Virus & Bacteria

5/19/2018 Virus & Bacteria

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



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



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



%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



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



The Discovery of Viruses:Scientific Inquiry

.obacco mosaic disease

$ Stunts the growth of tobacco plants and givestheir leaves a mosaic coloration

Figure 18.3



/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



Structure of Viruses

Viruses

$ "re very small infectious particles consisting ofnucleic acid enclosed in a protein coat and! in

some cases! a membranous envelope



Viral Genomes

Viral genomes may consist of

$ 4ouble5 or single5stranded 46"

$ 4ouble5 or single5stranded %6"


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



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



$acteriophages! also called phages

$ ave the most comple; capsids found amongviruses

Figure 18.4

-( **+ nm

+( nm

() &acteriophage T4

46"

ead

.ailfiber

.ail

sheath



General Features of Viral Reproductive Cycles

Viruses are obligate intracellular parasites

$ .hey can reproduce only within a host cell



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



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



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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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

.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


51/69


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


52/69


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 $ "


61/69


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

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