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MCB 250 - Dr. James M. Slauch - Dept of Microbiology

Overview of RNA Types

  • Three Types of RNA

    • Messenger RNA (mRNA)

    • Serves as a template for protein synthesis.

    • Characterized by instability, with a half-life of approximately 3 minutes in E. coli.

    • Transfer RNA (tRNA)

    • Functions as an adapter between mRNA and amino acids during translation.

    • More stable compared to mRNA.

    • Ribosomal RNA (rRNA)

    • Essential component of ribosomes, which are the sites of protein synthesis.

    • Major species in prokaryotes (bacteria): 5S, 16S, and 23S rRNA.

    • Major species in eukaryotes: 5.8S, 18S, and 28S rRNA.

    • Generally stable, ensuring the ribosomal structure is maintained.

Definition of a Gene

  • What is a Gene?

    • Genes are factors that control traits or confer phenotypes, serving as the fundamental unit of inheritance.

    • Each gene consists of a segment of DNA that comprises a transcribed region along with regulatory sequences that facilitate transcription.

    • The flow of genetic information can be summarized as follows:

    • DNA → mRNA → Protein → tRNA → rRNA

Processing of Ribosomal RNA Transcripts

  • rRNA Transcripts Processing

    • The precursor to rRNA (Pre-rRNA transcript) undergoes multiple cleavage steps to yield mature rRNAs:

    • Starts with a 30S pre-rRNA segment designated for processing.

    • Cleavage events ultimately produce:

      • 16S rRNA

      • tRNA

      • 23S rRNA

      • 5S rRNA

    • The intermediates formed during this process include 17S and 25S rRNAs.

    • In E. coli, there are 7 rRNA loci scattered throughout the chromosome that encode these rRNA molecules.

Structure of 16S rRNA

  • 16S rRNA Structure

    • Composed of 1542 nucleotides, the 16S rRNA adopts a specific structure that is critical for its function in the ribosome.

Structure of 23S rRNA

  • 23S rRNA Structure

    • Contains 2904 nucleotides, contributing to the ribosomal framework and enzymatic properties associated with ribosomal function.

Structure of 5S rRNA

  • 5S rRNA Structure

    • A smaller RNA molecule comprising 120 nucleotides, which play an important role in the assembly and function of ribosomal RNA.

Antitermination Mechanism

  • Antitermination in rRNA Operons

    • Antitermination allows for the expression of rRNA operons by preventing premature termination of transcription, typically mediated by the Rho protein.

    • Generally, rRNA operons are not translated; thus, Rho ordinarily terminates transcription. However, auxiliary proteins prevent this by interacting with RNA Polymerase.

    • Instead of pausing, RNA Polymerase continues transcription until it encounters strong intrinsic or Rho-independent terminators, achieving transcription speeds up to 100 nucleotides per second.

    • Antitermination was first discovered in bacteriophage Lambda, illustrating a significant point in understanding transcription regulation.

Eukaryotic rRNA Synthesis

  • Eukaryotic rRNA Characteristics

    • The transcriptional mechanism for rRNA in eukaryotes is notably more complex than in prokaryotes.

    • Multiple types of RNA Polymerase exist in eukaryotic cells; specifically, rRNA operons are transcribed by RNA Polymerase I (Pol I).

    • Unlike messenger RNA, Pol I transcripts are not subjected to capping, splicing, or polyadenylation processes.

Localization of rRNA Synthesis

  • Production of rRNA by Pol I

    • In eukaryotic cells, particularly within the nucleolus, rRNA is synthesized by Pol I.

    • rRNA genes are arranged as tandem repeats, with a total of approximately 400 rRNA genes clustered across 5 different chromosomes in humans.

    • Transcription and processing of rRNA occur locally in the nucleolus, underscoring its role as a focal point for ribosome biogenesis.