BIOL200 lecture PL4-final

Sxl Protein Production in Female Embryos

  • A student inquired about the initial production of the Sxl protein in female embryos without prior Sxl protein to assist its pre-mRNA splicing.

Sxl Early Promoter and Transcript

  • The Sxl early promoter resides in the intron between exons 1 and 2.

  • Unique first exon in the Sxl early transcript, referred to as exon E1.

  • Exon E1 is spliced directly to exon 4 and contains a start codon, ensuring the open reading frame (ORF) aligns correctly with exon 4.

  • Reference: Zhu, Urano, and Bell (1997) Molecular and Cellular Biology 17: 1674-1683.

Identification of Sxl Transcripts

  • Deep sequencing identified several Sxl transcripts including exons 8, 765, 4, 3, 2, E1, and 1.

  • Resource: www.flybase.org

RNA Surveillance Mechanisms

  • Improperly processed pre-mRNAs are detected in the nucleus and directed to the exosome for degradation.

  • Incompletely spliced pre-mRNAs that stay associated with spliceosome components are prohibited from being transported out of the nucleus.

Nonsense-Mediated Decay (NMD)

  • NMD targets mRNAs with stop codons located before the last splice junction.

  • This typically results from:

    • Splicing errors that create frameshifts.

    • Mutations leading to premature stop codons or frameshifts.

Impact of Frameshift Mutations

  • Frameshift mutations can dramatically decrease mRNA levels.

  • Example: A 1 bp deletion in β-globin genomic DNA leads to significant RNA reduction in affected patients.

  • Reference: Lodish, Fig. 9-36.

Mechanism Details in Nonsense-Mediated Decay

  • Key Players:

    • UPF3 remains bound to mRNA and enables interaction with other components like UPF1 and SMG1 during degradation.

  • Steps include initial binding of components at the mRNA's exon-exon junction, translation initiation, and detection of premature stop codons leading to decay.

  • Diagram reference: Lodish, Fig. 9-37a.

Other RNA Surveillance Mechanisms

  • Non-stop decay: Targets mRNAs missing an in-frame stop codon due to mutations or premature polyadenylation.

  • No-go decay: Targets mRNAs stalled in translation due to structural issues or damage.

Roles of RNA in Protein Synthesis

  • RNA has three main roles during protein synthesis: tRNA charging, mRNA serving as a template, and polypeptide chain elongation.

  • Illustration of the process with ribosome and tRNA interactions (Lodish, Fig. 5-29).

Ribosome Composition

  • Ribosomes comprise RNA and proteins, with variations among organisms:

    • E. coli: 2.0 MDa, 34 proteins, 3 rRNAs.

    • Yeast: 2.3 MDa, 54 proteins, 3 rRNAs.

    • Human: 4.3 MDa, 79 proteins, 4 rRNAs.

  • Key rRNA sizes vary, demonstrating evolutionary adaptations in ribosomal structure (Data from G. Yusupov & M. Yusupov, 2014).

Fundamental Structure of Ribosomes

  • All ribosomes share a core structural design with variations among different organisms.

  • Comparison of core structures includes measurements across bacteria, lower eukaryotes, and higher eukaryotes (Lodish, Fig. 5-35).

tRNA Interaction Sites in Ribosome

  • Ribosomes contain three key interaction sites for tRNA:

    • Aminoacyl (A), peptidyl (P), and exit (E) sites.

  • This design facilitates amino acid chain growth and polypeptide synthesis (Lodish, Fig. 5-34).

Aminoacyl-tRNA Synthetases

  • These enzymes play crucial roles by recognizing specific tRNAs and binding corresponding amino acids.

  • Example: The mechanism for binding phenylalanine (Phe) to its tRNA is detailed, showcasing the importance of this molecular pairing in translation initiation (Lodish, Fig. 5-31a).

The Degenerate Genetic Code

  • The genetic code is termed degenerate due to multiple tRNAs corresponding to one amino acid.

  • Some amino acids can attach to several codons via wobble pairing, allowing for redundancy.

  • This leads to several codons coding for the same amino acid (Lodish, Table 5-1).

tRNA Structure

  • tRNA consists of distinct regions crucial for function:

    • Accceptor stem for amino acid binding.

    • Anticodon loop that interacts with corresponding mRNA codons.

    • Additional loops play roles in stabilizing the structure and facilitating modifications (Lodish, Fig. 5-32).

Recognition of Codons by tRNA

  • The modification of the anticodon loop enables tRNA to recognize more than one codon effectively (Lodish, Fig. 5-33).

Initiating Translation

  • Special tRNA (tRNAi Met) is needed for the initiation of the polypeptide chain.

  • This tRNA binds to the ribosome's P site while other aminoacyl-tRNAs associate with the A site.

Translation Initiation Steps

  • The process involves the formation of the 43S pre-initiation complex that binds all necessary components for translation.

  • Translation initiation factors (eIFs) enable assembly of the complex (Lodish, Fig. 5-36).

Importance of the 5' Cap in Translation

  • The addition of a 5' cap is vital for effective translation of eukaryotic mRNAs.

  • Capping enzyme interacts with phosphorylated C-terminal domain (CTD) of RNA polymerase II, enhancing translational efficiency.

eIF4 Complex Contributions

  • eIF4's multi-subunit complex aids in recognizing the 5' cap, recruiting mRNA to the initiation complex.

  • Component functions include helicase activity to unwind mRNA and facilitating circularization of the transcript.

Scanning for the Start Codon

  • The UPF complex interacts with the AUG start codon, resulting in GTP hydrolysis and subsequent structural changes.

  • The 48S initiation complex forms as the small ribosomal subunit, initiator tRNA, and mRNA connect.

Subunit Joining in Translation Initiation

  • Initial binding establishes a stable interaction that prevents downstream dissociation, ensuring efficient translation until completion.

  • GTP hydrolysis is critical for proper assembly of the ribosomal subunits, stabilizing the translation process.