Gene expression in bacteria and bacteriophages

gene regulation

1. 台大農藝系 遺傳學601 20000Chapter 19 slide 1
CHAPTER 19
Regulation of Gene Expression in
Bacteria and Bacteriophages
Peter J. Russell
edited by Yue-Wen Wang Ph. D.
Dept. of Agronomy, NTU
A molecular Approach 2
nd
Edition

2. 台大農藝系 遺傳學601 20000Chapter 19 slide 2
The lacOperon of E. coli
1.Growth and division genes of bacteria are regulated genes. Their expression is
controlled by the needs of the cell as it responds to its environment with the
goal of increasing in mass and dividing.
2.Genes that generally are continuously expressed are constitutive genes
(housekeeping genes). Examples include protein synthesis and glucose
metabolism.
3.All genes are regulated at some level, so that as resources dwindle the cell can
respond with a different molecular strategy.
4.Prokaryotic genes are often organized into operons that are cotranscribed. A
regulatory protein binds an operator sequence in the DNA adjacent to the gene
array, and controls production of the polycis-tronic (polygenic) mRNA.
5.Gene regulation in bacteria and phage is similar in many ways to the emerging
information about gene regulation in eukaryotes, including humans. Much
remains to be discovered; even in E. coli, one of the most closely studied
organisms on earth, 35% of the genomic ORFs have no attributed function.

3. 台大農藝系 遺傳學601 20000Chapter 19 slide 3
The lacOperon of E. coli
Animation: Regulation of Expression of the lacOperon
Genes
1.An inducible operon responds to an inducer substance
(e.g., lactose). An inducer is a small molecule that joins
with a regulatory protein to control transcription of the
operon.
2.The regulatory event typically occurs at a specific DNA
sequence (controlling site) near the protein-coding
sequence (Figure 19.1).
3.Control of lactose metabolism in E. coliis an example of
an inducible operon.

4. 台大農藝系 遺傳學601 20000Chapter 19 slide 4
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.1 General organization of an inducible gene

5. 台大農藝系 遺傳學601 20000Chapter 19 slide 5
Lactose as a Carbon Source for E. coli
1.E. coliexpresses genes for glucose metabolism constitutively, but the genes for
metabolizing other sugars are regulated in a “sugar specific” sort of way. Presence of the
sugar stimulates synthesis of the proteins needed.
2.Lactose is a disaccharide (glucose 1 galactose). If lactose is E. coli’ssole carbon source,
three genes are expressed:
a.β-galactosidase has two functions:
i. Breaking lactose into glucose and galactose. Galactose is converted to glucose, and glucose
is metabolized by constitutively produced enzymes.
ii. Converting lactose to allolactose (an isomerization). Allolactose is involved in regulation
of the lacoperon (Figure 19.2).
b.Lactose permease (M protein) is required for transport of lactose across the cytoplasmic
membrane.
c.Transacetylase is poorly understood.
3.The lacoperon shows coordinate induction:
a.In glucose medium, E. colinormally has very low levels of the lacgene products.
b.When lactose is the sole carbon source, levels of the three enzymes increase coordinately
(simultaneously) about 1,000-fold.
i. Allolactose is the inducer molecule (Figure 16.2).
ii. The mRNA for the enzymes has a short half-life. When lactose is gone, lac transcription
stops, and enzyme levels drop rapidly.

6. 台大農藝系 遺傳學601 20000Chapter 19 slide 6
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.2 Reactions catalyzed by the enzyme -galactosidase

7. 台大農藝系 遺傳學601 20000Chapter 19 slide 7
Experimental Evidence for the Regulation of lac
Genes
1.The experiments of Jacob and Monod produced
an understanding of arrangement and control of
the lacgenes.

8. 台大農藝系 遺傳學601 20000Chapter 19 slide 8
Mutations in the Protein-coding Genes
1.Mutagens produced mutations in the lac structural genes that were used to
map their locations.
a. β-galactosidase is lacZ.
b. Permease is lacY.
c. Transacetylase is lacA.
d. The genes are tightly linked in the order: lacZ-lacY-lacA
2. The type of mutation made a difference in expression of the downstream genes:
a. Missense mutations affect only the product of the gene with the mutation.
b. Nonsense mutations show polarity (polar mutations), and affect translation of the
downstream genes as well.
3. The interpretation of gene polarity is that ribosomes translate the first gene in the
polycistronic (polygenic) RNA, and finish in proper position to initiate and
translate the next gene. Premature translation termination prevents this by
reducing translation of the downstream genes (Figure 19.3).

