virus & bacteria
DESCRIPTION
The genetics of viruses and bacteriaTRANSCRIPT
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1/69Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures forBiology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 18
he !enetics o" #iruses
and Bacteria
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Overview: Microbial Model Systems
Viruses called bacteriophages
$ Can infect and set in motion a genetic taeover
of bacteria! such as Escherichia coli
Figure 18.1
0.5 m
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E. coliand its viruses
$ "re called model systems because of theirfre#uent use by researchers in studies that
reveal broad biological principles
$eyond their value as model systems
$ Viruses and bacteria have uni#ue genetic
mechanisms that are interesting in their own
right
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%ecall that bacteria are proaryotes
$ &ith cells much smaller and more simplyorgani'ed than those of euaryotes
Viruses
$ "re smaller and simpler still
Figure 18.2
()*+ m
Virus
"nimalcell
$acterium
"nimal cell nucleus
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Concept ,-),: " virus has a genome but can
reproduce only within a host cell Scientists were able to detect viruses indirectly
$ Long before they were actually able to see
them
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The Discovery of Viruses:Scientific Inquiry
.obacco mosaic disease
$ Stunts the growth of tobacco plants and givestheir leaves a mosaic coloration
Figure 18.3
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/n the late ,-((s
$ %esearchers hypothesi'ed that a particlesmaller than bacteria caused tobacco mosaic
disease
/n ,01+! &endell Stanley
$ Confirmed this hypothesis when he crystalli'ed
the infectious particle! now nown as tobacco
mosaic virus 2.MV3
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Structure of Viruses
Viruses
$ "re very small infectious particles consisting ofnucleic acid enclosed in a protein coat and! in
some cases! a membranous envelope
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Viral Genomes
Viral genomes may consist of
$ 4ouble5 or single5stranded 46"
$ 4ouble5 or single5stranded %6"
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10/69Copyright 2005 Pearson Education, Inc. publishing as Benjamin CummingsFigure 18.4a, b
,- *+( mm 7(80( nm 2diameter3
*( nm +( nm
(a) Tobacco mosaic virus (b) Ae!oviruses
%6"46"Capsomere
9lycoprotein
Capsomere
of capsid
Capsids and Envelopes
" capsid
$ /s the protein shell that encloses the viral genome
$ Can have various structures
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Some viruses have envelopes
$ &hich are membranous coverings derivedfrom the membrane of the host cell
Figure 18.4c
-(8*(( nm 2diameter3
+( nm
(c) "!#$ue!%a viruses
%6"
9lycoprotein
Membranous
envelope
Capsid
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$acteriophages! also called phages
$ ave the most comple; capsids found amongviruses
Figure 18.4
-( **+ nm
+( nm
() &acteriophage T4
46"
ead
.ailfiber
.ail
sheath
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General Features of Viral Reproductive Cycles
Viruses are obligate intracellular parasites
$ .hey can reproduce only within a host cell
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Viruses use en'ymes! ribosomes! and small
molecules of host cells$ .o synthesi'e progeny viruses
V/%=S
Capsid
proteins
m%6"
Viral 46"
OS. C
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Reproductive Cycles of Phaes
Phages
$ "re the best understood of all viruses
$ 9o through two alternative reproductive
mechanisms: the lytic cycle and the lysogenic
cycle
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The ytic Cycle
.he lytic cycle
$ /s a phage reproductive cycle that culminatesin the death of the host
$ Produces new phages and digests the host>s
cell wall! releasing the progeny viruses
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.he lytic cycle of phage .?! a virulent phage
Phage assembly
ead .ails .ail fibersFigure 18.'
Attachme!t..he .? phage uses
its tail fibers to bind to specificreceptor sites on the outer
surface of an E. colicell)
1!tr o# phage *+A
a! egraatio! o# host *+A.
