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#rRNA synthesis

3 public questions tagged with this topic.

Cell Organelles

The primary function of the nucleolus is:

Electron microscopy reveals nucleolus as darkly staining subnuclear domain due to high RNA protein density, tripartite with fibrillar centers containing inactive rDNA, dense fibrillar component where transcription and early processing occurs, granular component assembling pre-ribosomes. RNA polymerase I with transcription factor UBF drives high-rate synthesis of forty seven S precursor containing eighteen S, five point eight S, twenty eight S RNAs flanked by five prime external transcribed spacer and internal transcribed spacers ITS1 and ITS2. Early cleavages by U3 snoRNP at five prime ETS occur in dense fibrillar component, subsequent modifications include methylation by fibrillarin and pseudouridylation by dyskerin guided by snoRNAs. Pre-forty S and pre-sixty S assembled with imported ribosomal proteins one hundred copies transit to granular component before export. DNA replication initiates at many origins in nucleoplasm during S phase, ATP production mitochondrial oxidative phosphorylation, RNA modification widespread but concentrated for rRNA here. Actinomycin intercalates rDNA halting Pol I causing nucleolar disruption, confirming function as ribosomal RNA processing and assembly center coordinating growth with protein synthesis demand.

Ref: Scheer Annu Rev Cell Biol; nucleolus FC DFC GC rRNA transcription processing assembly.

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Cell Organelles

Ribosomal RNA (rRNA) synthesis primarily occurs in:

Eukaryotic cells separate transcription spaces using distinct RNA polymerases. Precursor rRNAs eighteen S, five point eight S and twenty eight S originate from single forty seven S transcription unit driven by RNA polymerase I complex concentrated in nucleolus, organized around nucleolar organizer regions containing hundreds tandem rDNA repeats on acrocentric chromosomes. Accessory factors UBF and SL1 recruit Pol I to promoters within fibrillar centers, nascent transcripts emerge into dense fibrillar component where U3 snoRNP performs early cleavages and fibrillarin mediates methylation, dyskerin pseudouridylation guided by snoRNAs. Five S rRNA transcribed by Polymerase III in nucleoplasm later imports. Rough ER does not transcribe, mitochondria transcribe its own small rRNA for mitoribosomes, Golgi processes glycoproteins. Low-dose actinomycin D selectively inhibits Pol I leading to nucleolar segregation and stress activating p53. High rate of Pol I transcription meets demand for ribosome production, explaining prominent nucleolus in growing cells and upregulation in cancer driven by Myc and mTOR pathways.

Ref: Granneman Annu Rev Biochem; Pol I 47S rRNA in nucleolus, snoRNP processing, 5S Pol III.

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