Control of Gene Expression in Eukaryotes
Control of Gene Expression in Eukaryotes
Gene Expression Control in Eukaryotes
Eukaryotic gene expression can be regulated at multiple stages, offering fine-tuned control over protein production:
Transcription initiation: Controls when and how frequently a gene is transcribed into RNA. This is a primary regulatory point.
Post-transcriptional control: Involves modifications to RNA after transcription, such as splicing, capping, polyadenylation, and mRNA stability.
Translational control: Regulates the rate at which mRNA is translated into protein by ribosomes.
Post-translational control: Modifies the synthesized protein after translation, affecting its activity, stability, or localization.
Steps 1, 3, and 4 are unique to eukaryotes compared to prokaryotes due to several reasons:
Eukaryotic DNA is packaged into chromatin, requiring remodeling for transcription.
Eukaryotic genes often have introns that require splicing.
Eukaryotic mRNAs are processed (capped and polyadenylated) and transported out of the nucleus.
Eukaryotic cells have a more complex regulatory machinery, including a wider array of transcription factors and RNA interference mechanisms.
19.1 Initiating Transcription
Eukaryotic Promoters:
Far more complex than bacterial promoters, reflecting the need for sophisticated regulation in multicellular organisms.
Contain 2 or 3 regulatory sequences, crucial for precise gene expression control, often spread across a larger region.
Core Promoter:
The essential region where RNA polymerase II and general transcription factors assemble to begin transcription.
Common sequence: TATA box, typically located about -30 base pairs upstream from the transcription start site.
TATA-binding Protein (TBP) is a key component of the TFIID complex, which binds to the TATA box and is essential for positioning RNA polymerase II correctly to initiate transcription.
19.2 Transcription Factors
Function:
Transcription factors are proteins that bind to specific DNA sequences through hydrogen bonds and other non-covalent interactions with the exposed bases in the major and minor grooves of the DNA double helix.
This specific recognition allows them to differentiate between millions of possible DNA sequences.
Regulatory Sequences:
Promoter-Proximal Elements:
Located within a few hundred base pairs upstream of the core promoter; consist of short, specific regulatory DNA sequences that serve as binding sites for specific transcription factors.
These elements are crucial for the basal level of transcription and can often respond to specific cellular signals.
The first sequenced identified in studies on galactose metabolism in yeast, such as the GAL4 binding sites, highlighted their role in gene activation.
In eukaryotes, similar regulatory sequences are associated with co-regulated genes, allowing for coordinated expression of genes involved in a common pathway.
Enhancers:
Can be located significant distances from the core promoter, sometimes over 100,000 bases away, either upstream, downstream, or even within introns.
Function through positive control, dramatically enhancing transcription rates by binding activator proteins.
These activators often interact with the Mediator complex, which then facilitates the recruitment of RNA polymerase II and general transcription factors to the promoter, often involving DNA looping.
Can function independently of orientation or location relative to the promoter, indicating their flexible mode of action.
Silencers:
Similar to enhancers in their distant location and sequence specificity but function to repress transcription when bound by specific repressor proteins.
Repressors can inhibit transcription by blocking activator binding, competing with activators, or by recruiting chromatin-modifying enzymes that induce a more condensed chromatin structure.
19.3 Role of Transcription Factors in Gene Expression
Transcription Factors:
Include a diverse array of activators and repressors that play a pivotal role in determining which genes are expressed in different cell types or under varying environmental conditions.
The expression of these genes can be triggered by complex signaling cascades originating from external stimuli or signals from other cells, leading to precise and coordinated protein production necessary for cellular differentiation and function.
19.4 Model for Transcription Initiation
Process Steps:
Activators bind to DNA: Specific activator proteins bind to enhancer regions and promoter-proximal elements, initiating the process. These activators then recruit chromatin-remodeling complexes and histone acetyltransferases (HATs) to the promoter region.