.he sheath of the tail contracts!
in@ecting the phage 46" into
the cell and leaving an empty
capsid outside) .he cell>s
46" is hydroly'ed)
2
!thesis o# vira$ ge!omes
a! protei!s..he phage 46"
directs production of phage
proteins and copies of the phage
genome by host en'ymes! using
components within the cell)
3Assemb$..hree separate sets of proteinsself5assemble to form phage heads! tails!
and tail fibers) .he phage genome is
pacaged inside the capsid as the head forms)
4
-e$ease..he phage directs production
of an en'yme that damages the bacterial
cell wall! allowing fluid to enter) .he cell
swells and finally bursts! releasing ,((
to *(( phage particles)
5
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The yso!enic Cycle
.he lysogenic cycle
$ %eplicates the phage genome withoutdestroying the host
.emperate phages
$ "re capable of using both the lytic and
lysogenic cycles of reproduction
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.he lytic and lysogenic cycles of phage ! a
temperate phage
Many cell divisions
produce a large
population of bacteria
infected with theprophage)
.he bacterium reproducesnormally! copying the prophage
and transmitting it to daughter cells)
Phage 46" integrates into
the bacterial chromosome!
becoming a prophage)
6ew phage 46" and
proteins are synthesi'ed
and assembled into phages)
Occasionally! a prophage
e;its the bacterial chromosome!
initiating a lytic cycle)
Certain factorsdetermine whether
.he phage attaches to a
host cell and in@ects its 46")
Phage 46"
circulari'es
.he cell lyses! releasing phages)
Lytic cycle
is induced
Lysogenic cycle
is entered
soge!ic cc$etic cc$e
or Prophage
$acterial
chromosome
Phage
Phage
46"
Figure 18./
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Reproductive Cycles of !nimal Viruses
.he nature of the genome
$ /s the basis for the common classification ofanimal viruses
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Classes of animal viruses
Tab$e 18.1
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Viral Envelopes
Many animal viruses
$ ave a membranous envelope
Viral glycoproteins on the envelope
$ $ind to specific receptor molecules on thesurface of a host cell
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%6"
Capsid
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RN" as Viral Genetic #aterial
.he broadest variety of %6" genomes
$ /s found among the viruses that infect animals
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%etroviruses! such as /V! use the en'yme
reverse transcriptase$ .o copy their %6" genome into 46"! which
can then be integrated into the host genome
as a provirus
Figure 18.
%everse
transcriptase
Viral envelope
Capsid
9lycoprotein
%6"
2two identical
strands3
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.he reproductive cycle of /V! a retrovirus
Figure 18.10
m%6"
%6" genome
for the ne;t
viral generation
Viral %6"
%6"546"hybrid
46"
Chromosomal
46"
6=CLs 46")
4
Proviral genes are
transcribed into %6"molecules! which serve as
genomes for the ne;t viral
generation and as m%6"s
for translation into viral
proteins)
5
.he viral proteins include
capsid proteins and reverse
transcriptase 2made in the
cytosol3 and envelope
glycoproteins 2made in the
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"volution of Viruses
Viruses do not really fit our definition of living
organisms
Since viruses can reproduce only within cells
$ .hey probably evolved after the first cells
appeared! perhaps pacaged as fragments ofcellular nucleic acid
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Concept ,-)*: Viruses! viroids! and prions are
formidable pathogens in animals and plants
4iseases caused by viral infections
$ "ffect humans! agricultural crops! and
livestoc worldwide
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Viral Diseases in !nimals
Viruses may damage or ill cells
$ $y causing the release of hydrolytic en'ymesfrom lysosomes
Some viruses cause infected cells
$ .o produce to;ins that lead to disease
symptoms
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Vaccines
$ "re harmless derivatives of pathogenicmicrobes that stimulate the immune system to
mount defenses against the actual pathogen
$ Can prevent certain viral illnesses
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"merin Viruses
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Severe acute respiratory syndrome 2S"%S3
$ %ecently appeared in China
Figure 18.11 A, &
(a)Aoung ballet students in ong Bong
wear face mass to protect themselves
from the virus causing S"%S)
(b).he S"%S5causing agent is a coronavirus
lie this one 2colori'ed .
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Outbreas of newD viral diseases in humans
$ "re usually caused by e;isting viruses thate;pand their host territory
Vi l Di i Pl
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Viral Diseases in Plants
More than *!((( types of viral diseases of
plants are nown
Common symptoms of viral infection include
$ Spots on leaves and fruits! stunted growth! and
damaged flowers or roots
Figure 18.12
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Plant viruses spread disease in two ma@or
modes
$ ori'ontal transmission! entering through
damaged cell walls
$ Vertical transmission! inheriting the virus froma parent
Vi id d P i Th Si l t # f ti ! t
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Viroids and Prions: The Simplest #nfectious !ents
Viroids
$ "re circular %6" molecules that infect plantsand disrupt their growth
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Prions
$ "re slow5acting! virtually indestructibleinfectious proteins that cause brain diseases in
mammals
$ Propagate by converting normal proteins intothe prion version
Figure 18.13
Prion
6ormal
protein
Original
prion
6ew
prion
Many prions
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Concept ,-)1: %apid reproduction! mutation!
