Processing of mRNA, rRNA, tRNA

Processing of mRNA, rRNA, tRNA

tRNA splicing is

Transfer RNA splicing removes intervening introns typically located one nucleotide three prime to anticodon loop in eukaryotic pre-transfer RNAs. Unlike messenger RNA splicing mediated by spliceosomes or group I self-splicing, tRNA introns are excised by protein enzymes: heterotetrameric splicing endonuclease complex SEN comprises Sen2, Sen34 catalytic subunits, Sen15, Sen54 structural, cutting at both splice sites leaving two halves with 2',3' cyclic phosphate and five prime hydroxyl. Ligase complex containing Trl1 phosphorylates hydroxyl and joins ends via healing and sealing steps, adding phosphate from GTP before ligation, producing mature spliced tRNA ready for modifications.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: tRNA splicing enzyme mediated; Alberts 7th ed., Pre-tRNA processing and ligation

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Processing of mRNA, rRNA, tRNA

CCA sequence at 3' end of tRNA is added by

Mature transfer RNAs require universal CCA sequence at three prime terminus with terminal adenosine bearing amino acid attachment via ester linkage to two prime or three prime hydroxyl. In many eukaryotic genes, particularly vertebrate mitochondrial transfer RNAs, CCA not encoded and must be added post-transcriptionally. CCA-adding enzyme, member of class two nucleotidyltransferase beta-nucleotidyltransferase family, acts template-independently, sequentially adding cytidine, cytidine, adenosine using CTP and ATP as substrates while proofreading. Enzyme uses single active site with flexibility accommodating CTP versus ATP, flipping tRNA acceptor stem after each addition. Activity essential for translation fidelity.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: CCA-adding enzyme adds CCA; Berg et al., Biochemistry, CCA nucleotidyltransferase mechanism

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Processing of mRNA, rRNA, tRNA

RNase P is involved in processing of

RNase P is essential ribonucleoprotein endonuclease best known for processing five prime leader removal from precursor transfer RNAs across all domains of life. Bacterial RNase P consists of catalytic M1 RNA of approximately three hundred seventy-seven nucleotides capable of cleavage alone in vitro, classic ribozyme example, plus C5 protein enhancing specificity. Eukaryotic nuclear RNase P has H1 RNA plus ten protein subunits including Pop1, Pop5, Rpp20. Reaction cleaves phosphodiester at plus one position generating mature five prime end with phosphate, leaving three prime hydroxyl of leader. Human RNase P also processes long noncoding RNAs and regulates Pol I transcription.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: RNase P processes 5' of tRNA; Cech, Ribozyme RNase P mechanism

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Processing of mRNA, rRNA, tRNA

Box H/ACA snoRNAs guide

Box H/ACA small nucleolar RNAs contain two hairpin structures linked by H box ANANNA and terminated by ACA box three nucleotides from three prime end, forming hairpin-hinge-hairpin-tail architecture. Each hairpin harbors internal pseudouridylation pocket with antisense elements flanking target uridine left unpaired for isomerization. Core proteins include pseudouridine synthase dyskerin Cbf5, NOP10, Nhp2, Gar1. Dyskerin isomerizes uridine to pseudouridine via base rotation adding extra NH hydrogen donor strengthening base stacking and water-mediated interactions, enhancing ribosome stability. Mutations cause dyskeratosis congenita with bone marrow failure and ribosome biogenesis defects.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Box H/ACA guides pseudouridylation; Alberts 7th ed., Pseudouridine synthase function

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Processing of mRNA, rRNA, tRNA

Box C/D snoRNAs guide

Box C/D small nucleolar RNAs are defined by conserved motifs Box C RUGAUGA and Box D CUGA near five prime and three prime ends that fold into kink-turn structures bound by core proteins Snu13, NOP56, NOP58, and methyltransferase fibrillarin Nop1. Each snoRNA contains antisense guide element ten to twenty-one nucleotides complementary to pre-ribosomal RNA placing target ribose five nucleotides upstream of Box D or D' for methylation. Reaction transfers methyl group from SAM to 2'-hydroxyl creating 2'-O-methyl ribose enhancing hydrophobic stacking and nuclease resistance concentrated within functional centers like peptidyl transferase loop.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Box C/D snoRNAs guide methylation; Alberts 7th ed., snoRNP mediated methylation

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Processing of mRNA, rRNA, tRNA

rRNA chemical modification is guided by

In eukaryotes, precursor ribosomal RNA transcribed as forty-seven S transcript in nucleolus undergoes extensive covalent base modifications essential for ribosome assembly, stability, and decoding accuracy. Modifications include about one hundred 2'-O-methylations and one hundred pseudouridylations guided not by protein enzymes recognizing sequence alone but by small nucleolar RNAs snoRNAs providing sequence complementarity to target site and positioning catalytic proteins fibrillarin for methylation and dyskerin for pseudouridylation. SnoRNAs are intron-encoded, highly abundant within dense fibrillar component of nucleolus where rRNA transcription and early processing occur coordinated with assembly factors.

