Processing of rRNA and tRNA

Processing of rRNA and tRNA

Major role of rRNA processing is

Maturation of ribosomal RNA serves principal purpose of ribosome assembly rather than gene regulation or degradation alone. Processing cleaves polycistronic or 45S precursors, removes internal and external spacers, incorporates chemical modifications and coordinates binding of ribosomal proteins to produce functional 30S, 50S or 40S, 60S subunits. Modifications fine-tune decoding and peptidyl transferase function. Proper assembly guarantees translation competence, nucleolar surveillance, stress responses, and overall cellular growth control, linking rRNA biogenesis rate to cell proliferation demands. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, rRNA processing and ribosome assembly pathway

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Processing of rRNA and tRNA

rRNA introns are mainly found in

Bacterial ribosomal operons rarely harbor introns due to genome streamlining and rapid replication pressure favoring compactness. In contrast, archaeal and eukaryotic rRNA genes, including nuclear 28S and 18S, mitochondrial and chloroplast rRNAs, frequently contain Group I self-splicing introns located at highly conserved core regions. These introns are co-transcriptionally removed by self-splicing assisted by proteins, restoring functional rRNA. Distribution reflects ancestral intron richness in early life and selective loss in bacteria, while archaea and eukaryotes retained mobile intronic elements. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8, Distribution of rRNA introns in archaea and eukaryotes

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Processing of rRNA and tRNA

Retrohoming involves

Retrohoming describes mobility mechanism of Group II introns encoding reverse transcriptase and sometimes endonuclease in same polypeptide. Pathway initiates with intron RNA reverse splicing into single-stranded DNA target, then using target 3' end as primer, intron-encoded RT reverse-transcribes intron RNA into cDNA. Second strand synthesized by host repair polymerase. Thus retrohoming depends on reverse transcription converting RNA intermediate to DNA, producing permanent insertion. This retromobility inspired development of targetron gene editing biotechnology for site-specific gene disruption in bacteria. 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, Retrohoming of Group II introns via reverse transcription

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Processing of rRNA and tRNA

Homing endonuclease recognizes

Homing endonuclease recognizes an extended homing site, typically 14-40 base pairs, corresponding exactly to the intron insertion locus in intron-free allele. Recognition sequence spans cleavage site, ensuring endonuclease cuts only alleles lacking intron, because presence of intron disrupts site. Cleavage generates 4-base 3' overhangs or blunt ends, activating homologous recombination machinery. High specificity prevents widespread genome toxicity yet tolerates degeneracy for spread to related sites. Cellular repair copies intron plus flanking homology, propagating element efficiently. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: Homing endonuclease recognition of DNA homing site

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Processing of rRNA and tRNA

Homing introns encode

Mobile or homing introns are self-splicing Group I or Group II introns containing open reading frames encoding homing endonucleases, often belonging to LAGLIDADG or HNH families, and sometimes combined maturase-endonuclease functions. Homing endonuclease introduces site-specific double-strand break in intron-less allele, stimulating homologous recombination repair using intron-containing genome as donor, thereby duplicating intron into new allele. This selfish element drives super-Mendelian inheritance, spreads despite fitness cost, and links splicing to DNA mobility. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, Homing introns encoding homing endonucleases

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Processing of rRNA and tRNA

2′–5′ phosphodiester bond is formed during

Formation of unusual 2'-5' phosphodiester bond distinguishes Group II and spliceosomal splicing from Group I. During first transesterification, nucleophilic attack by 2' hydroxyl of branchpoint adenosine on phosphodiester at 5' splice junction creates branched RNA where intron 5' terminus connected to branch Adenosine via 2'-5' linkage, producing lariat with loop and 3' tail. Group I splicing retains standard 3'-5' linkages using external G. Detecting 2'-5' bond is diagnostic marker for lariat introns and spliceosomal catalysis in sequencing studies. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 32, 2'-5' phosphodiester bond in Group II lariat splicing