and genetic recombination contribute to the
genetic diversity of bacteria
$acteria allow researchers
$ .o investigate molecular genetics in thesimplest true organisms
Th $ t i l G d #t R li ti
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The $acterial Genome and #ts Replication
.he bacterial chromosome
$ /s usually a circular 46" molecule with fewassociated proteins
/n addition to the chromosome
$ Many bacteria have plasmids! smaller circular
46" molecules that can replicate
independently of the bacterial chromosome
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$acterial cells divide by binary fission
$ &hich is preceded by replication of thebacterial chromosome
%eplication
for
Origin of
replication
.ermination
of replication
Figure 18.14
% tation and Genetic Recombination as So rces of
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%utation and Genetic Recombination as Sources ofGenetic Variation
Since bacteria can reproduce rapidly$ 6ew mutations can #uicly increase a
population>s genetic diversity
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Eurther genetic diversity
$ Can arise by recombination of the 46" fromtwo different bacterial cells
Mutant
strain
arg
trp
F
67-"+T
Figure 18.15 Only the samples from the mi;ed culture! contained cells that gave rise to colonies on
minimal medium! which lacs amino acids)
-T
%esearchers had two mutant strains! one that could mae arginine but not
tryptophan 2arg+trp3 and one that could mae tryptophan but not arginine (argtrp+3)
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Colonies
grew
Mutant
strain
argFtrp8
Mutant
strain
arg8trpF
6ocolonies
2control3
6ocolonies
2control3
Mi;ture
$ecause only cells that can mae both arginine and tryptophan 2arg+trp+cells3 can grow into
colonies on minimal medium! the lac of colonies on the two control plates showed that no further mutations had
occurred restoring this ability to cells of the mutant strains) .hus! each cell from the mi;ture that formed a colony on the
minimal medium must have ac#uired one or more genes from a cell of the other strain by genetic recombination)
C9+C"9+
%echanisms of Gene Transfer and Genetic
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%echanisms of Gene Transfer and GeneticRecombination in $acteria
.hree processes bring bacterial 46" fromdifferent individuals together
$ .ransformation
$ .ransduction
$ Con@ugation
Transformation
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Transformation
.ransformation
$ /s the alteration of a bacterial cell>s genotypeand phenotype by the uptae of naed! foreign
46" from the surrounding environment
Transduction
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Transduction
/n the process nown as transduction
$ Phages carry bacterial genes from one hostcell to another
1
Figure 18.1'
4onor
cell
%ecipient
cell
AF BF
AF
AF B8
A8 B8
AF
%ecombinant cell
Crossing
over
Phage infects bacterial cell that has allelesA+and B+
ost 46" 2brown3 is fragmented! and phage 46"
and proteins are made) .his is the donor cell)
" bacterial 46" fragment 2in this case a fragment with
theA+allele3 may be pacaged in a phage capsid)
Phage with theA+allele from the donor cell infectsa recipientABcell! and crossing over 2recombination3
between donor 46" 2brown3 and recipient 46"
2green3 occurs at two places 2dotted lines3)
.he genotype of the resulting recombinant cell 2A+B3
differs from the genotypes of both the donor 2A+B+3 and
the recipient 2AB3)
2
3
4
5
Phage 46"
AF BF
Con$u!ation and %lasmids
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Con$u!ation and %lasmids
Con@ugation
$ /s the direct transfer of genetic material betweenbacterial cells that are temporarily @oined
Figure 18.1/ Se; pilus , m
The F Plasmid and Con&uation
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The F Plasmid and Con&uation
Cells containing the E plasmid! designated EF
cells
$ Eunction as 46" donors during con@ugation
$ .ransfer plasmid 46" to an Erecipient cell
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Con@ugation and transfer of an E plasmid from
an EFdonor to an Erecipient
Figure 18.18a
" cell carrying an E plasmid
2an EFcell3 can form a
mating bridge with an E8cell
and transfer its E plasmid)
" single strand of the
E plasmid breas at a
specific point 2tip of blue
arrowhead3 and begins to
move into the recipient cell)
"s transfer continues! the
donor plasmid rotates2red arrow3)
* 46" replication occurs in
both donor and recipient
cells! using the single
parental strands of the
E plasmid as templates
to synthesi'e complementary
strands)
1 .he plasmid in the
recipient cell
circulari'es) .ransfer
and replication result
in a compete E plasmid
in each cell) .hus! both
cells are now EF)
?
