Download 17_Lecture_Presentation

Chapter
17
Regulation of Gene
Expression in
Eukaryotes
Lecture Presentation by
Dr. Cindy Malone,
California State University Northridge
© 2015 Pearson Education, Inc.
Repaso de histonas y acetilación
https://youtu.be/N53WZP_ILj4
© 2015 Pearson Education, Inc.
Chapter Contents
17.1 Eukaryotic Gene Regulation Can Occur at Any of the
Steps Leading from DNA to Protein Product
17.2 Eukaryotic Gene Expression Is Influenced by
Chromatin Modifications
17.3 Eukaryotic Transcription Initiation Requires Specific
Cis-Acting Sites
Continued
© 2015 Pearson Education, Inc.
Chapter Contents
17.4 Eukaryotic Transcription Initiation Is Regulated by
Transcription Factors That Bind to Cis-Acting Sites
17.5 Activators and Repressors Interact with General
Transcription Factors and Affect Chromatin Structure
17.6 Gene Regulation in a Model Organism: Transcription
of the GAL Genes of Yeast
Continued
© 2015 Pearson Education, Inc.
Chapter Contents
17.7 Posttranscriptional Gene Regulation Occurs at All
the Steps from RNA Processing to Protein
Modification
17.8 RNA Silencing Controls Gene Expression in Several
Ways
17.9 Programmed DNA Rearrangements Regulate
Expression of a Small Number of Genes
17.10 ENCODE Data Are Transforming Our Concepts of
Eukaryotic Gene Regulation
© 2015 Pearson Education, Inc.
17.1 Eukaryotic Gene Regulation Can
Occur at Any of the Steps Leading from
DNA to Protein Product
© 2015 Pearson Education, Inc.
Section 17.1: Eukaryotic Gene Regulation
 Eukaryotic gene regulation is more complex
than that in prokaryotes
– Greater amount of DNA that is associated with
histones and other proteins
– mRNAs must be spliced, capped, and
polyadenylated prior to transport from nucleus
– Genes on numerous chromosomes are enclosed
in a double membrane nucleus
continued
© 2015 Pearson Education, Inc.
Section 17.1: Eukaryotic Gene Regulation
continued
 mRNAs have wide range of half-lives (t1/2)
 Modulation of mRNA translation, protein
processing, modification, and degradation
(Figure 17-1)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-1
17.2 Eukaryotic Gene Expression Is
Influenced by Chromatin Modifications
© 2015 Pearson Education, Inc.
Section 17.2: Gene Expression Influenced by
Chromatin Modifications
 Two structural features of eukaryotes
distinguish them from prokaryotes
– Eukaryotic genes are situated on
chromosomes that occupy a distinct location
– Eukaryotic DNA is combined with histones and
nonhistone proteins to form chromatin
– Compact chromatin structure inhibits
transcription, replication, and DNA repair
© 2015 Pearson Education, Inc.
Section 17.2: Transcription Factory
 Transcription factories
– Feature within nucleus that may contribute to
gene expression
– Nuclear sites that contain most of the active RNA
polymerase and transcription regulatory
molecules
– Dynamic structures that form rapidly and
disassemble upon stimulation and repression of
transcription
© 2015 Pearson Education, Inc.
Section 17.2: Nucleosomal Modifications
 Nucleosomes modified via change in
composition
– Important step in gene regulation; involves
changes to either nucleosome or DNA
– Histones contain normal histone H2A
– Variant histones (H2A.Z) affect nucleosome
mobility and positioning on DNA
– Nucleosome position may repress or activate
transcription via gene promoter
© 2015 Pearson Education, Inc.
Section 17.2: Histone Modification
 Histone modification
– Covalent bonding of functional groups onto Nterminal tails of histone proteins
 Most common: acetyl, methyl, phosphate
– Acetylation decreases positive charge, which
reduces affinity of histone to DNA
– Histone acetylation of nucleosome is catalyzed by
histone acetyltransferase enzymes (HATs):
associated with increased transcription
(Figure 17-2)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-2
Section 17.2: Chromatin Remodeling
 Chromatin remodeling
– Involves repositioning or removal of nucleosomes
on DNA
– Repositioned nucleosomes make chromosome
regions accessible to
 Transcription regulatory proteins
 Transcription activators
 RNAP II (RNA polymerase II)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-2
Section 17.2: DNA Methylation
 DNA methylation
– Associated with decreased gene expression
– Methylation occurs most often on cytosine of CG
doublets in DNA
– Methylation can repress transcription by binding
to transcription factors of DNA
© 2015 Pearson Education, Inc.
17.3 Eukaryotic Transcription Initiation
Requires Specific Cis-Acting Sites
© 2015 Pearson Education, Inc.
Section 17.3: Cis-Acting Sequences
 Cis-acting sequence
– Located on same chromosome as gene that it
regulates
– Required for accurate regulated transcription of
genes
 Promoters
 Enhancers
 Silencers
© 2015 Pearson Education, Inc.