Chromatin remodeling opens up chromatin structure: Chromatin-remodeling complexes use ATP to reposition or evict nucleosomes, making the DNA more accessible. Histone acetyltransferases (HATs) add acetyl groups to the positively charged lysine residues of histone tails, neutralizing their charge and weakening their interaction with the negatively charged DNA, thereby loosening the chromatin structure and revealing core promoters and regulatory sequences.
Additional activators bind: With the chromatin now decondensed, more activator proteins bind to the newly exposed enhancer and promoter-proximal elements, stabilizing the open chromatin state.
General transcription factors and RNA polymerase assemble: General transcription factors (e.g., TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH) assemble on the core promoter, along with RNA polymerase II, through the crucial involvement of the Mediator complex. The Mediator complex acts as a bridge, conveying signals from activators bound at enhancers to the RNA polymerase II and general transcription factors at the promoter, facilitating phosphorylation of RNA polymerase II and initiating transcription.
Pathway visually represented with diagrams showing depicted steps in transcription initiation, illustrating the sequential recruitment of protein complexes.
19.5 Post-Transcriptional Control
Post-transcriptional control involves various intricate processes that occur after RNA synthesis but before protein production, primarily focusing on mRNA regulation:
mRNA alterations including splicing, translation regulation, and mRNA stability.
RNA Processing:
Essential in eukaryotes for producing mature mRNA that can be successfully translated and exported from the nucleus.
Involves the addition of a 5' cap (a modified guanine nucleotide) that protects the mRNA from degradation and aids in ribosome binding for translation initiation.
Also involves the addition of a 3' poly-A tail (a string of adenine nucleotides) that confers stability to the mRNA, influences its nuclear export, and affects translation efficiency.
Crucially, it involves the splicing out of non-coding introns and the ligation of coding exons, performed by the spliceosome complex.
19.6 Alternative Splicing
Alternative Splicing:
A sophisticated mechanism that allows the production of different mature mRNA molecules, and consequently different protein isoforms, from the same primary RNA transcript by selectively including/excluding certain exons.
This process is tightly regulated by specific RNA-binding proteins that either promote or repress the recognition of splice sites by the spliceosome.
Example: The tropomyosin gene, with its 14 exons, produces diverse protein isoforms in different muscle cells (e.g., smooth muscle vs. striated muscle) due to tissue-specific alternative splicing patterns, leading to varied protein functions from a single gene.
This mechanism is highly significant as over 90% of human genes with multiple exons undergo this process, leading to a much greater diversity of proteins (the proteome) than the number of genes in the genome.
19.7 RNA Interference (RNAi)
Functionality of RNAi:
A highly conserved biological process that controls the lifespan and translation of many mRNAs through mechanisms like direct degradation of target mRNA or inhibiting its translation via small non-coding RNAs.
Specifically, microRNAs (miRNAs), a class of smallRNA molecules, are crucial regulators of gene expression, typically repressing it.
Mechanism of miRNA:
miRNAs are transcribed as primary miRNAs (pri-miRNAs), which are then processed in the nucleus by the enzyme Drosha into pre-miRNAs. These pre-miRNAs are exported to the cytoplasm where the enzyme Dicer further processes them into short (approximately 21-23 nucleotide) double-stranded miRNA duplexes.
One strand of the duplex is incorporated into the RNA-induced silencing complex (RISC).
The RISC complex then binds to target mRNAs based on sequence complementarity. If the complementarity is extensive, RISC typically cleaves and degrades the mRNA. If the complementarity is partial, RISC inhibits translation of the mRNA or triggers its deadenylation, leading to decay over time.
19.8 Post-Translational Control
Mechanisms Involved:
Provides the fastest responses to cellular stimuli by directly modifying existing proteins.
Key modifications include phosphorylation, where phosphate groups are added by kinases (or removed by phosphatases) to specific amino acid residues (like serine, threonine, or tyrosine), often changing protein conformation and activity.
