RNA Processing and snoRNP Complexes
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Questions and Answers

What is the function of polyadenylate polymerase in eukaryotes?

  • Removal of introns from nascent RNA
  • Transcription termination
  • Addition of a poly(A) tail to the 3' end of nascent RNA (correct)
  • Base-pairing with target RNA

    • In eukaryotes, transcription terminates when Pol II releases the nascent RNA.

    False

    What is the function of snoRNPs in RNA processing?

    Aiding in endonucleolytic cleavage and chemical modification of primary transcripts of tRNAs, rRNAs, and miRNAs.

    In eukaryotes, the exosome is involved in many steps of RNA ______________________.

    <p>decay</p> Signup and view all the answers

    Match the following RNA processing mechanisms with their functions:

    <p>Alternative splicing = Creating many different transcripts from a single gene Endonucleolytic cleavage = Removal of introns from nascent RNA Poly(A) tail addition = Stabilizing mRNA by preventing degradation Decapping and poly(A) tail shortening = Initiating mRNA degradation</p> Signup and view all the answers

    What is the result of alternative splicing and alternative poly(A) site choice in eukaryotes?

    <p>Many different transcripts from a single gene</p> Signup and view all the answers

    What is the effect of mutations in the SMN1 gene?

    <p>Decreased functional protein</p> Signup and view all the answers

    The calcitonin gene in rats has only one poly(A) site.

    <p>False</p> Signup and view all the answers

    What is the result of alternative splicing of the calcitonin gene in the thyroid?

    <p>The hormone calcitonin</p> Signup and view all the answers

    Posttranscriptional enzymatic reactions create ______________ derivatives in tRNA and pre-rRNA.

    <p>96</p> Signup and view all the answers

    What is the role of the cofactor S-adenosylmethionine in bacterial rRNA modification?

    <p>Methylating bases or ribose 2’-OH groups</p> Signup and view all the answers

    Match the following RNA modifications with their characteristics:

    <p>Pseudouridine = Found in 16S and 23S rRNA Methylated bases = Require S-adenosylmethionine Dihydrouridine = Found in 23S rRNA Ribose 2’-OH groups = Found in 16S and 23S rRNA</p> Signup and view all the answers

    Eukaryotic rRNA processing is similar to bacterial rRNA processing.

    <p>False</p> Signup and view all the answers

    What is the effect of the drug nusinersen on the SMN2 gene?

    <p>Corrects splicing to create active SMN2 protein</p> Signup and view all the answers

    What is the role of snoRNP in RNA processing?

    <p>To modify one or two rRNA nucleotides</p> Signup and view all the answers

    In eukaryotes, tRNA does not have introns that need to be excised.

    <p>False</p> Signup and view all the answers

    What is the function of Rnase P in tRNA processing?

    <p>Rnase P is an RNP with catalytic RNA that cleaves the 5' end of tRNA precursors.</p> Signup and view all the answers

    The 3' CCA trinucleotide, to which the respective amino acid is attached, is mostly added by _______________________.

    <p>tRNA nucleotidyltransferase</p> Signup and view all the answers

    What is the typical half-life of vertebrate mRNA?

    <p>~3 h</p> Signup and view all the answers

    The rate of synthesis is always equal to the rate of degradation, resulting in a constant mRNA level.

    <p>False</p> Signup and view all the answers

    Match the following enzymes with their functions in RNA processing:

    <p>Rnase P = Cleaves the 5' end of tRNA precursors Rnase D = Cleaves the 3' end of tRNA precursors tRNA nucleotidyltransferase = Adds the 3' CCA trinucleotide to tRNA</p> Signup and view all the answers

    What is the role of pseudouridine in RNA modification?

