12.3 - 12.4

Section 12.4: RNA Modifications

Overview of RNA Modifications

  • Importance of RNA modifications in gene expression
  • Objective of the section: Understanding different RNA processing aspects and their occurrence in bacteria and eukaryotes
  • Exam preparation: review materials for questions and understanding

RNA Processing

  • RNA modifications are categorized.
  • Occurrence: Distinction made between bacterial and eukaryotic processes.
    • Both Bacteria and Eukaryotes: RNA processing & base modification
    • Only Eukaryotes: Four additional processes described
  • Visual Aid: Table 12.2 - summarizes RNA modification processes.
Types of RNA Modifications

  1. RNA Processing

    • Involves taking a primary RNA transcript and cutting it into functional RNA molecules.

    • Occurs for non-coding RNA molecules such as ribosomal RNA (rRNA) and transfer RNA (tRNA).

      • Key Functional RNAs: 18S, 5.8S, and 28S rRNA

    • Mechanism:

      • Starts with RNA transcription from a DNA sequence, which includes a promoter for RNA polymerase recruitment and a terminator that signals the end of transcription.
      • Following transcription, the primary RNA transcript (pink in illustrations) must be processed.
      • Process includes cutting at phosphodiester bonds.

    • Enzymatic Action: Requires specific enzymes or complexes to facilitate the cutting of bonds.

    • Specific enzymes: RNA polymerase I primarily transcribes rRNA.

    • RNA folding: RNA molecules fold into specific shapes that are recognized by these enzyme complexes, affecting processing.

      • Visualization: Cartoon representation of process demonstrating how RNA folds and gets cleaved into separate parts.

  2. Cleavage

    • Involves cutting the bonded RNA to separate into functional rRNA molecules.
    • Visualization: Depicts where the cleavage occurs and its relevance in separating vital RNA units.
    • Context of eukaryotes versus prokaryotes: in eukaryotes, nucleolus structure inside the nucleus is important for rRNA biogenesis.
    • Processing of rRNA occurs within the nucleolus, with various RNA/protein complexes performing this action.

  3. Examples of Enzyme Complexes for Cleavage:

    • Composed of small RNA and proteins: e.g. RNase P and RNase D as discussed in other sources. Focus on general roles rather than specific names.

  4. tRNA Processing and Structure

    • Similar processing as rRNA. Example shown includes seven tRNAs concatenated in primary transcript.
    • Importance of RNA folding as tRNA adopts a cloverleaf structure for functionality and specific binding.
    • Nuclease types:
      • Exonucleases: Cut phosphodiester bonds from the ends of nucleic acids.
      • Endonucleases: Cut within nucleic acid streams, which processes tRNAs effectively.
      • Examples of endo and exonucleases showcased.

  5. Ribozymes

    • RNA molecules capable of catalyzing reactions, identified in the early 1980s; Nobel Prize attributed to Sydney Altman.
    • Further discussion on ribozymes in the context of RNA catalysis.
    • Reflections on enzymatic specificity and recognition of RNA structures involved in cleavage.

Splicing Mechanism

  • Introduction to splicing of RNA to remove introns and link exons.

  • Comparison between bacterial and eukaryotic processes with emphasis on eukaryotic splicing.

  • Description of splicing for group 3 introns - focused mainly on eukaryotic organisms.

    • Steps in Splicing:
    1. Identify and Cut Out Introns: RNA sequence has intervening sequences.
    2. Bringing Together Exons: Highlighted in figures depicting mechanics and how splicing is executed with the use of spliceosomes composed of small nuclear RNAs (snRNAs) and associated proteins.
    3. Functional Mechanism of Spliceosome: Detailed inner workings of spliceosome role and the involvement of snRNPs during the action of splicing which links exons together post-intron removal.
Alternative Splicing
  • Definition: Mechanism to create multiple protein variants from a single gene by different combinations of exons.
  • Importance: Involves up to 70% of human genes, demonstrating adaptability in function across different cell types.
  • Example in Muscle Cells:
    • Alpha tropomyosin has variations due to alternative splicing showing capabilities within smooth versus striated muscle types, leading to functional protein differences.

Capping and Polyadenylation

5’ Capping
  • Addition of a 7-methylguanylate to the 5’ end of eukaryotic mRNA.
  • Mechanism:
    1. Triphosphatase action breaks down one phosphate from RNA molecule.
    2. Guanylyltransferase introduces GMP attaching to RNA, forming amine bonds.
    3. Methyltransferase adds a methyl group on nitrogen-7 of guanine, establishing the cap.
  • Functions: Capping promotes mRNA transport, stabilizes mRNA against degradation, and aids in splicing and initiation of translation.
3’ Poly A Tail
  • Post-transcriptional addition of adenine nucleotides not encoded in DNA.
  • Mechanism:
    • Endonuclease cuts the transcript, followed by the addition of adenines via poly(A) polymerase, resulting in a tail of typical length 200-250 bases.
  • Importance: Facilitates stabilization and longevity of mRNA, aids in nuclear export, and is involved in initiation of translation.

RNA Editing

  • Definition: Base sequence changes through addition/deletion or conversion by enzymatic activity.
  • Example of changing cytosine to uracil or adenine to hypoxanthine changing the reading sequence during translation.
  • Rare occurrences in eukaryotic RNA sequences.
  • Significance: Provides infrequent but critical alterations affecting protein synthesis from edited mRNA.

Base Modification

  • Common in both eukaryotes and prokaryotes, especially in tRNA.
  • Mechanism includes covalent modifications influencing properties but not changing the base itself (e.g., methylation).
  • Lack of modifications can lead to translation errors, impacting function and health.

Comparative Summary

  • Climate in the transcription and post-transcription processes in bacteria versus eukaryotes.
  • Processes discussed include initiation, elongation, termination, RNA processing types, and key distinctions, e.g., presence of introns in eukaryotic mRNAs.
  • Highlighting complexities introduced with gene regulation, especially in eukaryotes.

Questions for Review

  • Consider questions on specific enzyme actions, RNA processing types, and scenarios in both bacteria and eukaryotic contexts.