E Plasmid $acterial chromosome
$acterialchromosome
EF cell
EF cell
EF
cell
Mating
bridge
,
Co!:ugatio! a! tra!s#er o# a!
F p$asmi #rom a! F;
o!or toa! F
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Chromosomal genes can be transferred during
con@ugation
$ &hen the donor cell>s E factor is integrated into the
chromosome
" cell with the E factor built into its chromosome
$ /s called an fr cell
.he E factor of an fr cell
$ $rings some chromosomal 46" along with it when itis transferred to an E8cell
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Con@ugation and transfer of part of the
bacterial chromosome from an fr donor
to an E8recipient! resulting in recombination
EF cell fr cell
E factor.he circular E plasmid in an EFcell
can be integrated into the circular
chromosome by a single crossover
event 2dotted line3)
1
.he resulting cell is called an fr cell
2for igh fre#uency of recombination3)
2
Since an fr cell has all
the E5factor genes! it can
form a mating bridge with
an E8cell and transfer 46")
1 " single strand of the E factor
breas and begins to move
through the bridge) 46"
replication occurs in both donor
and recipient cells! resulting in
double5stranded 46"
? .he location and orientation
of the E factor in the donor
chromosome determine
the se#uence of gene transfer
during con@ugation) /n this
e;ample! the transfer se#uence
for four genes is "5$5C54)
+ .he mating bridge
usually breas well
before the entire
chromosome and
the rest of the
E factor are transferred)
G
.wo crossovers can result
in the e;change of similar
2homologous3 genes between
the transferred chromosome fragment
2brown3 and the recipient cell>s
chromosome 2green3)
/ .he piece of 46" ending up outside the
bacterial chromosome will eventually be
degraded by the cell>s en'ymes) .he recipient
cell now contains a new combination of genes
but no E factorH it is a recombinant E8cell)
8
.emporary
partial
diploid
%ecombinant E8
bacterium
A+B+ C+
D+
E8 cell AB
CD
AB
CD D
A
C8B
A+B+C+
D+A+B+
D+C+
A+
A+
B+
AB
CD
AB+
CD
A+$F B
A+
fr cell
D
A
CB
A+B+C+
D+
A+B+
Co!:ugatio! a! tra!s#er o# part
o# the bacteria$ chromosome #rom
a! #r o!or to a! F
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R plasmids and !ntibiotic Resistance
% plasmids
$ Confer resistance to various antibiotics
Transposition of Genetic "lements
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Transposition of Genetic "lements
.ransposable elements
$ Can move around within a cell>s genome
$ "re often called @umping genesD
$ Contribute to genetic shuffling in bacteria
Insertion Sequences
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Figure 18.1a
(a) /nsertion se#uences! the simplest transposable elements in bacteria! contain a single gene that
encodes transposase! which cataly'es movement within the genome) .he inverted repeats are
bacward! upside5down versions of each otherH only a portion is shown) .he inverted repeat
se#uence varies from one type of insertion se#uence to another)
"!sertio! se=ue!ce
.ransposase gene/nverted
repeat
/nverted
repeat
1
+
1
+
" . C C 9 9 .I
. " 9 9 C C " I
" C C 9 9 " .I
. 9 9 C C . " I
Insertion Sequences
"n insertion se#uence contains a single gene
for transposase
$ "n en'yme that cataly'es movement of the
insertion se#uence from one site to another
within the genome
Transposons
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Transposons
$acterial transposons
$ "lso move about within the bacterial genome
$ ave additional genes! such as those for
antibiotic resistance
Figure 18.1b
(b) .ransposons contain one or more genes in addition to the transposase gene) /n the transposon
shown here! a gene for resistance to an antibiotic is located between twin insertion se#uences)
.he gene for antibiotic resistance is carried along as part of the transposon when the transposon
is inserted at a new site in the genome)
/nverted repeats .ransposase gene
/nsertion
se#uence/nsertion
se#uence
"ntibiotic
resistance gene
Tra!sposo!