    <p>Pseudouridine is produced by removing the base and re-attaching it through its C5, which helps to control the interaction of tRNA with proteins and the ribosome.</p> Signup and view all the answers

    Study Notes

    Pre-rRNA Processing

    • Pre-rRNAs are processed in the nucleolus by processing complexes containing small nucleolar RNPs (snoRNP).
    • In yeast, the process involves the pre-rRNA, 170 nonribosomal proteins, 70 snoRNPs, and 78 ribosomal proteins.
    • Each snoRNP catalyzes the modification of one or two rRNA nucleotides (nt).

    snoRNPs

    • Each snoRNP combines a snoRNA of 60 to 300 nt with four or five proteins/enzymes.
    • 10 to 21 nt of a snoRNA is complementary to a sequence on the rRNA – identifying a base for modification.
    • One snoRNP type produces pseudouridine.

    tRNA Processing

    • Cells have 40 to 50 tRNAs, which are processed from longer precursors by removing 5’ and 3’ nt.
    • Rnase P, an RNP with catalytic RNA, cleaves the 5’ end.
    • Rnase D cleaves the 3’ end.
    • In eukaryotes, some tRNA have introns that need to be excised.
    • Specific nt are methylated, deaminated, or reduced to control its interaction with proteins and the ribosome.
    • The 3’ CCA trinucleotide, to which the respective amino acid is attached, is mostly added by tRNA nucleotidyltransferase.

    Degradation of Cellular mRNAs

    • RNA lifetime is a method of gene regulation.
    • Half-lives of mRNA vary from seconds to hours.
    • Typical half-life of vertebrate mRNA is ~3 h.
    • The pool of each mRNA turns over ~10 fold per cell generation.
    • Bacterial mRNA half-lives are only ~1.5 min.
    • Degradation occurs via ribonucleases.
    • In eukaryotes, transcription terminates when an endonuclease cleaves the nascent RNA from Pol II.
    • A poly(A) tail is added to the 3’ end by polyadenylate polymerase.

    Alternative Splicing and Poly(A) Site Choice

    • Alternative splicing and alternative poly(A) site choice in eukaryotes create many different transcripts from a single gene.
    • Primary transcripts of tRNAs, rRNAs, and miRNAs are extensively processed by endonucleolytic cleavage and chemical modification aided by snoRNPs.

    RNA Degradation

    • RNA degradation is tightly regulated.
    • In bacteria, endonucleases fragment mRNA for destruction by exonucleases.
    • In eukaryotes, mRNAs are decapped and the poly(A) tail shortened before degradation.
    • The exosome is a supramolecular complex of exo- and endonucleases involved in many steps of eukaryotic RNA decay.

    Mutations in SMN1 and Splicing

    • Mutations in SMN1 create too little functional protein, resulting in neuromuscular degeneration.
    • The drug nusinersen, an 18 nt oligomer, corrects the splicing of SMN2, creating active SMN2 protein.

    Alternative Splicing of the Calcitonin Gene

    • The combination of alternative splicing and different poly(A) sites greatly increases the variety of proteins in higher eukaryotes.
    • The calcitonin gene transcript in rats has two poly(A) sites, one used in the brain and the other in the thyroid.

    Posttranscriptional Modifications

    • Bases in tRNA and pre-rRNA are chemically modified by posttranscriptional enzymatic reactions, creating 96 derivatives: 81 in tRNAs, 30 in rRNAs.

    Bacterial Ribosomal RNAs (rRNA)

    • In bacteria, a single 30S RNA precursor of 6500 nt is cleaved by RNases III, P, and E to create the 16S, 23S, and 5S ribosomal RNAs (rRNA) – as well as some tRNAs.
    • The 16S rRNA includes a pseudouridine and 10 methylated bases or ribose 2’‐OH groups.
    • The 23S rRNA has 10 pseudouridines, 1 dihydrouridine, and 12 methylated nt.

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    Description

    This quiz covers the processing of pre-rRNA in the nucleolus, involving snoRNP complexes and their components. It explores the roles of snoRNAs, proteins, and enzymes in modifying rRNA nucleotides.

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