Regulation of gene expression in prokaryotes

Regulation of Gene Expression in
Prokaryotes
Masih Sherafatian

Copyright © 2009 Pearson Education, Inc.

Prokaryotes Regulate Gene Expression
in Response to Environmental
Conditions

•  Many Prokaryotic Genes Are Clustered
and Regulated in Operons


Lactose Metabolism in E. coli
Is Regulated by an Inducible
System
•  Genes coding for the primary structure of
an enzyme are called structural genes.


Constitutive Mutations

•  Enzymes are produced regardless of the
presence or absence of lactose
•  lacI – : repressor gene
•  lac OC : operator region


The major predictions to be tested

(1) the I gene produces a diffusible product
(that is, a trans-acting product)
(2) the O region is involved in regulation but
does not produce a product (it is cis-acting);
and
(3) the O region must be adjacent to the
structural genes in order to regulate
transcription.


Genetic Proof of the Operon Model

•  Creation of partially diploid bacteria
•  The use of F’ plasmid makes it possible,
for example, to introduce an I+ gene into a
host cell whose genotype is I– , or to
introduce an O+ region into a host cell of
genotype OC.
•  Mutation, IS, “superrepressed,” cannot
interact with the inducer, lactose


The Catabolite-Activating
Protein (CAP) Exerts Positive
Control over the lac Operon


Predict the level of genetic activity of the lac
operon as well as the status of the lac
repressor and the CAP protein under the
cellular conditions listed in the
accompanying table


(a) With no lactose and no glucose, the operon is off because
the lac repressor is bound to the operator and although CAP is
bound to its binding site, it will not override the action of the
repressor.
(b) With lactose added to the medium, the lac repressor is
inactivated and the operon is transcribing the structural genes.
With no glucose, the CAP is bound to its binding site, thus
enhancing transcription.
(c) With no lactose present in the medium, the lac repressor is
bound to the operator region, and since glucose inhibits adenyl
cyclase, the CAP protein will not interact with its binding site.
The operon is therefore “off.”
(d) With lactose present, the lac repressor is inactivated;
however, since glucose is also present, CAP will not interact
with its binding site. Under this condition transcription is
severely diminished and the operon can be considered to be
“off.”


Crystal Structure Analysis of
Repressor Complexes Has
Confirmed the Operon Model


The Tryptophan (trp) Operon in
E. coli Is a Repressible Gene
System
•  This operon is regulated by two
mechanisms
•  Tryptophan is Corepressor
•  constitutive mutations: trpR–


Riboswitches Utilize Metabolite-
sensing RNAs to Regulate Gene
Expression

•  mRNA sequences (or elements), present
in the 5’-untranslated region (5’-UTR)
upstream from the coding sequences.
•  capable of binding with small molecule
ligands, such as metabolites,
•  creating a terminator structure.


The two important domains within a
riboswitch

•  Aptamer: binds to the ligand
•  Expression platform: capable of forming
the terminator structure


The ara Operon Is Controlled by a
Regulator Protein That Exerts Both
Positive and Negative Control


In a theoretical operon, genes A, B, C, and D
represent the repressor gene, the promoter
sequence, the operator gene, and the structural
gene, but not necessarily in the order named. This
operon is concerned with the metabolism of a
theoretical molecule (tm). From the data provided
in the accompanying table, first decide whether
the operon is inducible or repressible. Then
assign A, B, C, and D to the four parts of the
operon. Explain your rationale. (AE = active
enzyme; IE = inactive enzyme; NE = no enzyme)


The system is inducible. C codes for the
structural gene. B must be the promoter.
The A locus is the operator, and the
D locus is the repressor gene.


Regulation of gene expression in prokaryotes

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