Listeria Infections and RNA Folding

  • Listeria Overview

    • Listeria is a type of bacteria that infects host cells.

    • The regulatory gene PRFA controls its infection capability.

    • This gene is temperature-responsive, influencing its expression depending on the temperature of the environment.

  • Temperature Effects on RNA Structure

    • At 30°C, RNA structures from the PRFA gene adopt specific folds due to stable hydrogen bonding.

    • RNA Structure Description:

    • Features a 5' end and 3' end.

    • Initially single-stranded before adopting secondary structures post-transcription.

    • Diagram shown in transcript indicating hydrogen bonds (H-bonds) made between bases in RNA.

    • Heating to 37°C destabilizes these structures, resulting in:

    • Loss of H-bonds

    • Disruption of RNA secondary folding.

  • Role of Temperature in Gene Regulation

    • The importance of temperature regulation ties to accessibility of RNA sequences.

    • Higher temperatures expose important ribosomal binding sites essential for translation.

    • Shine-Dalgarno Sequence:

    • A ribosomal binding site necessary for translation initiation.

    • At 30°C, the sequence remains inaccessible due to RNA folding.

    • At 37°C, the sequence is exposed, allowing ribosome recruitment and facilitating protein synthesis.

  • Gene Expression State

    • At 30°C: The gene expression status is off, and translation does not occur due to ribosome inability to bind.

    • Prevents protein synthesis despite transcription occurring.

    • At 37°C: The expression status shifts to on, as the ribosome binds to exposed sequences, enabling translation.

Additional RNA Regulatory Mechanisms

  • Riboswitch Concepts

    • Defined as RNA elements that can bind to metabolites, influencing their own transcription.

    • Example: Thiamine Pyrophosphate binding alters RNA structure and can switch the state to off when binding.

  • Riboswitch Functionality:

    • Off State:

    • Metabolite binding causes structural changes that obscure binding sites for ribosomes (e.g., Shine-Dalgarno).

    • On State:

    • In absence of the metabolite, ribosome binding sequences are available, enabling translation.

  • Eukaryotic Variant of Riboswitch

    • i. Eukaryotic RNAs have a cap, distinct from bacterial RNAs.

    • ii. Eukaryotic riboswitches react to iron levels, utilizing Iron Responsive Protein (IRP).

    • iii. When iron binds IRP, it prevents IRP from binding to RNA, allowing gene activation for iron detoxification.

Attenuation Mechanism

  • Introduction to Attenuation

    • A regulatory mechanism that halts transcription prematurely based on environmental conditions.

    • Demonstrated through RNA structures dictating translation availability based on folding.

  • General Structure of the RNA in Regulation

    • The Open Reading Frame (ORF) refers to the sequences in RNA that encode proteins.

    • The integrity of the ORF determines whether translation occurs.

  • Examples of Attenuation Dynamics

    • 5 regions in RNA can contribute to secondary structure formation leading to either gene activation (on) or suppression (off).

    • SAM (S-adenosyl methionine) levels influence availability of necessary structures in RNA.

Tryptophan Operon Example

  • Operational Overview

    • The operon is controlled under one promoter and contains genes needed for tryptophan synthesis.

    • Transcription results in either long (active) or short (inactive) mRNAs depending on environmental metabolites such as tryptophan.

  • Regulatory Feedback

    • Low Tryptophan Levels:

    • Ribosomal stalling at specific codons prevents termination hairpin formation, allowing transcription completion.

    • High Tryptophan Levels:

    • Ribosome scans quickly through adjacent codons, preventing necessary structural change for further transcription.

  • Mechanism of Regulation

    • Ribosome stalling signals availability of charged tRNA and thus affects RNA polymerase activity, contributing to transcriptional regulation via structural adaptations.

Summary of Key Concepts

  • Conclusions:

    • RNA folding plays a critical role in regulation of gene expression by controlling access to necessary translation initiation sites.

    • Regulatory mechanisms like riboswitches and attenuation showcase how cells interact with and respond to environmental conditions.