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Processing of rRNA and tRNA

Group II intron splicing forms

Group II introns share mechanistic ancestry with spliceosomes, forming lariat structure rather than linear product. First step uses internal bulged adenosine within intron domain VI as nucleophile; its 2' OH attacks 5' splice site, generating branched intermediate where intron 5' end linked via 2'-5' phosphodiester to branch point, forming loop. Second step, 5' exon attacks 3' splice site, ligating exons and releasing lariat intron. This lariat pathway underpins evolution of spliceosomal snRNA-mediated splicing in eukaryotic nuclei. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8, Group II intron lariat formation via branch-point adenosine

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Processing of rRNA and tRNA

Group I intron is released as

Following two consecutive transesterifications, Group I intron is released as linear molecule carrying exogenous guanosine at its 5' terminus added during first step. The product is not branched because nucleophile is external, not internal branch adenosine. Linear intron may subsequently undergo circularization reactions. Ligated exons join via canonical 3'-5' phosphodiester bond to yield functional mRNA, rRNA or tRNA. Reaction underscores energy-independent splicing using RNA structure itself for catalysis, classic ribozyme chemistry characterized in Tetrahymena rRNA intron. 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, Linear excision product of Group I introns

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Processing of rRNA and tRNA

Group I intron splicing requires

Group I introns are self-splicing ribozymes catalyzing their own excision via two transesterification steps that require external guanosine cofactor. Free guanosine or GMP, GDP, GTP binds specific G-binding pocket in P7 paired region of intron tertiary structure. Its 3' hydroxyl serves as nucleophile attacking 5' splice site, becoming covalently attached to intron 5' end, freeing upstream exon. Second attack by upstream exon 3' OH on 3' splice site ligates exons. No ATP hydrolyzed, reversible, aided in vivo by maturases. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Watson Molecular Biology of the Gene, 7th ed., Chapter 15, Group I intron self-splicing requiring external guanosine

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Processing of rRNA and tRNA

tRNA intron splicing is

Eukaryotic and archaeal pre-tRNAs often carry intervening sequences in anticodon loop between positions 37 and 38. Unlike spliceosomal or self-splicing introns, tRNA intron excision requires protein enzymes: heterotetrameric tRNA splicing endonuclease TSEN complex makes two precise cuts excising intron, generating 5' and 3' halves with unusual cyclic phosphate termini. Subsequent healing by kinase, phosphodiesterase and ligase activity (RtcB or Trl1) rejoins exons using ATP. This enzyme-mediated pathway occurs in nucleus and distinguishes tRNA splicing from Group I self-splicing. 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, Eukaryotic tRNA splicing by TSEN endonuclease complex

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Processing of rRNA and tRNA

CCA addition to tRNA is

CCA synthesis by tRNA nucleotidyltransferase occurs without nucleic acid template, termed template-independent polymerization. Specificity arises from protein-side templating: active site pockets formed by amino acid side chains and backbone hydrogen bonds discriminate CTP versus ATP, with conformational remodeling after each addition switching nucleotide preference from CTP to ATP after two incorporations. Thus enzyme itself encodes CCA sequence. This mechanism allows universal addition to all tRNAs regardless of gene sequence and enables repair of cleaved or degraded tRNA ends in quality control.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 32, Template-independent CCA addition mechanism

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Processing of rRNA and tRNA

CCA sequence at 3′ end of tRNA is added by

All mature tRNAs require conserved CCA terminal trinucleotide at 3' end where amino acid attaches via ester linkage. In most bacteria and eukaryotes, CCA is absent from tRNA gene and added post-transcriptionally by ATP(CTP):tRNA nucleotidyltransferase, CCA-adding enzyme. This enzyme binds T-loop and acceptor stem, sequentially polymerizing C-C-A using CTP and ATP. It also surveys tRNA quality, adding CCA to correctly folded tRNAs and engaging in repair cycles where damaged termini are excised and resynthesized, ensuring translational competence. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.

Ref: Watson et al., Molecular Biology of the Gene, 7th ed., Chapter 15, tRNA nucleotidyltransferase CCA-adding enzyme

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