9. 台大農藝系 遺傳學601 20000Chapter 19 slide 9
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.3 Translation of the polygenic mRNA encoded by lacutilization genes

10. 台大農藝系 遺傳學601 20000Chapter 19 slide 10
Mutations Affecting the Regulation of Gene
Expression
1.Jacob and Monod found mutants that produced all three lacoperon proteins
constitutively, and hypothesized that regulatory mutations had affected the normal
control of gene expression for the operon. Constitutive mutants are in two classes
(Figure 19.4):
a.Mutations in the lacoperator (lacO) just upstream from the lacZgene.
b.Mutations further upstream in the lac repressor gene (lacI).
2.Operator-constitutive (lacO
c
) mutations were defined by experiments using partial
diploid E. coliF’ strains.
a.An example is the partial diploid F’ lacO
+
lacZ
-
lacY
+
/lacO
c
lacZ
+
lacY
-
. (Promoters are normal
for both operons, and the lacAgene is irrelevant to the study.)
b.This strain was tested for β-galactosidase and lactose permease, both in the presence and
absence of the inducer.
c.Without inducer, β-galactosidase is produced, but only inactive permease is made.
i. β-galactosidase is produced from the chromosomal gene under control of the constitutive
promoter.
ii. Permease is produced from the chromosomal gene also, but is inactive because the gene is
mutated.
iii. No products are produced from the F’ DNA, because its promoter is wild-type, and
requires induction for gene expression.

11. 台大農藝系 遺傳學601 20000Chapter 19 slide 11
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.4 Organization of the lacgenes of E. coliand the associated regulatory
elements: the lacoperon

12. 台大農藝系 遺傳學601 20000Chapter 19 slide 12
d.With inducer, functional molecules of both β-galactosidase and permease are
produced. This indicates that:
i. β-galactosidase is under constitutive control (on the chromosome).
ii. Permease is under inducible control (on the F’ DNA).
e.In both cases, the promoter controls genes downstream from it on the same DNA
molecule, showing cis-dominance.
3. lacIconstitutive genes were also discovered in experiments with partial diploid
strains.
a.An example is the strain lacI
+
lacO
+
lacZ
-
lacY
+
/lacI
-
lacO
+
lacZ
+
lacY
-
, in which
both gene sets have normal operators and promoters.
b.Without inducer, no β-galactosidase or permease is produced.
c.With inducer, both are produced. A lacI
+
gene can overcome the lacI
-
mutation.
d.Since lacI
+
and lacI
-
genes are on different DNA molecules, lacI
+
is trans-dominant.
e.Jacob and Monod proposed that lacI
+
produces a repressor that controls expression
of both lac operons, making them both inducible.
4. The lac promoter is also affected by mutations (P
lac-). Most affect
all three structural genes, which are not made, even when the inducer is present.
a.P
lac-mutations affect RNA polymerase binding to the start of the operon.
b.Only genes in the same DNA strand are affected, so P
lac-mutations are cis-ominant.
iActivity: Mutations and Lactose Metabolism

13. 台大農藝系 遺傳學601 20000Chapter 19 slide 13
Jacob and Monod’s Operon Model for the
Regulation of lac Genes
1.Jacob and Monod’s model of regulation, with more recent information,
follows:
a.An operon is a cluster of genes that are regulated together. The order of the lac
genes is shown in Figure 19.4, and Figure 19.5 shows the operon when lactose is
absent.
b.The lacIgene has its own constitutive weak promoter and terminator, and repressor
protein is always present in low concentration.
i. The repressor functions as a tetramer (Figure 19.6).
ii. Repressor protein binds the operator (lacO
+
), and prevents RNA polymerase
initiation to transcribe the operon genes (negative control).
iii. Binding of the repressor to the operator is not absolute, and so an occasional
transcript is made, resulting in low levels of the structural proteins.
c.β-galactosidase in wild-type E. coligrowing with lactose as the
sole carbon source converts lactose into allolactose (Figures 19.2 and 19.7).
i. Repressor bound with allolactose bound changes shape (allosteric shift) and
dissociates from the lac operator. Free repressor-allolactose complexes are
unable to bind the operator.
ii. Allolactose induces expression of the lacoperon, by removing the repressor
and allowing transcription to occur.

14. 台大農藝系 遺傳學601 20000Chapter 19 slide 14
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.5 Functional state of the lacoperon in wild-type E. coligrowing in the absence
of lactose

15. 台大農藝系 遺傳學601 20000Chapter 19 slide 15
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.6 Molecular model of the lacrepressor tetramer

16. 台大農藝系 遺傳學601 20000Chapter 19 slide 16
Fig. 19.7 Functional state of the lacoperon in wild-type E. coligrowing in the presence
of lactose as the sole carbon source

17. 台大農藝系 遺傳學601 20000Chapter 19 slide 17
2.The lacO
c
mutations result in constitutive gene expression. They are
cis-dominant to lacO
+
,because repressor cannot bind to the lacO
c
operator sequence (Figure 19.8).
3.The lacI
-
mutations change the repressor protein’s conformation and
prevent it from binding the operator, resulting in constitutive expression
of the operon (Figure 19.9).
a.In a partial diploid (lacI
+
lacO
+
lacZ
-
lacY
+
/lacI
-
lacO
+
lacZ
+
lacY
-
),
the wild-type repressor (lacI
+
) is dominant over lacI
-
mutants.
b.Defective lacI
-
repressor can’t bind either operator, but normal
repressor from lacI
+
binds both operators and regulates transcription,
resulting in functional β-galactosidase and permease.

