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#rRNA

12 public questions tagged with this topic.

Basics of Cell Biology

Which organelle is responsible for the production of ribosomal RNA?

Nucleolus is non-membrane-bound condensate within nucleus formed by liquid-liquid phase separation around nucleolar organizer regions on chromosomes 13,14,15,21,22 containing tandem arrays of 45S rRNA genes 400 copies in humans. RNA polymerase I, with transcription factors UBF and SL1, transcribes 47S pre-rRNA precursor containing 18S, 5.8S, 28S sequences separated by internal and external transcribed spacers. Co-transcriptional processing involves snoRNPs C/D box guiding 2'-O-methylation and H/ACA box guiding pseudouridylation, endonucleolytic cleavages generating mature rRNAs. 5S rRNA transcribed by Pol III in nucleoplasm imports. Ribosomal proteins, 33 for 60S and 21 for 40S plus assembly factors, imported from cytoplasm, combine hierarchically to pre-60S and pre-40S particles exported via CRM1 and RanGTP. Tripartite morphology fibrillar center containing Pol I, dense fibrillar component where processing occurs, granular component for assembly reflects this vectorial flow. Nucleolar stress with impaired rRNA synthesis stabilizes p53 via MDM2 sequestration. Rough ER, Golgi, mitochondria perform translation, glycosylation, and respiration, not ribosomal RNA synthesis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Nucleolus and rRNA Synthesis.

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Cell Organelles

The dense fibrillar component (DFC) of the nucleolus is involved in:

Nucleolar organization reflects sequential ribosome assembly. Fibrillar centers contain transcriptionally inactive rDNA, border to dense fibrillar component active transcription. Dense fibrillar component appears as tight fibrillar mesh containing fibrillarin methyltransferase, Nop56, Nop58 and U3 snoRNP required for early cleavages of forty seven S precursor at sites A0, A1 and A2 within external and internal transcribed spacers, plus two prime O methylation directed by box C/D snoRNAs and pseudouridylation by box H/ACA snoRNAs. This processing trims precursor into eighteen S, five point eight S, twenty eight S RNAs. Granular component then hosts pre-ribosome assembly. In situ hybridization shows pre-rRNA rapidly moves from dense fibrillar component outward. Inhibition with five-fluorouridine labels dense fibrillar component preferentially, low actinomycin D disrupts its structure blocking processing while transcription persists transiently. DNA replication occurs at many foci throughout nucleoplasm S phase, protein folding ER, chromosome condensation at mitosis via condensin, distinguishing dense fibrillar component role specifically in ribosomal RNA maturation crucial for ribosome supply.

Ref: Boisvert Annu Rev Biochem; nucleolus DFC fibrillarin early cleavage methylation pseudouridylation.

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Translation in Prokaryotes and Eukaryotes

Peptidyl transferase activity resides in

Genetic, biochemical, and crystallographic studies established that peptide bond formation is catalyzed by ribosomal RNA, not protein side chains. The peptidyl transferase center resides within domain V of large subunit 23S rRNA in prokaryotes, conserved as 28S rRNA in eukaryotes, where nucleotides position the 3' CCA ends of P-site and A-site tRNAs for nucleophilic attack of amino group on ester carbonyl. Key adenine A2451 contributes orientation and pKa perturbation rather than direct acid-base chemistry. Deproteinized large subunit retains catalytic activity, demonstrating the ribosome is a true ribozyme, the largest natural RNA catalyst with synthetic polymerase activity known.

Ref: Alberts Molecular Biology of the Cell Fig 6-80; NCBI Bookshelf NBK21760 - peptidyl transferase center composed of 23S rRNA, ribosome is ribozyme

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Translation in Prokaryotes and Eukaryotes

Which rRNA contains anti–Shine Dalgarno sequence?

Anti-Shine-Dalgarno sequence resides at 3' terminal region of 16S rRNA in 30S small subunit, conserved nucleotides 1534 to 1540 with sequence CCUCCU complementary to Shine-Dalgarno AGGAGG in mRNA. Located in a single-stranded tail of 16S, it becomes accessible for base-pairing upstream of start codon, anchoring mRNA with optimal spacing five to nine nucleotides from AUG for initiator tRNA engagement. Mutations altering complementarity impair translation of canonical mRNAs, while engineered systems exploit altered anti-SD for orthogonal translation regulating gene expression synthetically.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, 16S rRNA anti-Shine-Dalgarno sequence CCUCCU at 3' end

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Processing of rRNA and tRNA

rRNA chemical modifications mainly occur in

The bulk of rRNA nucleotide modifications, pseudouridylation and 2'-O-methylation totaling over 200 sites in human rRNA, occurs in nucleolus, the site of rRNA synthesis and early ribosome assembly. Modification is mediated by small nucleolar RNPs, H/ACA guiding pseudouridine synthase dyskerin and C/D guiding fibrillarin methyltransferase, base-pairing with nascent 45S pre-rRNA co-transcriptionally. Nucleolar concentration of factors couples transcription, modification, folding and protein binding, ensuring modified nucleotides cluster at functional centers like decoding and peptidyl transferase for accuracy and stability. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6, Nucleolar snoRNP-guided rRNA modifications

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Processing of rRNA and tRNA

rRNA processing in prokaryotes involves which type of RNAs?

Bacterial ribosomal RNA operons (rrn) are transcribed as long polycistronic precursors rather than individual genes. The primary transcript typically around 30S contains 16S, 23S, 5S rRNAs and interspersed tRNAs co-transcribed in one unit. This polycistronic organization allows stoichiometric production of rRNAs. The transcript folds into extensive double-stranded processing stems recognized by RNase III for initial cleavage, followed by RNase E, RNase G, RNase T and other nucleases for secondary trimming, ultimately releasing mature rRNAs ready for ribosomal assembly and coordinated growth regulation.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 32, Bacterial rRNA operons and polycistronic transcription

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Processing of rRNA and tRNA

Chemical modification of rRNA in eukaryotes is mainly guided by

Chemical modification of ribosomal RNAs in eukaryotes involves predominantly 2'-O-methylation and pseudouridylation clusters concentrated in decoding center, peptidyl transferase center, and intersubunit bridges, enhancing translation accuracy and stability. These modifications are guided by small nucleolar RNAs snoRNAs residing in nucleolus and Cajal bodies that base pair via antisense elements to pre-rRNA positioning methyltransferase fibrillarin or pseudouridine synthase dyskerin. Box C/D family guides methylation while Box H/ACA guides pseudouridylation. Majority of vertebrate snoRNAs encoded within introns of protein-coding genes or noncoding gas5-like hosts, excised during splicing and assembled into snoRNPs.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: snoRNA guides rRNA modification; Lodish 9th ed., Overview rRNA chemical modifications

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Transcription in Eukaryotes

RNA polymerase I transcribes

RNA polymerase I is localized to fibrillar center of nucleolus and devoted exclusively to ribosomal RNA production, accounting for up to sixty percent of total cellular transcription in proliferating cells. It transcribes a single tandem repeat locus into large 47S precursor in mammals, 45S in mice, containing 18S, 5.8S, and 28S sequences separated by external and internal transcribed spacers. Spacer regions are removed by endo and exonucleases guided by snoRNAs. Promoter includes core element and upstream control element bound by selectivity factor SL1 and UBF architectural factor.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Pol I transcribes 5.8S, 18S, 28S rRNA; Lodish 9th ed., Nucleolar transcription

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