+
1
+
1
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Concept ,-)?: /ndividual bacteria respond to
environmental change by regulating their gene
e;pression
E. coli! a type of bacteria that lives in the
human colon$ Can tune its metabolism to the changing
environment and food sources
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.his metabolic control occurs on two levels
$ "d@usting the activity of metabolic en'ymesalready present
$ %egulating the genes encoding the metabolic
en'ymes
Figure 18.20a, b
(a) -egu$atio! o# e!%me
activit
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'perons: The $asic Concept
/n bacteria! genes are often clustered into
operons! composed of
$ "n operator! an on5offD switch
$ " promoter
$ 9enes for metabolic en'ymes
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"n operon
$ /s usually turned onD
$ Can be switched off by a protein called a
repressor
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.he trpoperon: regulated synthesis of
repressible en'ymes
Figure 18.21a
(a) Trptopha! abse!t, repressor i!active, opero! o!.%6" polymerase attaches to the 46" at the
promoter and transcribes the operon>s genes)
9enes of operon
/nactive
repressorProtein
Operator
Polypeptides that mae upen'ymes for tryptophan synthesis
Promoter
%egulatory
gene%6"
polymerase
Start codon Stop codon
Promoter
trpoperon
+
1m%6" +
trpDtrpE trpC trpB trpAtrpR46"
m%6"
< 4 C $ "
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46"
m%6"
Protein
.ryptophan
2corepressor3
"ctive
repressor
6o %6" made
Trptopha! prese!t, repressor active, opero! o##."s tryptophan
accumulates! it inhibits its own production by activating the repressor protein)
(b)
Figure 18.21b
Repressible and #nducible 'perons: T(o Types of
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p p yp)eative Gene Reulation
/n a repressible operon
$ $inding of a specific repressor protein to the
operator shuts off transcription
/n an inducible operon
$ $inding of an inducer to an innately inactive
repressor inactivates the repressor and turns
on transcription
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.he lacoperon: regulated synthesis of
inducible en'ymes
Figure 18.22a
46"
m%6"
Protein"ctive
repressor
%6"
polymerase
6o
%6"
made
lacZlacl
%egulatorygene
Operator
Promoter
actose abse!t, repressor active, opero! o##..he lacrepressor is innately active! and in
the absence of lactose it switches off the operon by binding to the operator)
(a)
+
1
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m%6" +J
46"
m%6"
Protein
"llolactose
2inducer3
/nactive
repressor
lacl lacz lacY lacA
%6"
polymerase
Permease .ransacetylase59alactosidase
+
1
(b) actose prese!t, repressor i!active, opero! o!."llolactose! an isomer of lactose! derepresses
the operon by inactivating the repressor) /n this way! the en'ymes for lactose utili'ation are induced)
m%6" +
lac operon
Figure 18.22b
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/nducible en'ymes
$ =sually function in catabolic pathways
%epressible en'ymes
$ =sually function in anabolic pathways
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%egulation of both the trpand lacoperons
$ /nvolves the negative control of genes!because the operons are switched off by the
active form of the repressor protein
Positive Gene Reulation
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Some operons are also sub@ect to positive
control
$ Via a stimulatory activator protein! such as
catabolite activator protein 2C"P3
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Promoter
actose prese!t, g$ucose scarce (cA7 $eve$ high)> abu!a!t lacm-+A s!thesi%e.
/f glucose is scarce! the high level of c"MP activates C"P! and the lacoperon produces
large amounts of m%6" for the lactose pathway)
(a)
CA7?bi!i!g site Operator%6"
polymerase
can bind
and transcribe
/nactive
C"P
"ctive
C"Pc"MP
46"
/nactive lac
repressor
lacl lacZ
Figure 18.23a
/n E. coli! when glucose! a preferred food
source! is scarce
$ .he lacoperon is activated by the binding of a
regulatory protein! catabolite activator protein
2C"P3
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&hen glucose levels in an E. colicell increase
$ C"P detaches from the lacoperon! turning itoff
Figure 18.23b
(b) actose prese!t, g$ucose prese!t (cA7 $eve$ $o)> $itt$e lacm-+A s!thesi%e.
&hen glucose is present! c"MP is scarce! and C"P is unable to stimulate transcription)
/nactive lacrepressor
/nactive
C"P
46"
%6"
polymerase
can>t bind
Operator
lacl lacZ
CA7?bi!i!g site
Promoter