OPERONS
Chapter 18
Campbell and Reece
E. Coli synthesizes
tryptophan
5 genes clustered
together
 each reaction in
pathway catalyzed
by specific enzyme
 one promoter (site
where RNA
polymerase can
attach) serves all 5
genes

E. Coli synthesizes
tryptophan

transcription of these 5 genes  1
very long mRNA that codes for 5
polypeptides that make up the 5
enzymes in tryptophan pathway
◦ mRNA is punctuated with stops & starts
E. Coli synthesizes
tryptophan
advantage of grouping genes: can use
1 “switch” controls cluster of
functionally related genes: they are
“coordinately controlled”
 “switch” is a segment of DNA called
an operator

Operator
positioned w/in or between promoter
& genes
 controls access of RNA polymerase to
genes

Operon

includes: promoter, operator, & genes
How does the “switch” work?
just by itself: trp operon is in “on”
position
 to switch it off: trp repressor must
bind to operator which blocks
attachment of RNA polymerase

◦ repressor proteins are specific for each
operator
◦ repressors are protein product of a
regulatory gene called trpR
 located some distance from trp operon
 has own promoter
Regulatory Genes
continuously expressed @ low rate
 trp repressor is allosteric (2 alternate
shapes: active & inactive)

◦ trp repressor made in its inactive form
with low affinity for trp operator

repressor binding is reversible
◦ operator vacillates between 2 states
 1 w/out repressor bound
 1 with repressor bound
 duration of each state depends on # of active
repressor molecules around
Regulatory Genes

trp repressor is allosteric (2 alternate
shapes: active & inactive)
◦ trp repressor made in its inactive form
with low affinity for trp operator
◦ when tryptophan binds to trp repressor @
allosteric site  protein changes to active
form shape  can attach to operator
turning operon off
Tryptophan

functions as corepressor:
◦ small molecule that cooperates with
repressor protein to switch operon off
◦ if cell’s tryptophan levels high more
molecules will associate with a repressor
protein to switch it off
◦ if cell’s tryptophan levels low 
transcription of operon’s genes resume
trp operon animation

http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/
animation28.html
2 Types of (-) Gene Regulation
Repressible Operon
1.
◦
◦
transcription usually in “on” position
but it can be repressed when specific
molecule binds to regulatory protein
ex: trp operon
Inducible Operon
2.
◦
◦
usually in “off”position but can be
induced when small molecule interacts
with regulatory protein
ex: lac operon
Lac operon
lactose: disaccharide in milk available
to E. coli in human intestine when
person drinks milk
 lactose  glucose + galactose

◦ hydrolysis reaction by enzyme βgalactosidase
◦ in absence of lactose E. coli has only a few
molecules of β-galactosidase
◦ + milk  w/in 15 min bacteria has 1,000s
x more
Lac Operon

3 genes:
◦ 1: β-galactosidase
◦ 2: enzymes that function in lactose
utilization
Lac Operon
entire transcription unit is under
control of 1 main operator & promoter
 lacl: regulatory gene outside of operon
codes for allosteric repressor protein
that can switch off lac operon by
binding to operator
 by itself lac repressor active  binds
to operator  switching lac operon off

Repressor

achieved by specific small molecule
called an inducer  inactivates
repressor
◦ lac operon: its inducer is allolactose
(isomer of lactose: sm amt made when
lactose enters cell)
◦ no lactose  no allolactose  lac
repressor in active shape  genes of lac
operon not being transcribed
Inactivation of Repressor

+ lactose  allolactose binds to lac
repressor  it changes shape 
nullifying repressor’s ability to attach
to operator  transcription of lactoseutilizing enzymes
Lac Repressor
Inducible Enzymes
ex: lac operon
 their synthesis is induced by a
chemical signal (allolactose)
 generally, function in catabolic
pathways

Repressible Enzymes
ex: tryptophan synthesis
 generally function in anabolic
pathways


both repressible & inducible enzymes
involves negative control of genes
(operons switched off by active form
of repressor protein)
http://highered.mcgrawhill.com/sites/007337797x/student_view0/
chapter13/animation_quiz__combination_of_switches__the_lac_ope
ron.html
 http://www.wwnorton.com/college/biolog
y/mbio/animations/main.asp?chno=ch10a0
1
