Chapter 11
Molecular
Mechanisms
of Gene
regulation
Jones and Bartlett Publishers © 2005
Categories of Protein-Coding Genes in
Arabidopsis
Regulation of Gene
Expression
Transcriptional
RNA processing
Translational
mRNA stability
Posttranslational control
DNA rearrangements
Prokaryotic
transcriptional regulation
• How ‘off’ is off?
• Coordinate regulation
Negative / Inducible / Repressible
Positive regulation of gene expression
Negative
Control
Inducible
System
Negative
Control
Repressible
System
Positive
Control
Inducible
System
Positive
Control
Repressible
System
Structure of an Operon
Inducible
Operon
Repressible
Operon
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.
Kinetics of induction of lactose operon
mRNA and proteins
Characteristics of partial diploids containing
several combinations of lacI, lacO and lacP alleles
Mutations of lac operon
Mutation
Effect
lacIRepressor 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, cont.
Mutation
Effect
lacPRNA polymerase cannot bind,
no transcription results.
lacZNo b-galactosidase synthesis.
lacYNo permease synthesis.
Mutations of lac operon, cont.
Mutation
Polar
mutations
crp-
Effect
Nonsense- termination of
transcription
Catabolite activator protein
cannot bind to crp site, no RNA
binding, no transcription
The 3 structural genes in the lac operon and the
mechanism of their regulation by the lac repressor
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.
Positive regulation of lactose operon
• In presence of glucose, lac operon is ‘off’.
How?
Structure of cyclic adenosine
monophosphate (cAMP)
Lac operon is negatively regulated by the lac repressor
and positively regulated by the cAMP-CRP complex
The 4 critical sequences in the lac operon bound by
CRP, RNA polymerase, repressor and the ribosome
Structure of the tryptophan (trp) operon showing
regulatory elements and the structural genes
Binding of tryptophan (the co-repressor) activates
an inactive repressor into an active form
capable of binding to the trp operator site
Structure of the 3’-end of a mRNA terminated at a
rho-independent termination site
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
Alternative conformations that the trp leader RNA
can assume which are important in attenuation
Other operons with repeated amino acid sequences
that act as “stalling sequence” during attenuation