins was induced while the nucleo lar remnant was disorganized. UBF became abundant and showed a necklace-lik e distribution on the decondensed ribosomal genes. Fibrillarin, nucleolin, and snoRNAs U3 and U8, also largely imported from the extract, were associa ted in large prenuclear bodies scattered in the nucleoplasm. B23/NO38 was p resent in different small bodies formed only in the most decondensed nuclei . In these remodeled erythrocyte nuclei, there was no imported preribosomal RNA and the initial presence of a residual nucleolar structure containing several partners of ribosome biogenesis was not sufficient to promote reass embly of newly imported nucleolar machineries. These nuclei, which reproduc e the early events of nucleogenesis are also transcriptionally silent and t hus compare to the early embryonic nuclei of Xenopus laevis. (C) 2001 Acade mic Press.