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Control of Gene Expression

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Control of Gene Expression Chapter 16 Biology Dual Enrollment Mrs. Mansfield * – PowerPoint PPT presentation

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Title: Control of Gene Expression


1
Control of Gene Expression
  • Chapter 16

Biology Dual Enrollment Mrs. Mansfield
1
2
Control of Gene Expression
  • Controlling gene expression is often accomplished
    by controlling transcription initiation
  • Regulatory proteins bind to DNA
  • May block or stimulate transcription
  • Prokaryotic organisms regulate gene expression in
    response to their environment
  • Eukaryotic cells regulate gene expression to
    maintain homeostasis in the organism

3
Regulatory Proteins
  • Gene expression is often controlled by regulatory
    proteins binding to specific DNA sequences
  • Regulatory proteins gain access to the bases of
    DNA at the major groove
  • Regulatory proteins possess DNA-binding motifs

4
DNA-binding motifs
  • Regions of regulatory proteins which bind to DNA
  • Helix-turn-helix motif
  • Homeodomain motif
  • Zinc finger motif
  • Leucine zipper motif

5
Prokaryotic regulation
  • Control of transcription initiation
  • Positive control increases frequency of
    initiation of transcription
  • Activators enhance binding of RNA polymerase to
    promoter
  • Effector molecules can enhance or decrease
  • Negative control decreases frequency
  • Repressors bind to operators in DNA
  • Allosterically regulated
  • Respond to effector molecules enhance or
    abolish binding to DNA

6
  • Prokaryotic cells often respond to their
    environment by changes in gene expression
  • Genes involved in the same metabolic pathway are
    organized in operons
  • Induction enzymes for a certain pathway are
    produced in response to a substrate
  • Repression capable of making an enzyme but does
    not

7
lac operon
  • Contains genes for the use of lactose as an
    energy source
  • -b-galactosidase (lacZ), permease (lacY), and
    transacetylase (lacA)
  • Gene for the lac repressor (lacI) is linked to
    the rest of the lac operon

8
  • The lac operon is negatively regulated by a
    repressor protein
  • lac repressor binds to the operator to block
    transcription
  • In the presence of lactose, an inducer molecule
    (allolactose) binds to the repressor protein
  • Repressor can no longer bind to operator
  • Transcription proceeds
  • Even in the absence of lactose, the lac operon is
    expressed at a very low level

9
Glucose repression
  • Preferential use of glucose in the presence of
    other sugars
  • Mechanism involves activator protein that
    stimulates transcription
  • Catabolic activator protein (CAP) is an
    allosteric protein with cAMP as effector
  • Level of cAMP in cells is reduced in the presence
    of glucose so that no stimulation of
    transcription from CAP-responsive operons takes
    place
  • Inducer exclusion presence of glucose inhibits
    the transport of lactose into the cell

10
trp operon
  • Genes for the biosynthesis of tryptophan
  • The operon is not expressed when the cell
    contains sufficient amounts of tryptophan
  • The operon is expressed when levels of tryptophan
    are low
  • trp repressor is a helix-turn-helix protein that
    binds to the operator site located adjacent to
    the trp promoter

11
  • The trp operon is negatively regulated by the trp
    repressor protein
  • trp repressor binds to the operator to block
    transcription
  • Binding of repressor to the operator requires a
    corepressor which is tryptophan
  • Low levels of tryptophan prevent the repressor
    from binding to the operator

12
Eukaryotic Regulation
  • Control of transcription more complex
  • Major differences from prokaryotes
  • Eukaryotes have DNA organized into chromatin
  • Complicates protein-DNA interaction
  • Eukaryotic transcription occurs in nucleus
  • Amount of DNA involved in regulating eukaryotic
    genes much larger

13
Transcription factors
  • General transcription factors
  • Necessary for the assembly of a transcription
    apparatus and recruitment of RNA polymerase II to
    a promoter
  • TFIID recognizes TATA box sequences
  • Specific transcription factors
  • Increase the level of transcription in certain
    cell types or in response to signals

14
  • Promoters form the binding sites for general
    transcription factors
  • Mediate the binding of RNA polymerase II to the
    promoter
  • Enhancers are the binding site of the specific
    transcription factors
  • DNA bends to form loop to position enhancer
    closer to promoter

