15.2 - 15.3

Regulatory Transcription Factors

  • Regulatory transcription factors can be modulated by small molecule effectors.

    • These effectors can be:

    • Homodimers (polypeptides that form a dimer to function).

    • Modified through covalent modification.

Combinatorial Control in Gene Regulation

  • The concept of combinatorial control addresses multiple factors that can alter gene regulation in eukaryotes.

    • The discussion aims to cover the remaining points, specifically points four and five, regarding alterations of nucleosomes and modifications affecting transcription.

Nucleosomes

  • Definition: Nucleosomes consist of a histone octamer made up of four specific proteins around which DNA is wrapped.

  • Nature of Interaction:

    • The interaction between nucleosomes and DNA is driven by:

    • Charge—histones are positively charged, while DNA is negatively charged.

    • The sequence of DNA bases (adenine, thymine, guanine, cytosine) does not impact the formation of nucleosomes.

  • Key Function:

    • Nucleosomes can obstruct transcription by tightly wrapping DNA, particularly around crucial regions such as promoters.

Chromatin Remodeling

  • Chromatin Definition:

    • Chromatin refers to the complex of DNA and proteins (nucleosomes) in the cell nucleus and can exist in a

    • Closed conformation: When DNA is tightly wrapped around nucleosomes and is not readily accessible for transcription.

    • Open conformation: When DNA is less securely bound, allowing accessibility for transcriptional mechanisms.

  • Distinction in Terminology:

    • The term closed conformation in eukaryotes differs from the bacterial context, where it referred to hydrogen bonds in DNA strands.

  • Mechanisms for Alteration:

    • Movement of nucleosomes can occur via ATP-dependent chromatin remodeling complexes, which include:

    1. Changing Position of Nucleosomes: Nucleosomes can shift to create longer linker regions between DNA segments.

    2. Removing Histone Octamers: Removing histones can open up DNA regions for transcription, especially if they are wrapped around promoter sequences.

    3. Replacing Histone Variants: Incorporating different histone variants can alter interaction properties and assist in recruiting transcription machinery.

Nucleosome Structure and Dynamics

  • DNA Wrapped Around Nucleosome:

    • Approximately 150 base pairs of DNA wrap around a single histone octamer.

    • The total stretch may be approximately 300-400 base pairs considering linker regions.

  • Impact of Nucleosome Alteration:

    • Modifications to nucleosome structure promote accessibility for transcription.

Histone Modification

  • Histones: The four types of histones are H2A, H2B, H3, and H4, forming the histone octamer.

  • Amino Acid Composition:

    • Histone tails contain positively charged amino acids, specifically arginine and lysine, which facilitate interaction with negatively charged DNA.

  • Covalent Modifications:

    • Functional groups can be added to the R groups of arginine and lysine, impacting interaction intensity with DNA.

Types of Modifications

  1. Methylation:

    • Addition of a methyl group (CH3CH_3) to histone tails.

    • Methylation masks the positive charge, potentially loosening histone-DNA interactions.

  2. Acetylation:

    • Attachments of acetyl groups that introduce partial negative charges, further loosening DNA interactions.

  3. Phosphorylation:

    • This involves adding phosphate groups (e.g., to serine), which introduce negative charges, influencing DNA accessibility.

  • Enzymatic Actions:

    • Modifications are enacted by specific enzymes (e.g., histone acetyltransferases for acetylation and histone deacetylases for removal).

Nucleosome-Free Regions (NFRs)

  • Definition: NFRs are segments of DNA not associated with histones; these regions are crucial for transcription initiation.

  • Location:

    • Typically found surrounding promoter and terminator regions of genes in eukaryotes.

  • Histone Variants: Although NFRs are devoid of core histones, histone variants may still associate in proximity, aiding in recruitment of transcription factors.

Roles of Enhancers and Activator Proteins

  • Enhancers: Regions of DNA that can bind activator proteins to increase transcription rates.

  • Upon binding a specific enhancer sequence:

    • Activators recruit ATP-dependent chromatin remodeling complexes, including acetyltransferases.

    • This action loosens nucleosome interactions, enhancing accessibility of DNA for RNA polymerase and transcription factors.

  • Mechanism Workflow:

    • Activator binding → chromatin remodeling complex recruitment → acetylation of histones → enhanced transcriptional activity.

Transcription Cycle Mechanism

  • Transcription Initiation:

    • RNA polymerase and general transcription factors recognize the now accessible DNA template post chromatin remodeling.

    • Mediators may assist in RNA polymerase's engagement with the target DNA.

  • Histone Repackaging:

    • After transcription occurs, there may be a need to remove modifications like acetylation, leading to re-wrapping of DNA tightly around histones.

  • Histone Deacetylases: Enzymes that remove acetyl groups to revert histones to a more compact state following transcription.

Overview of Methylation Effects

  • Histone Methylation:

    • Generally encourages transcription in eukaryotic genes.

  • DNA Methylation:

    • Typically involves methylation of cytosine bases in DNA, which often results in transcriptional repression.

  • Clarification for Examination:

    • Distinctions should be made between histone methylation and DNA methylation when addressing questions in assessments.