chapter 11 molecular mechanisms of gene regulation jones and bartlett publishers © 2005
TRANSCRIPT
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Chapter 11
Molecular Mechanisms
of Gene regulation
Jones and Bartlett Publishers © 2005
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Categories of Protein-Coding Genes in Arabidopsis
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Regulation of Gene Expression
Transcriptional
RNA processing
Translational
mRNA stability
Posttranslational control
DNA rearrangements
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Prokaryotic transcriptional regulation
• How ‘off’ is off?
• Coordinate regulation
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Negative / Inducible / Repressible
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Positive regulation of gene expression
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Negative Control
Inducible System
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Negative Control
Repressible System
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Positive Control
Inducible System
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Positive Control
Repressible System
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Structure of an Operon
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Inducible Operon
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Repressible Operon
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The lac operon
• In E. coli, glucose is the preferred carbon source when both glucose and lactose are present.
• Jacob and Monod, 1950s, studied lactose metabolism and mutants, and won a Nobel Prize in 1965.
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Kinetics of induction of lactose operon mRNA and proteins
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Characteristics of partial diploids containing several combinations of lacI, lacO and lacP alleles
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Mutation Effect lacI- Repressor protein cannot bind,
constitutive expression results.lacIs Repressor binds tightly to
operator, not inducible.lacOc Repressor cannot bind to the
operator site; constitutive expression.
(cis-dominant)
Mutations of lac operon
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Mutation Effect
lacP- RNA polymerase cannot bind,
no transcription results.
lacZ- No -galactosidase synthesis.
lacY- No permease synthesis.
Mutations of lac operon, cont.
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Mutations of lac operon, cont.
Mutation Effect
Polar Nonsense- termination of
mutations transcription
crp- Catabolite activator protein cannot bind to crp site, no RNA binding, no transcription
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The 3 structural genes in the lac operon and the mechanism of their regulation by the lac repressor
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lac operon model
• 2 kinds of genes: structural, regulatory elements.• Polycistronic structural genes, with promoter and
operator constitute the lac operon.• Promoter mutants make no lac mRNA.• lacI gene makes a repressor, which binds to the
operator.• When operator is ‘repressed’ no transcription
occurs.• Inducers bind to repressor, lac mRNA is made.
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Positive regulation of lactose operon
• In presence of glucose, lac operon is ‘off’. How?
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Structure of cyclic adenosine
monophosphate (cAMP)
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Lac operon is negatively regulated by the lac repressor and positively regulated by the cAMP-CRP complex
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The 4 critical sequences in the lac operon bound by CRP, RNA polymerase, repressor and the ribosome
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Structure of the tryptophan (trp) operon showing regulatory elements and the structural genes
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Binding of tryptophan (the co-repressor) activates an inactive repressor into an active form capable of binding to the trp operator site
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Structure of the 3’-end of a mRNA terminated at a rho-independent termination site
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Structure of the leader polypeptide in the trp operon
The two tandem tryptophans in the leader peptide act as “stalling sequences” in the absence of tryptophan in the cell
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Alternative conformations that the trp leader RNA
can assume which are important in attenuation
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Other operons with repeated amino acid sequences that act as “stalling sequence” during attenuation