Another crucial mechanism is ubiquitination, where ubiquitin (a small regulatory protein) is covalently attached to a target protein, typically marking it for destruction.
Ubiquitin tags proteins for destruction, which is mediated through the proteasome, a large protein complex in the cytoplasm and nucleus that degrades unneeded or damaged proteins by proteolysis. This process is vital for cell cycle control, DNA repair, and other cellular processes.
19.9 Linking Gene Regulation to Cancer
Cancers typically arise from an accumulation of mutations affecting critical genes involved in cell growth, division, and death:
Tumor suppressor genes (e.g., p53, Rb): These genes normally inhibit cell cycle progression and promote apoptosis when damage occurs. Mutations in these genes often lead to uncontrolled cell proliferation because the brakes on cell division are disabled.
Proto-oncogenes (e.g., Ras, Myc): These genes normally promote cell growth and division. Mutations can convert them into oncogenes, which act like accelerators for cell division, leading to uncontrolled growth and proliferation even in the absence of normal signals.
Cancer development often requires multiple somatic mutations in a single cell over time (the multi-hit hypothesis), as a single mutation is usually insufficient to cause cancer. This explains why cancer incidence increases with age.
19.10 Comparison of Gene Expression in Bacteria and Eukaryotes
Four primary differences highlight the increased complexity of eukaryotic gene regulation:
DNA Packaging: In eukaryotes, DNA is extensively wrapped around histone proteins to form nucleosomes, which are further condensed into chromatin. This chromatin structure (especially heterochromatin) provides a significant layer of negative control, as it restricts access for transcription factors and RNA polymerase. Bacterial DNA is not complexed with histones and lacks this level of condensation.
Complexity of Transcription: Eukaryotic transcription initiation involves many more general transcription factors (GTFs) and the Mediator complex, assembling at more complex promoters, whereas bacterial transcription can often initiate with just RNA polymerase and a sigma factor.
Coordinated Transcription: In bacteria, genes coding for functionally related proteins are often organized into operons, allowing for coordinated expression from a single promoter. In eukaryotes, related genes are typically dispersed across the genome but can be coordinately regulated by sharing common regulatory sequences (enhancers or promoter-proximal elements) that bind the same set of transcription factors.
Post-Transcriptional Control: Significantly more pronounced and diverse in eukaryotes compared to bacteria. Eukaryotic pre-mRNAs undergo extensive processing (5' capping, 3' polyadenylation, and splicing) in the nucleus before export, regulating mRNA stability, transport, and translation efficiency. Bacteria generally lack introns and extensive mRNA processing beyond simple cleavage, and thus do not utilize mechanisms like alternative splicing or RNA interference for developmental controls.
19.11 Summary of Elements of Transcriptional Regulation
Core Promoter: The minimal DNA sequence required for basal transcription; serves as the binding site for RNA polymerase II and general transcription factors to initiate transcription (e.g., TATA box).
Promoter-Proximal Elements: Short regulatory DNA sequences located within a few hundred base pairs upstream of the core promoter; serve as binding sites for specific transcription factors (activators and repressors) to modulate gene expression levels.
Enhancers: Distant regulatory DNA sequences that can be thousands of base pairs away from the promoter; bind activator proteins to significantly boost transcription rates, often by interacting with the Mediator complex through DNA looping.
Silencers: Regulatory DNA sequences that bind repressor proteins to inhibit transcription initiation, counteracting the effects of activators or stabilizing condensed chromatin states.
General Transcription Factors: A set of proteins (TFIIA, B, D, E, F, H) universally required for RNA polymerase II to bind to the core promoter and initiate transcription from most eukaryotic protein-coding genes.
RNA Regulation Mechanisms: mRNA stability, translation, and overall gene expression outcomes can be regulated through processes like alternative splicing, RNA interference (e.g., miRNAs), and the addition of 5' caps and 3' poly-A tails, which control mRNA longevity and translation efficiency.