Nucleolus

About: Nucleolus is a research topic. Over the lifetime, 5873 publications have been published within this topic receiving 232435 citations. The topic is also known as: GO:0005730 & cell nucleolus.

The nucleolus as a component of the cell nucleus essential to life

Abstract: 1. The two sibling species, Chironomus tentans and C. pallidivittatus, differ genetically with respect to the number and the chromosomal location of their nucleolus organizers. These differences are not due to any structural rearrangement of the chromosomes. With respect to location therefore, the two organizers of C. tentans are not homologous to the one organizer of C. pallidivittatus. 2. Since hybrids are fertile, recombinant genotypes may be obtained which show any combination from 3 homozygous nucleolar organizers to none at all. All but the last are viable and fertile. This demonstrates the functional equivalence of the three organizers. The absence of any of the three organizers as such is not deleterious. 3. A balanced heterozygous strain has been established which regularly segregates approximately 25% “nucleolusless“ progeny. The actual segregation data in a cross of nucleolus heterozygotes often deviate significantly from the 1∶2∶1 ratio in favor of the nucleolus heterozygotes and homozygotes. 4. Nucleolusless zygotes regularly die as embryos. Development ceases to be normal shortly after the formation of the germ band following blastoderm and may be described as a general failure of gastrulation. No endoderm has been found. In spite of the desorganization of the nucleolusless embryo, mitotic division and differentiation of a few cell types may go on. 5. Nuclei of the nucleolusless embryos show numerous small granules of spherical shape instead of the typical, large nucleoli of irregular outline. The distinction is clear even at the beginning of blastoderm when the nucleoli just begin to be formed in the normal embryo. 6. An accessory nucleolus organizer occurs in a mutant 4th chromosome of C. tentans. and C. pallidivittatus. The accessory nucleolus is different in structure from the normal nucleoli and does not fuse with these. When present in an otherwise nucleolusless embryo, the accessory nucleolus is unable to sustain normal development. Such embryos degenerate in the same manner as completely nucleolusless embryos. 7. By irradiation the normal nucleolar organizers may be broken into fragments of different length which, when recombined with other chromosome breaks, still form a nucleolus each. The capacities of two such partial organizers have been tested by introducing them singly into an otherwise nucleolusless genome. Both were shown to fulfill the functions of a complete organizer, so that one partial organizer alone was sufficient to sustain normal development. 8. The possible functional significance of organizers and nucleoli is considered. It is suggested that the nucleolus organizer synthesizes a special type of RNA. One of the functions of this RNA would be to precipitate and temporarily store a protein of basic importance for all types of „growth“, perhaps the carrier protein of the ribosomes (as suggested by Brenner 1959). The nucleolus may be an essential link in a feed back mechanism involving RNA-synthesis and protein-synthesis.

SIRT7-dependent deacetylation of the U3-55k protein controls pre-rRNA processing.

Abstract: SIRT7 is an NAD(+)-dependent protein deacetylase with important roles in ribosome biogenesis and cell proliferation. Previous studies have established that SIRT7 is associated with RNA polymerase I, interacts with pre-ribosomal RNA (rRNA) and promotes rRNA synthesis. Here we show that SIRT7 is also associated with small nucleolar RNP (snoRNPs) that are involved in pre-rRNA processing and rRNA maturation. Knockdown of SIRT7 impairs U3 snoRNA dependent early cleavage steps that are necessary for generation of 18S rRNA. Mechanistically, SIRT7 deacetylates U3-55k, a core component of the U3 snoRNP complex, and reversible acetylation of U3-55k modulates the association of U3-55k with U3 snoRNA. Deacetylation by SIRT7 enhances U3-55k binding to U3 snoRNA, which is a prerequisite for pre-rRNA processing. Under stress conditions, SIRT7 is released from nucleoli, leading to hyperacetylation of U3-55k and attenuation of pre-rRNA processing. The results reveal a multifaceted role of SIRT7 in ribosome biogenesis, regulating both transcription and processing of rRNA.

mRNA transport in yeast: time to reinvestigate the functions of the nucleolus.

Abstract: Nucleocytoplasmic transport of mRNA is vital to gene expression and may prove to be key to its regulation. Genetic approaches in Saccharomyces cerevisiae have led to the identification of conditional mutants defective in mRNA transport. Mutations in approximately two dozen genes result in accumulation of transcripts, trapped at various sites in the nucleus, as detected by in situ hybridization. Phenotypic and molecular analyses of many of these mRNA transport mutants suggest that, in yeast, the function of the nucleus is not limited to the biogenesis of pre-ribosomes but may also be important for transport of poly(A)+ RNA. A similar function of the animal cell nucleolus is suggested by several observations.