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

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Processing of mRNA, rRNA, tRNA

Guide RNAs are required for RNA editing in

In mitochondria of trypanosomatids including Trypanosoma brucei, many protein-coding genes exist as cryptogenes requiring extensive uridine insertion and deletion editing guided by small noncoding guide RNAs encoded in kinetoplast DNA. Guide RNAs form short anchor duplex with pre-messenger RNA upstream of editing sites, specifying positions via base pairing where editosome complex containing endonuclease, 3' terminal uridylyltransferase TUTase adding uridines, and RNA ligase executes insertions. Editing can add hundreds of uridines and delete dozens, dramatically remodeling open reading frames to generate functional cytochrome oxidase subunits and other mitochondrial proteins essential for respiration.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Guide RNAs required for editing mitochondria of trypanosomes; Cell, Trypanosome editosome structure

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Processing of mRNA, rRNA, tRNA

A to I RNA editing is catalyzed by

Adenosine to inosine editing, most frequent editing in mammalian transcriptomes, catalyzed by ADAR family adenosine deaminases acting on RNA containing double-stranded regions. Enzymes ADAR1 p110 p150 and ADAR2 deaminate adenosine at C6 position via hydrolytic deamination producing inosine, which ribosome and spliceosome read as guanosine due to base pairing with cytidine. Targets include glutamate receptor GluA2 Q/R site altering calcium permeability, serotonin 5HT2C receptor fine-tuning signaling. ADARs contain dsRNA binding domains, deaminase domain, editing complementary sequences often within Alu repeats. Deficiency triggers MDA5-mediated interferon response.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: ADAR catalyzes A-to-I editing; Nature Rev Mol Cell Biol, ADAR editing functions

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Processing of mRNA, rRNA, tRNA

C to U RNA editing occurs in

Classic example of mammalian C-to-U editing occurs in apolipoprotein B messenger RNA in small intestine, converting glutamine codon CAA at codon two thousand one hundred fifty-three to stop codon UAA, yielding truncated ApoB48 protein of two thousand one hundred fifty-two amino acids versus full-length ApoB100 of four thousand five hundred thirty-six amino acids synthesized in liver. ApoB48 lacks low-density lipoprotein receptor binding domain, essential for chylomicron formation. Editing mediated by APOBEC1 cytidine deaminase and ACF cofactor recognizing mooring sequence eleven nucleotides downstream of target cytidine within editosome complex.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: C-to-U editing apolipoprotein B mRNA; Berg et al., Biochemistry, APOBEC1 mechanism

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Processing of mRNA, rRNA, tRNA

RNA editing involves

RNA editing encompasses post-transcriptional processes altering nucleotide sequence of primary transcript after synthesis, thereby changing coding potential without modifying genome. Mechanisms include site-specific insertion or deletion of nucleotides, such as uridine insertion guided by guide RNAs in trypanosome mitochondria creating correct reading frames, and deaminase-mediated base conversions C-to-U in apolipoprotein B mRNA or A-to-I in neuronal receptors catalyzed by ADAR enzymes converting codon and affecting splicing patterns. Editing can affect stability, localization, and translation efficiency and contributes to transcriptome diversification beyond genomic information.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: RNA editing involves addition/deletion of nucleotides; Lodish 9th ed., Overview of editing types

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Processing of mRNA, rRNA, tRNA

Trans-splicing is commonly seen in

Trans-splicing is process joining exons encoded on different precursor RNA molecules into single mature messenger RNA, contrasting with conventional cis-splicing within same transcript. Widespread in kinetoplastid protozoa Trypanosoma brucei, Leishmania, where spliced leader RNA donor provides thirty-nine nucleotide mini-exon capped at five prime end to all messenger RNAs, enabling polycistronic transcription processing. Requires spliceosome similar machinery using SL RNA as five prime splice site donor. Also observed in nematode Caenorhabditis elegans where about seventy percent of pre-mRNAs receive spliced leader, and rarely in mammals.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Trans-splicing Trypanosoma spliced leader; Alberts et al., Trans-splicing in protozoa

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Processing of mRNA, rRNA, tRNA

AU-AC introns are spliced by

Most eukaryotic introns follow GT-AG rule with GU at five prime end and AG at three prime end recognized by major U2-dependent spliceosome comprising U1, U2, U4, U5, U6 small nuclear RNPs. Rare introns with AU at five prime and AC at three prime, as well as subset of GU-AG introns with distinct branch site consensus, are removed by minor or U12-dependent spliceosome comprising U11, U12, U4atac, U6atac, U5 snRNPs. U11-U12 di-snRNP functionally analogous to U1-U2. Minor introns occur in genes encoding ion channels, DNA repair proteins, and are highly conserved evolutionarily.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: AU-AC introns spliced by minor spliceosome; Alberts et al., U12-dependent splicing

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