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MCB 250 - mRNA Processing Study Notes

Instructor Information

  • Course: MCB 250

  • Lecture: Vcast 33

  • Instructor: Dr. James M. Slauch

  • Department: Microbiology

Key Literature

  • Study Referenced:

    • Meinhart, et al. (2005), Genes and Development, 19:1401

    • Focus on the role of the C-terminal domain (CTD) of RNA polymerase II (Pol II) during transcription.

RNA Polymerase II C-terminal Domain (CTD)

  • Function: The CTD serves as a binding platform for various proteins involved in transcription.

  • Phosphorylation State Changes: The phosphorylation state of the CTD varies during the transcription process, which determines the specific proteins that can bind to it.

mRNA Processing in Eukaryotes

  • Eukaryotic Characteristics:

    • Eukaryotes have nuclei which separate transcription and translation into distinct cellular compartments.

  • mRNA Processing Steps: Once synthesized, mRNA undergoes several processing steps before it is transported to the cytoplasm:

    • 5' Cap Addition

    • 3' Polyadenylation

    • Splicing

  • Variability: There is variability in the structures and signals associated with these mRNA processing events among different eukaryotes, with a focus on mammals in this study.

Structure of the 5' Cap of Eukaryotic mRNAs

  • The 5' cap consists of:

    • 7-Methyl-guanosine attached by a 5',5'-Triphosphate linkage.

    • Structural representation-

    • 7-Methyl-Guanylate

    • Unique molecular structure highlighting methylation at various positions (2' O-methylation).

Enzymatic Process of 5' Cap Synthesis

  • Capping Enzymes: Enzymes responsible for the capping of mRNA are associated with the CTD of Pol II.

    • Guanylyl transferase: The primary enzyme that catalyzes the addition of the cap.

    • Its role ensures that the mRNA is capped during transcription.

  • Cap-Binding Complex (CBC): Once the cap is synthesized, guanylyltransferase dissociates, and the CBC binds to the cap.

Functions of 5' mRNA Capping

  • Timing: Capping occurs immediately after synthesis of the 5' end and prior to the completion of transcription.

  • Key Functions:

    • Important for Splicing: Capping is crucial for the splicing of introns from the mRNA transcript.

    • Protection from RNases: The cap provides protection against degradation by RNases.

    • Increased Stability: Capping enhances the overall stability of the mRNA molecule.

    • Translation Efficiency: The cap is vital for the efficient initiation of translation by recruiting the cap-binding protein.

    • Nuclear Transport: The cap-binding protein is also required for the effective transport of the mRNA from the nucleus into the cytoplasm.

Polyadenylation of mRNA

  • Definition: Polyadenylation refers to the addition of a poly(A) tail at the 3' end of the mRNA transcript.

  • Process: Involves two main steps:

    1. Cleavage of the mRNA at a specific sequence:

    • Recognition Sequence: The sequence recognized is 5'−AAUAAA−~20 nt (GU-rich)−3'.

    1. Addition of Poly(A) Tail: About 200 adenine nucleotides are added after the cleavage.

  • Timing: Polyadenylation can occur prior to the completion of transcription and may be positioned more than 500 nucleotides from the actual 3' end of the transcript.

Factors in Polyadenylation

  • Enzymatic Role:

    • Pol II: RNA polymerase II transcribes the poly(A) addition site which necessitates specific polyadenylation factors.

    • Endonuclease: Recognizes the poly(A) signal (AAUAAA) and initiates the cleavage of the mRNA.

    • Poly-A Polymerase (PAP): Synthesizes the poly(A) tail once cleavage occurs.

    • Poly(A) Binding Protein (PABP): Binds to the poly(A) tail to protect it from degradation by nucleases.

Functions of Polyadenylation

  • Stability: The addition of the poly(A) tail is critical for protecting mRNA from RNase degradation; the loss of the tail correlates with mRNA degradation.

  • Translation: The presence of the poly(A) tail enhances translation efficiency, mediated by PABP which binds to the poly(A) tail.

Visual Aids

  • Figures referenced include detailed molecular structures and mechanisms of mRNA processing which elucidate the aforementioned processes (Figures 16-2a, 16-2b, 16-4 from Molecular Biology: Principles and Practice).