Eukaryotic transcriptional regulation is a complex process involving multiple layers of control. From DNA elements like enhancers and silencers to transcription factors and epigenetic modifications, cells have numerous tools to fine-tune gene expression.
Understanding these mechanisms is crucial for grasping how cells respond to their environment and maintain proper function. This topic builds on earlier concepts of gene expression, highlighting the intricate ways eukaryotes control which genes are active and when.
Regulatory DNA Elements
Enhancers and Silencers
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Enhancers boost gene transcription by binding activator proteins
Located far from the promoter, up to thousands of base pairs away
Can function in either direction and on multiple genes
Form DNA loops to interact with promoter regions
Silencers repress gene transcription by binding repressor proteins
Similar to enhancers in location and function, but with opposite effects
Can act over long distances and through chromatin structures
Both enhancers and silencers contain specific DNA sequences recognized by regulatory proteins
Tissue-specific enhancers and silencers contribute to cell type-specific gene expression patterns
TATA box serves as a binding site for RNA polymerase II and general transcription factors
Located about 25-35 base pairs upstream of the transcription start site
Consists of a consensus sequence TATAAA
Helps position RNA polymerase II correctly for transcription initiation
Promoter-proximal elements regulate transcription from nearby genes
Located within ~200 base pairs upstream of the transcription start site
Include CAAT box, GC box, and other regulatory sequences
Bind specific transcription factors to modulate gene expression
Promoter strength varies depending on the combination and arrangement of these elements
Some genes lack a TATA box and rely on other promoter elements for transcription initiation
Transcription Factors and Machinery
Transcription Factor Structure and Function
Transcription factors regulate gene expression by binding to specific DNA sequences
Consist of two main domains:
DNA-binding domain recognizes and binds to specific DNA sequences
Activation domain interacts with other proteins to influence transcription
Various types of DNA-binding domains (zinc finger, helix-turn-helix, leucine zipper)
Can act as activators or repressors of gene expression
Often work in combination to fine-tune gene expression levels
Basal Transcription Machinery
RNA polymerase II synthesizes mRNA from DNA template in eukaryotes
Composed of 12 subunits
Requires additional proteins for accurate transcription initiation
General transcription factors assist RNA polymerase II in transcription initiation
Include TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH
Form the preinitiation complex (PIC) at the promoter
TFIID contains the TATA-binding protein (TBP) which recognizes the TATA box
TFIIH possesses helicase activity to unwind DNA and kinase activity to phosphorylate RNA polymerase II
Stepwise assembly of the PIC ensures accurate transcription initiation
Epigenetic Regulation
Chromatin Remodeling and Histone Modifications
Chromatin remodeling alters DNA accessibility for transcription factors
ATP-dependent chromatin remodeling complexes (SWI/SNF, ISWI, CHD, INO80)
Slide, eject, or restructure nucleosomes to expose or conceal DNA sequences
Histone modifications affect chromatin structure and gene expression
Include acetylation, methylation, phosphorylation, and ubiquitination
Histone acetyltransferases (HATs) add acetyl groups, promoting open chromatin
Histone deacetylases (HDACs) remove acetyl groups, promoting closed chromatin
Histone methylation can activate or repress transcription depending on the specific residue and degree of methylation
Histone code hypothesis suggests combinations of modifications determine gene activity
DNA Methylation and Gene Silencing
DNA methylation involves addition of methyl groups to cytosine bases
Occurs primarily at CpG dinucleotides in mammals
Catalyzed by DNA methyltransferases (DNMTs)
Methylated DNA generally associated with transcriptional repression
Interferes with transcription factor binding
Recruits methyl-CpG-binding proteins that promote chromatin compaction
DNA methylation patterns are heritable and contribute to cell type-specific gene expression
Plays crucial roles in genomic imprinting and X chromosome inactivation
Aberrant DNA methylation associated with various diseases, including cancer