ap biology chapter 13: gene regulation. gene regulation in bacteria and eukaryotes bacterial cells...
TRANSCRIPT
Gene Regulation in Bacteria and Eukaryotes
Bacterial cells• Genetic Organization?• Grow rapidly and have short life span• Controlling transcription is the most economical way for the
cell to regulate gene expression
Eukaryote cells• Long life span/respond to many different stimuli• A single gene is regulated in different ways in different types
of cells• Gene regulation complex• Although transcriptional-level control predominates, control
at other levels of gene expression is also important
Gene Regulation in Bacteria Prokaryotic DNA is organized into units called operons,
which contain functionally related genes Operons regulated as units, so functionally related proteins
are synthesized simultaneously only when needed Each operon consists of:
Regulatory gene, controls transcription of other genes Promoter, RNA polymerase recognizes as place to start transcribing Operator, governs access of RNA polymerase to promoter
Structural genes, encode for related proteins
Inducible, Repressible, and Constitutive Genes in Bacteria
Inducible operon• Normally turned off• Catabolic pathways
Repressible operon• Normally turned on• Operate via feedback inhibition• Anabolic pathways
Constitutive genes• Constantly needed and therefore constantly transcribed• Examples: Ribosomal proteins, tRNAs, RNA polymerase, glycolysis
enzymes• Neither inducible nor repressible and active at all times• The activity of constitutive genes is controlled by how efficiently RNA
polymerase binds to their promoter regions.
Inducible Operons: lac operon Intestinal bacterium Escherichia coli (E.coli) lives on what its
host eats Specific enzymes are needed to metabolize the type of food
that comes along e.g. in newborn mammals, E.coli are bathed in milk,
containing the milk sugar lactose The lactose operon contains three structural genes, each
coding for an enzyme that aids in lactose metabolism lactose not present: repressor active, operon off; no
transcription for lactose enzymes lactose present: repressor inactive, operon on; inducer
molecule inactivates protein repressor (allolactose) transcription is stimulated when inducer binds to a
regulatory protein Lac Operon Video
Lac Operon Video
Allolactose bound to repressor
Repressible Operons:trp Operon
Tryptophan synthesis (anabolism) Promoter: RNA polymerase binding
site; begins transcription Operator: controls access of RNA
polymerase to genes (tryptophan not present)
Repressor: binds to operator preventing attachment of RNA polymerase ~ when tryptophan is present ~ acts as a co-repressor)
Transcription is repressed when tryptophan binds to a regulatory protein
Trp Operon Video
Negative Control in the Regulation of an Operon
Negative regulators inhibit transcriptionRepressible and inducible operons are under
negative controlWhen repressor protein binds to operator,
transcription is turned offSeen in lac and trp operons
Positive regulators stimulate transcription Some inducible operons (lac) are also under
positive controlA separate protein binds to DNA and stimulates
transcription of the genePositive control of lac operon requires that the cell
is able to sense the presence of glucose• More efficient for cell to utilize glucose before lactose• Only when lactose is present and glucose is in short
supply does E.coli use lactose as energy source
Positive Control in the Regulation of an Operon
Positive Regulation of lac Operon
Lac operon always has low affinity for RNA polymerase Involves Two Proteins:
• CAP (catabolite activator protein) • cAMP (cyclic AMP)
Together, CAP and cAMP cause RNA polymerase to bind tightly to promoter region Levels of cAMP increase as levels of glucose decrease Lac operon is fully active only when lactose is available and glucose levels are low
A Regulon Group of functionally related operons controlled
by a common regulator Example: CAP regulates the catabolism of
lactose, galactose, arabinose and maltose
Comprehension CheckMatch these components of the lac operon with their functions.
______ b-galactosidase A. is inactivated when attached to
lactose______ cAMP-CAP complex B. codes for synthesis of
repressor______ lactose C. hydrolyzes lactose______ operator D. stimulates gene
expression______ promoter E. repressor attaches here______ regulator gene F. RNA polymerase attaches
here______ repressor G. acts as inducer that
inactivates repressor______ structural gene H. codes for an enzyme
C
D
G
E
F
B
A
H
Comprehension CheckListed below are characteristics of repressible
and inducible enzymes. Identify each of the following as true of repressible or inducible enzymes.
______ genes are switched off until a specific metabolite inactivates the repressor
______ genes are switched on until a specific metabolite activates the repressor
______ Generally function in anabolic pathways______ Usually function in catabolic pathways______ Pathway end product switches off its own
production______ Enzyme synthesis is switched on by the
nutrient in used in the pathway
Inducible
Repressible
RepressibleInducibleRepressible
Inducible
Comprehension CheckThe events listed below describe how the lac operon functions
when lactose is present and glucose is absent. Put the steps in the correct order.______ Allolactose binds to repressor______ cAMP accumulates______ cAMP activates CAP______ cAMP binds to CAP______ cAMP/CAP complex binds to CAP site in
promoter______ CAP concentration increases______ Genes transcribed______ Repressor inactivated______ RNA polymerase binds to promoter
1
2
3
4
5
6
7
8
9
Eukaryotic Gene Expression
Not organized into operons Typical human cell only expresses about 20%
of its genes at any given time Remember: All body cells contain identical
genome Cells rely on differential gene expression
Regulation allows cell differentiation and organization into tissues/organs
Each gene has regulatory sequences essential to the control of transcription
Gene Regulation in Eukaryotic Cells
Gene regulation occurs at the levels of
• Transcription
• mRNA processing
• Translation
• The protein product
Eukaryotic Promoters Vary in Efficiency, Depending on UPE’s
Like prokaryotes, transcription requires an initiation and promoter sites
Eukaryotic Promoter consists of:• RNA Polymerase-binding Site (TATA box)• Upstream Promoter Elements (UPE’s)
• 8-12 bases upstream from TATA box
Types/Number of UPE’s determine efficiency of promoter
• UPE’s required for accurate and efficient initiation of transcription
In addition to UPE’s, eukaryotic genes also controlled by Enhancers
• Enhancers facilitate RNA polymerase binding to promoter
• Increase rate of transcription
Eukaryotic Regulatory Proteins
Called “Transcription Factors”
Similar to repressors and CAP’s in prokaryotes
Usually act as activators
Enhancers only become functional when bound to specific transcription factors
Chromosome Organization may Affect Gene Expression
Genes are inactivated by changes in chromosome structure• Heterochromatin is tightly wound and not transcribed (ex. Barr body)• Euchromatin is loosely packed and easily transcribed
DNA methylation• Methyl groups added to cytosines• Make transcription impossible
Multiple copies of some genes present in one chromosome• Tandemly Repeated Gene Sequences (VTNR’s)
Gene amplification• Cells produce multiple copies of a gene by selective replication
Differential mRNA Processing(Posttranscriptional Control)
Cells in each tissue produce own version of mRNA Same gene can be used to produce a certain protein in one tissue and a
related, but slightly different protein in another tissue Example: troponin
• a protein that regulates muscle contraction• produced in different muscle tissues
Other Methods of Posttranscriptional Control
Proteolytic Protein ProcessingProteins produced in inactive formBecome active via removal of a portion of their
polyepeptide chain Chemical Modification
Addition or removal of functional groupsAffects enzyme activityKinases (add phosphate groups)Phosphatases (remove phosphate groups)