15
  • Coactivators and mediators are also required for
    the function of transcription factors
  • Bind to transcription factors and bind to other
    parts of the transcription apparatus
  • Mediators essential to some but not all
    transcription factors
  • Number of coactivators is small because used with
    multiple transcription factors

16
Transcription complex
  • Few general principles
  • Nearly every eukaryotic gene represents a unique
    case
  • Great flexibility to respond to many signals
  • Virtually all genes that are transcribed by RNA
    polymerase II need the same suite of general
    factors to assemble an initiation complex

17
Eukaryotic chromatin structure
  • Structure is directly related to the control of
    gene expression
  • DNA wound around histone proteins to form
    nucleosomes
  • Nucleosomes may block access to promoter
  • Histones can be modified to result in greater
    condensation

18
  • Methylation once thought to play a major role in
    gene regulation
  • Many inactive mammalian genes are methylated
  • Lesser role in blocking accidental transcription
    of genes turned off
  • Histones can be modified
  • Correlated with active versus inactive regions of
    chromatin
  • Can be methylated found in inactive regions
  • Can be acetylated found in active regions

19
  • Some coactivators have been shown to be histone
    acetylases
  • Transcription is increased by removing higher
    order chromatin structure that would prevent
    transcription
  • Histone code postulated to underlie the control
    of chromatin structure

20
  • Chromatin-remodeling complexes
  • Large complex of proteins
  • Modify histones and DNA
  • Also change chromatin structure
  • ATP-dependent chromatin remodeling factors
  • Function as molecular motors
  • Catalyze 4 different changes in DNA/histone
    binding
  • Make DNA more accessible to regulatory proteins

21
Posttranscriptional Regulation
  • Control of gene expression usually involves the
    control of transcription initiation
  • Gene expression can be controlled after
    transcription with
  • Small RNAs
  • miRNA and siRNA
  • Alternative splicing
  • RNA editing
  • mRNA degradation

22
Micro RNA or miRNA
  • Production of a functional miRNA begins in the
    nucleus
  • Ends in the cytoplasm with a 22 nt RNA that
    functions to repress gene expression
  • miRNA loaded into RNA induced silencing complex
    (RISC)
  • RISC is targeted to repress the expression of
    genes based on sequence complementarity to the
    miRNA

23
siRNA
  • RNA interference involves the production of
    siRNAs
  • Production similar to miRNAs but siRNAs arise
    from long double-stranded RNA
  • Dicer cuts yield multiple siRNAs to load into
    RISC
  • Target mRNA is cleaved

24
miRNA or siRNA?
  • Biogenesis of both miRNA and siRNA involves
    cleavage by Dicer and incorporation into a RISC
    complex
  • Main difference is target
  • miRNA repress genes different from their origin
  • Endogenous siRNAs tend to repress genes they were
    derived from

25
Alternative splicing
  • Introns are spliced out of pre-mRNAs to produce
    the mature mRNA
  • Tissue-specific alternative splicing
  • Same gene makes calcitonin in the thyroid and
    calcitonin-gene related peptide (CGRP) in the
    hypothalamus
  • Determined by tissue-specific factors that
    regulate the processing of the primary transcript

26
RNA editing
  • Creates mature mRNA that are not truly encoded by
    the genome
  • Involves chemical modification of a base to
    change its base-pairing properties
  • Apolipoprotein B exists in 2 isoforms
  • One isoform is produced by editing the mRNA to
    create a stop codon
  • This RNA editing is tissue-specific

27
  • Initiation of translation can be controlled
  • Ferritin mRNA only translated if iron present
  • Mature mRNA molecules have various half-lives
    depending on the gene and the location (tissue)
    of expression
  • Target near poly-A tail can cause loss of the
    tail and destabilization

28
Protein Degradation
  • Proteins are produced and degraded continually in
    the cell
  • Lysosomes house proteases for nonspecific protein
    digestion
  • Proteins marked specifically for destruction with
    ubiquitin
  • Degradation of proteins marked with ubiquitin
    occurs at the proteasome
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