#ribosome biogenesis

Cell Organelles

The ribosome biogenesis pathway in eukaryotic cells involves:

Eukaryotic ribosome biogenesis is highly compartmentalized energy intensive process. Initiation occurs in nucleolus where RNA polymerase I synthesizes forty seven S precursor containing eighteen S, five point eight S, twenty eight S rRNAs, processed by small nucleolar ribonucleoproteins catalyzing cleavages, methylations and pseudouridylations concurrent with association of imported ribosomal proteins. Resulting pre-forty S and pre-sixty S particles undergo remodeling by assembly factors such as Nob1, Rio2, Rix1 in nucleoplasm ensuring correct folding. These immature subunits are exported through nuclear pores via exportin CRM1 interacting with adapter Nmd3 and Ran-GTP. Cytoplasmic maturation involves final eighteen S cleavage by Nob1, removal of anti-association factors Tif6 and eIF6, binding of functional ligands and quality proofreading. Rough ER hosts translation by mature ribosomes but not biogenesis; lysosomes, peroxisomes, Golgi degradative and modifying roles not synthesis. Disruption at any stage triggers nucleolar stress stabilizing p53 via MDM2 sequestration, linking growth to ribosome production capacity regulated by mTOR signaling.

Ref: Thomson Annu Rev Biochem; ribosome biogenesis nucleolus export pre-subunits CRM1 cytoplasmic maturation.

View Details

Processing of rRNA and tRNA

Primary rRNA transcript in E. coli is processed by

In Escherichia coli, seven rrn operons produce about 30S primary precursor containing leader, 16S, spacer tRNAs, 23S, 5S and trailer. Maturation initiates with RNase III, a Mg2+-dependent double-strand specific endoribonuclease that cleaves long inverted repeat stems flanking 16S and 23S, co-transcriptionally. Cleavage liberates pre-16S 17S, pre-23S and 9S 5S precursors carrying extra nucleotides at both ends. Subsequent processing by RNase E, RNase G, RNase PH, RNase T, PNPase removes leaders and trailers, generating mature termini essential for subunit assembly and translation activity.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6, Processing of bacterial 30S pre-rRNA by RNase III

View Details

Processing of rRNA and tRNA

Box C/D snoRNAs primarily guide

Box C/D small nucleolar RNAs guide site-specific ribose methylation of ribosomal RNA and small nuclear RNAs. Family defined by conserved sequence motifs Box C UGAUGA and Box D CUGA forming kink-turn structure recognized by Snu13 fifteen-point-five kilodalton protein, NOP56, NOP58, and fibrillarin Nop1 methyltransferase transferring methyl group from S-adenosylmethionine to 2' hydroxyl. Guide duplex upstream of Box D positions target five nucleotides upstream for methylation. Each snoRNA may harbor two guide sequences, D and D' guides, allowing dual modification. Modifications enhance hydrophobic stacking, steric stabilization of helices, and protection from nucleases within catalytic RNA cores.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Box C/D snoRNAs primarily guide methylation; NCBI Bookshelf, snoRNP mechanism

View Details

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

View Details