Ch 16 Regulation of Gene Expression in Eukaryotes – Study Notes

Ch 16 Regulation of Gene Expression in Eukaryotes

Introduction to Eukaryotic Gene Regulation

Gene expression in eukaryotes is a highly regulated process, ensuring that genes are expressed in the right cell type, at the right time, and in the correct amount. This regulation is essential for cellular differentiation, response to environmental signals, and prevention of diseases such as cancer.

• Cell-type specific expression: Different cell types express unique sets of genes (e.g., keratin in skin cells, myosin in muscle cells).

• Conditional expression: Genes can be upregulated or downregulated in response to physiological conditions (e.g., erythropoietin in response to low oxygen).

• Misregulation: Incorrect gene expression can lead to developmental defects, cancer, or cell death.

• Cis-acting and trans-acting factors: These are fundamental for regulating transcription initiation.

Levels of Gene Regulation in Eukaryotes

Gene expression in eukaryotes is regulated at multiple levels, including transcription, RNA processing, mRNA stability, translation, and posttranslational modification.

• Transcriptional regulation

• RNA splicing and processing

• mRNA degradation

• Translational regulation

• Posttranslational modifications

Chromatin Structure and Its Role in Gene Regulation

Chromatin Organization

Eukaryotic DNA is packaged with histones and nonhistone proteins to form chromatin. The basic repeating unit is the nucleosome, which can inhibit processes such as DNA replication, repair, and transcription when chromatin is in a compact state.

• Nucleosomes: DNA-histone complexes that form the fundamental unit of chromatin.

• Chromatin compaction: Inhibits access of transcription machinery to DNA.

Chromatin Alterations: Nucleosome and Histone Modifications

Chromatin structure can be dynamically altered to regulate gene expression. This includes changes in nucleosome composition and covalent modifications of histone tails.

• Histone variants: Replacement of standard histones (e.g., H2A) with variants (e.g., H2A.Z) can affect nucleosome stability and transcription.

• Histone modifications: Covalent addition of acetyl, methyl, or phosphate groups to histone tails alters chromatin structure and gene accessibility.

• Acetylation: Decreases histone-DNA affinity, generally associated with increased transcription.

• Enzymes: HATs (histone acetyltransferases) add acetyl groups; HDACs (histone deacetylases) remove them, often recruited by repressors.

DNA Methylation

DNA methylation is a key epigenetic modification that typically represses gene expression. It occurs mainly at cytosines in CpG dinucleotides, often clustered in CpG islands.

• Function: Silences transcription, especially when present in promoter regions.

• Location: Most commonly at the 5-position of cytosine (5-methylcytosine) in CpG islands.

Transcriptional Regulation

Promoters and Promoter Elements

Promoters are DNA sequences recognized by RNA polymerase and transcription factors, essential for transcription initiation. They are classified as core promoters (site of transcription initiation) and proximal-promoter elements (enhance basal transcription).

• Core promoter: Contains elements necessary for accurate initiation (e.g., TATA box, Inr, BRE, MTE, DPE).

• Proximal-promoter elements: Located upstream, include CAAT box and GC boxes, enhance transcription levels.

Classification of Core Promoters

• Focused promoters: Direct transcription from a single start site; common in lower eukaryotes.

• Dispersed promoters: Initiate transcription from multiple weak start sites over a region; common in higher eukaryotes.

Enhancers and Silencers

Enhancers and silencers are cis-acting regulatory elements that can be located far from the gene they regulate. Enhancers increase, while silencers decrease, the rate of transcription initiation, often in a tissue- or time-specific manner.

• Enhancers: Increase transcription, can function at a distance and in either orientation.

• Silencers: Repress transcription, also act in a position- and orientation-independent manner.

Transcription Regulatory Proteins

Transcription factors bind to promoters, enhancers, and silencers to modulate gene expression. Activators increase, and repressors decrease, transcription initiation. Multiple factors can fine-tune gene expression.

Pre-Initiation Complex (PIC) Formation

The assembly of the pre-initiation complex (PIC) is a critical step in transcription initiation. It involves RNA polymerase II and general transcription factors (GTFs), with TFIID binding to the TATA box as the first step.

Mechanism of Transcription Activation and Repression

Transcription activators and repressors modulate gene expression by facilitating or hindering the assembly of the PIC. DNA looping allows distant enhancers or silencers to interact with the core promoter region.

Post-Transcriptional Regulation

Alternative Splicing

Alternative splicing allows a single gene to produce multiple mRNA variants (spliceforms), increasing protein diversity. Different exons may be included or excluded, resulting in protein isoforms with distinct functions.

• Types of alternative splicing: Cassette exons, alternative splice sites, intron retention, mutually exclusive exons, alternative promoters, and polyadenylation.

• Example: The Drosophila Dscam gene can generate over 38,000 different proteins through alternative splicing.

RNA Binding Proteins (RBPs) and Spliceopathies

RNA-binding proteins regulate alternative splicing by binding to specific RNA sequences, promoting or inhibiting the use of particular splice sites. Mutations affecting splicing can cause genetic disorders known as spliceopathies (e.g., myotonic dystrophy).

mRNA Stability and Degradation

The steady-state level of mRNA in a cell is determined by the balance between synthesis and degradation. mRNA decay is often initiated by deadenylation (removal of the poly-A tail) and carried out by exoribonucleases.

• Nonsense-mediated decay (NMD): Eliminates mRNAs with premature stop codons to prevent production of truncated, potentially harmful proteins.

mRNA Localization

Some mRNAs are transported to specific cellular locations for localized translation. For example, actin mRNA is localized to the leading edge of migrating cells, guided by zip code sequences and RNA-binding proteins.

Posttranslational Regulation

Posttranslational Modifications

Proteins can be regulated after translation by covalent modifications such as phosphorylation and ubiquitination.

• Phosphorylation: Addition of phosphate groups by kinases; removal by phosphatases. Regulates protein activity, localization, and interactions.

• Ubiquitination: Attachment of ubiquitin molecules marks proteins for degradation by the proteasome.

Summary Table: Key Regulatory Elements in Eukaryotic Gene Expression

Regulatory Element	Location	Function
Core Promoter (TATA box, Inr, BRE, MTE, DPE)	Immediately upstream of gene	Transcription initiation site
Proximal Promoter Elements (CAAT, GC box)	Upstream of core promoter	Enhance basal transcription
Enhancers	Variable (upstream, downstream, intronic)	Increase transcription rate
Silencers	Variable	Repress transcription
CpG Islands	Promoter regions	DNA methylation silences genes
RNA-binding Proteins	mRNA	Regulate splicing, stability, localization
Ubiquitin Ligases	Proteins	Target proteins for degradation