L6 Transcription & RNA Processing

The Central Dogma of Molecular Biology

The central dogma outlines the flow of genetic information within a biological system, summarized as follows:
DNA→RNA→Protein\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}
This fundamental framework indicates that information encoded in DNA is transcribed into RNA, which in turn is translated into protein.

Transcription Directionality

Transcription involves the synthesis of an RNA chain that is identical in sequence to the coding strand of DNA. The process proceeds in the 5’ to 3’ direction, making the RNA transcript complementary to the DNA template strand, which runs in the 3’ to 5’ direction.

Major Types of RNA

  1. rRNA (Ribosomal RNA): This is the major structural component of ribosomes and constitutes more than 85% of the total RNA in cells.

  2. tRNA (Transfer RNA): Acts as an intermediary in protein synthesis, interpreting the genetic code for amino acid incorporation into polypeptides.

  3. mRNA (Messenger RNA): Carries the genetic information from DNA to the ribosomes for protein synthesis; it is less stable and typically short-lived compared to rRNA and tRNA.

Prokaryotic RNA Polymerase

The prokaryotic RNA polymerase (Rpol) is responsible for the transcription of DNA into RNA. It is characterized by:

  • Components: Comprising subunits including α2, β, β’, and ω, with interactions facilitating its function.

  • Core Enzyme: The core enzyme structure plays a pivotal role in its transcriptional activity in prokaryotes.

  • Promoter Recognition: The enzyme interacts with regulatory factors to initiate transcription efficiently.

Eukaryotic RNA Polymerase Overview

Eukaryotic RNA polymerases show greater complexity compared to their prokaryotic counterparts:

  1. RNA Polymerase I: Predominantly responsible for rRNA synthesis.

  2. RNA Polymerase II: Synthesizes heterogeneous nuclear RNA (hnRNA), which is the precursor for mRNA (pre-mRNA).

  3. RNA Polymerase III: Produces tRNAs and other small RNA molecules.
    Eukaryotic RNA polymerases are large complexes, generally over 500 kDa and comprising about 12 subunits, compared to 5 subunits in bacteria.

Prokaryotic Promoter Structures

Promoters in prokaryotes are critical as they dictate where RNA polymerase binds and initiates transcription. Key structural elements include:

  • -35 Region: Typically contains a consensus sequence that facilitates binding.

  • -10 Region (Pribnow box): Essential for promoter recognition by RNA polymerase.

  • +1 Site: The transcription start point.

  • UP-Element: Enhances RNA polymerase binding in certain promoters.

  • Variability exists among prokaryotic promoters, which contain distinct signal sequences.

Eukaryotic Promoter Structures

Eukaryotic promoters may contain:

  • TFIIB Recognition Element (BRE): Acknowledged by specific transcription factors.

  • TATA Box: A conserved element fundamental to initiation.

  • Initiator Element: Involves transcription startss.

  • Downstream Core Elements (DCE) and Downstream Promoter Element (DPE): These are features present in some eukaryotic promoters but not all necessarily contain every element.

Transcription Initiation Mechanisms

Transcription initiation in both domains of life requires the cooperation of RNA polymerases with various transcription factors:

  • Prokaryotic System: Sigma factors are vital for recognizing promoter regions. The formation of the holoenzyme (RNA polymerase + sigma factor) is crucial for specificity in promoter recognition. Sigma factors can be classified based on the function and the number of promoters they control:

    • Sigma Factor σ70 (RpoD): The major sigma factor for normal growth.

    • Alternative Sigma Factors: Such as σ54 for nitrogen assimilation or σ38 for stress responses. These factors govern the differential expression of gene families based on availability and activity modulation.

  • Eukaryotic System: Uses general transcription factors that assemble to form a pre-initiation complex that helps RNA polymerase initiate transcription.

Important Concepts in Promoter Recognition

  • Electrostatic Interactions: The prokaryotic RNA polymerase has a natural affinity for DNA due to electrostatic interactions but requires a bound sigma factor for specific promoter recognition.

  • Closed Complex to Open Complex Transition: This is the conversion of the RNA polymerase-promoter complex from a closed to an open configuration, allowing DNA unwinding at the transcription start site.

  • Initiation of RNA Synthesis: RNA polymerase begins synthesizing RNA and undergoes a period of abortive transcription, where it produces short RNA transcripts until it effectively transitions into elongation, allowing for true transcription to proceed.

Conclusions and Summary

In conclusion, the lecture provided a foundational understanding of both prokaryotic and eukaryotic transcriptional mechanisms, focusing on RNA polymerases and promoter interactions. The necessary components for successful initiation of transcription were highlighted, with particular attention to the roles of sigma factors and their potential as antimicrobial targets. The distinction between the structures and functions of eukaryotic and prokaryotic RNA polymerases was also covered.