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

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

Mutation Effect

lacP- RNA polymerase cannot bind,

no transcription results.

lacZ- No -galactosidase synthesis.

lacY- No permease synthesis.

Mutations of lac operon, cont.

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

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