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#ribosome assembly

6 public questions tagged with this topic.

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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Cell Organelles

Which organelle is responsible for ribosome assembly?

Ribosome biogenesis requires coordinated synthesis of forty seven S pre-ribosomal RNA containing eighteen S, five point eight S and twenty eight S sequences separated by internal transcribed spacers. This transcription occurs in nucleolus, a nonmembranous nuclear condensate nucleated by tandem repeats of ribosomal DNA located on nucleolar organizer regions of acrocentric chromosomes. RNA polymerase I machinery with upstream binding factor and selectivity factor SL1 concentrates in fibrillar centers, nascent transcripts enter dense fibrillar component where snoRNP-guided cleavages, two prime O methylation and pseudouridylation occur guided by fibrillarin and dyskerin. Granular component then assembles pre-forty S and pre-sixty S particles with imported ribosomal proteins. Pre subunits export via CRM1 for final maturation. Endoplasmic reticulum and Golgi modify secretory proteins, peroxisomes detoxify lipids, none produce rRNA. Actinomycin D at low doses selectively inhibits Pol I disrupting nucleoli, proving synthetic role. Consequently nucleolar hypertrophy correlates with growth rate and is upregulated in transformed cells requiring high protein synthesis.

Ref: Hernandez-Verdun Trends Cell Biol; nucleolus Pol I 47S pre-rRNA ribosome factory, phase separated.

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Translation in Prokaryotes and Eukaryotes

48S complex is formed after

48S pre-initiation complex forms after cap recognition by eIF4F and recruitment of 43S pre-initiation complex to mRNA 5' end. eIF4E binds m7G cap, eIF4G bridges to polyA binding protein circularizing mRNA, then 43S joins near cap and scans 5' to 3' direction inspecting codons in P site. Upon locating AUG in favorable Kozak context, perfect codon-anticodon pairing triggers eIF1 release, GTP hydrolysis by eIF2, Pi release, locking stable 48S complex containing mRNA, 40S and initiator tRNA. Subsequent eIF5B mediated 60S joining yields elongation-competent 80S ribosome.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, 48S complex formation after cap recognition and scanning

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Translation in Prokaryotes and Eukaryotes

43S pre-initiation complex contains

Eukaryotic 43S pre-initiation complex represents key intermediate before mRNA recruitment, comprising 40S small subunit bound to eIF1, eIF1A, eIF3 multi-subunit complex, eIF5, and ternary complex eIF2 GTP Met-tRNAiMet positioned in P site. Assembly occurs independently of mRNA and ensures initiator tRNA ready to base-pair with start codon. This complex interacts with eIF4F capped mRNA to form 48S scanning complex. Formation of ternary complex is rate-limiting regulated by eIF2 alpha phosphorylation during integrated stress response reducing global initiation while permitting selective ATF4 translation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7, 43S pre-initiation complex containing 40S, eIFs, Met-tRNAi

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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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