Regulation of Gene Expression Part 2: Gene Regulation in Prokaryotes and Eukaryotes

Regulation of Gene Expression

Part 2:Gene

Regulation in Prokaryotes and

Eukaryotes

Gene Regulation in Prokaryotes

The lac operon is also subject to positive regulation What happens if both glucose and

lactose are present? Involves catabolite repressor Represses genes for catabolism of other

sugars if glucose is present Mediated by cAMP and CAP

CAP: catabolite gene activator protein also CRP: cAMP receptor protein

Effects of glucose and lactose levels on the expression of the lac operon


The lac operon is also subject to positive regulation (cont): Mechanism:

[increase] cAMP and [decreased] glucose: allows CAP binding to DNA

Stimulates transcription of lac operon Lactose-metabolizing enzymes produced

[increased] glucose depresses [cAMP] Restricts expression of lac Supresses use of secondary sugars

Regulon: coordinates regulated operons (CAP and cAMP)

cAMP receptor protein (CRP)

Activation of lac operon by CAP


The ara operon is (+) and (-) regulated by a single protein E. coli arabinose operon

One protein exerts both + and – regulation Binding a signal molecule alters

conformation from repressor form Repressor binds one DNA regulatory site Activator, without signal molecules, binds

to another DNA sequence

Ara operon

Regulation of Ara operon




The ara operon is (+) and (-) regulated by a single protein

Ara C : regulates its own synthesis Represses transcription of its gene Called Autoregulation

Effects of some regulatory DNA sequences can be exerted from a distance via DNA looping

Gene Regulation in Prokaryotes Transcription Attenuation: regulates

genes for a.a. biosynthesis Genes for amino acid synthesizing enzymes

are clustered in operons Operons expressed when [a.a.] are

inadequate Trp operon of E. coli:

5 genes for conversion of chorismate to tryptophan mRNA from trp operon has 3 min half-life When [trp] increases, trp binds to trp repressor Causes conformational change in repressor protein

that permits binding to the operator Trp operator overlaps promoter, binding repressor

blocks RNA polymerase A ‘self-regulation’ mechanism

The trp operon

The trp operon

Trp receptor


Transcription Attenuation: regulates genes for a.a. biosynthesis (cont) Transcription attenuation is a second trp

regulating mechnism Uses translation termination site “leader” blocks transcription Halts transcription before operon; halts RNA –

polymerase Couples transcription to translation via leader

peptide Attenuation of transcripts increases as [trp]

increases due to sensitivity of leader peptide to [trp]

Transcriptional attenuation in the trp operon

Transcriptional attenuation in the trp operon


Transcription Attenuation: regulates genes for a.a. biosynthesis (cont):

Each a.a. biosynthetic operon uses a similar strategy

Induction of SOS response requires destruction of repressor


Induction of SOS response requires destruction of repressor

SOS response is induced if chromosome is damaged

An example of coordinated regulation of unlinked genes

Multiple unlinked genes repressed by Lex A protein

All genes induced simultaneously when DNA is damaged

Triggers lysis of repressor Mediated by Rec A protein Rec A only binds to single stranded DNA

SOS response in E. coli


Regulated Developmental Switch: bacteriophage

Objective is assembly of new viruses without cell destruction

Choices are lysis or lysogeny Lysis: results in destruction of infected cell Lysogenic cycle

Virus may inhabit host cell for generations Viral DNA inserts into host, replicates passively Phage in this state: Prophage Some trigger induction Virus enters lytic phase


Regulated Developmental Switch: bacteriophage lamda (cont):

Bacteriophage lambda has a complex regulatory circuit

Oversees ‘choice’ between pathways Involves many lambda proteins Two (N and Q) act as anti-terminators Modify host RNA polymerase to by-pass

termination sites Other proteins serve as promoters or activators


Some genes are regulated by genetic recombination

Occurs spontaneously in prokaryotes Called: Phase Variation Physically moves promoters relative to

genes regulated Mechanism used by some pathogens as

defense against host immune system E.g.:Salmonella

Salmonella typhimurium

Regulation of flagellin genes in Salmonella: Phase variation

Regulation of flagellin genes in Salmonella: Phase variation

Gene Regulation in Eukaryotes

Mechanisms resemble those in prokaryotes Positive regulation more common Involves selective binding of proteins

to control sequences Effect is modulation of rate of

transcription initiation


Mechanisms resemble those in prokaryotes

Gene Regulation in Eukaryotes Eukaryotic promoter and enhancer

elements mediate expression of cell-specific genes

Cells contain factors that recognize promoters and enhancers in the genes they transcribe

Transcription is accompanied by changes in chromosomal structure Lampbrush chromosomes Chromosome “puffs”


Transcription activator proteins required for binding RNA polymerase Some have general function Others are specific:

TATA-binding protein at “TATA-box” Activators required because eukaryotic

RNA-polymerase lacks ability to bind promoters

Gene Regulation in Eukaryotes Complex regulatory problems seen in

development of multicellular animals Genes function temporally and spatially Must act in succession Must be highly coordinated

Most genes expressed early in development Genes must be turned “off”, “on” in cell to

facilitate function Regulation involves expression and location of

genes and their products in developing organisms

Regulation of Gene Expression

End of part two

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Regulation of Gene Expression Part 2: Gene Regulation in Prokaryotes and Eukaryotes