Section 17.3: Promoter Structure
 Promoter structure
– Made up of DNA sequence elements including:
 Initiator (Inr)
 TATA box
 TFIIB recognition element (BRE)
 Downstream promoter element (DPE)
 Motif ten element (MTE)
(Figure 17-4)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-4
Section 17.4: Proximal-Promoter Elements
 Promoters contain proximal-promoter
elements
– Located upstream of TATA and BRE motifs
– Enhance levels of basal transcription
– Examples: CAAT and GC boxes
(Figure 17-5)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-5
Section 17.3: Enhancers and Silencers
 Enhancers and silencers regulate
transcription of eukaryotic genes
– Cis-acting transcription regulatory elements
 Enhancers: Located on either side of gene,
some distance from gene, or even within gene
– Important in reaching maximum level of
transcription
 Silencers: Repress the level of transcription
initiation
© 2015 Pearson Education, Inc.
Section 17.4: Transcription Factors
 Transcription factors
– Transcription regulatory proteins
– Target cis-acting sites of genes regulating
expression
– Activators increase transcription initiation
– Repressors decrease transcription initiation
 Multiple transcription factors bind to several
different enhancers and promoter elements and
fine-tune the level of transcription initiation
© 2015 Pearson Education, Inc.
Section 17.4: Functional Domains
 Transcription factors (proteins)
– Have two functional domains (clusters of amino
acids with a specific function):
– DNA-binding domain
 Binds to specific DNA sequences in the cis-acting
regulatory site
– Trans-activating domain
 Activates or represses transcription by binding to
other transcription factors or RNA polymerase
© 2015 Pearson Education, Inc.
Section 17.5: Formation of RNA Pol II
Initiation Complex
Formation of RNA Pol II initiation complex
 General transcription factors (proteins)
– Required at promoter to initiate basal or
enhanced levels of transcription
– Assembly of proteins in specific order forms
pre-initiation complex (PIC)
– PIC provides platform for RNAP II to recognize
transcription start sites
© 2015 Pearson Education, Inc.
Section 17.5: Core Promoter TFIID
 Core promoter TFIID
– General transcription factor that assists RNA Pol
II at core promoter TFIID
– First step in forming PIC – binding of TFIID to
TATA box via TATA binding protein (TBP)
(Figure 17-7)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-7
Section 17.5: Coactivators and
Enhanceosome
 Coactivators
– Interact with proteins and enable activators to
make contact with promoter-bound factors
– Coactivators form complex “enhanceosome”
 Enhanceosome
– Interacts with transcription complex (Figure 17-8)
 Repressor proteins at silencer elements
decrease rate of PIC assembly and RNA Pol II
release
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-8
Section 17.7: Posttranscriptional Regulation
 Posttranscriptional regulation plays an equal,
if not more significant, role compared to
transcriptional control (perhaps the major type of
regulation in eukaryotes)
 Mechanisms of posttranscriptional gene
regulation
– Control of alternative splicing
– mRNA stability
– Translation
– RNA silencing
© 2015 Pearson Education, Inc.
Section 17.7: Alternative Splicing
 Alternative splicing
– Generates different forms of mRNA from identical
pre-mRNA (increases number of proteins)
– Expression of one gene gives rise to numerous
proteins with similar and different functions
– Increases number of proteins made from one
gene (Figure 17-11 and Figure 17-12)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-11
Section 17.7: Three Pathways of Degradation
 Pathways of degradation
1) Enzymes shorten length of poly-A tail
• Binding of poly-A binding protein to tail stabilizes
mRNA
2) Decapping enzymes removes 7-methylguanine
cap – mRNA now unstable
3) Endonuclease cleaves mRNA internally
© 2015 Pearson Education, Inc.
Section 17.7: p53 Protein and Ubiquitin
Translational and posttranslational regulation
 p53 protein
– p53 levels increase if cell suffers DNA damage or
metabolic stress
– p53 is a transcription factor that induces
transcription of Mdm2 gene (ubiquitin ligase,
blocks transcription)
– Ubiquitin tags proteins for degradation by
enzymes
© 2015 Pearson Education, Inc.
Section 17.8: RNA Interference
 RNA interference (RNAi)
– Sequence specific posttranscriptional regulation
– Short RNA molecules regulate gene expression in
cytoplasm of plants, animals, and fungi; repress
translation and trigger mRNA degradation
 Phenomena known as RNA-induced gene
silencing
© 2015 Pearson Education, Inc.
Section 17.8: Molecular Mechanisms
 Molecular mechanisms of RNA-induced gene
silencing
 siRNA and microRNAs
– Short, double-stranded ribonucleotides
– siRNAs: Arise in cell due to virus infection –
produce double-stranded RNA, which is
recognized and cleaved by Dicer
– microRNAs: Noncoding RNAs that negatively
regulate gene expression
(Figure 17-14)
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Figure 17-14