18. 台大農藝系 遺傳學601 20000Chapter 19 slide 18
Fig. 19.8a Cis-dominant effect of lacOcmutation in a partial-diploid strain of E. coli

19. 台大農藝系 遺傳學601 20000Chapter 19 slide 19
Fig. 19.8b Cis-dominant effect of lacOcmutation in a partial-diploid strain of E. coli

20. 台大農藝系 遺傳學601 20000Chapter 19 slide 20
Fig. 19.9a Effects of a lacI-mutation

21. 台大農藝系 遺傳學601 20000Chapter 19 slide 21
Fig. 19.9b Effects of a lacI-mutation

22. 台大農藝系 遺傳學601 20000Chapter 19 slide 22
Fig. 19.9c Effects of a lacI-mutation

23. 台大農藝系 遺傳學601 20000Chapter 19 slide 23
4. Additional lacI mutants have been identified:
a. Superrepressor (lacI
S
) mutants that produce no lacenzymes.
i. The mutant repressor cannot bind allolactose, so lactose does not induce the operon.
ii. The lacI
S
allele is trans-dominant in partial diploids (lacI
+
/lacI
S
) (Figure 19.10). The superrepressor protein
binds both operators and transcription cannot occur.
iii. Normal repressor cannot compete, because superrepressor cannot be induced to fall off.
iv. Low levels of transcription will occur (superrepressor is not covalently bound to the DNA operator sequence),
but lacI
S
E. coli
cannot use lactose as a carbon source.
b. Dominance (lac
-d
) mutants have missense mutations at the 5’ end of the lacI gene.
i. In haploid cells, the phenotype is constitutive expression of the lac operon.
ii. In partial diploids, lac
-d
is trans-dominant. The operon is expressed even in the presence of normal repressor.
iii. Normal repressor functions as a tetramer. Mutant and normal subunits combine randomly.
iv. A tetramer containing one or more mutant subunits cannot bind to operator DNA. Repression does not occur,
and so gene expression is constitutive.
c. Mutations in the lad promoter can increase or decrease the gene¡¦s transcription rate, by altering its affinity for RNA
polymerase. Examples are lacI
Q
and lac
SQ
:
i. Both of these mutations raise the transcription rate of the repressor gene. (Q stands for “quantity” and SQ for
“super quantity”)
ii. Large amounts of repressor are produced in these mutants, reducing the efficiency of lac operon induction so
that high levels of lactose are needed.
lii. Overproduction of the lad gene has been useful for isolating and characterizing the repressor molecule.
5. These mutants indicate that repressor has three different recognition interactions:
a. Binding to the operator region.
b. Binding with the inducer (allolactose).
c. Binding of repressor polypeptide subunits to form an active tetramer.

24. 台大農藝系 遺傳學601 20000Chapter 19 slide 24
Fig. 19.10 Dominant effect oflacI
s
mutation over wild-type lacI
+

25. 台大農藝系 遺傳學601 20000Chapter 19 slide 25
Positive Control of the lacOperon
Animation: Positive Control of the lac Operon
1.Repressor exerts negative control by preventing transcription. Positive control of
this operon also occurs when lactose is E. coli’ssole carbon source (Figure
19.11).
a.Catabolite activator protein (CAP) binds cyclic AMP (cAMP) (Figure 19.12).
b.CAP-cAMP complex is a positive regulator of the lacoperon. It binds the CAP-site, a
DNA sequence upstream of the operon’s promoter.
c.Binding of CAP-cAMP complex causes the DNA to bend, facilitating protein-protein
interactions between CAP and RNA polymerase, and leading to transcription.
2.When both glucose and lactose are in the medium, E. colipreferentially uses
glucose, due to catabolite repression.
a.Glucose metabolism greatly reduces cAMP levels in the cell.
b.The CAP-cAMP level drops, and is insufficient to maintain high transcription of the lac
genes.
c.Even when allolactose has removed the repressor protein from the operator, lacgene
transcription is at very low levels without CAP-cAMP complex bound to the CAP-site.
d.Experimental evidence supports this model. Adding cAMP to cells restored
transcription of the lacoperon, even when glucose was present.

26. 台大農藝系 遺傳學601 20000Chapter 19 slide 26
Fig. 19.11 Role of cyclic AMP (cAMP) in the functioning of glucose-sensitive operons
such as the lacoperon of E. coli

27. 台大農藝系 遺傳學601 20000Chapter 19 slide 27
Fig. 19.12 Structure of cyclic AMP (cAMP)

28. 台大農藝系 遺傳學601 20000Chapter 19 slide 28
3. The model is that catabolite repression targets adenylate
cyclase (the enzyme that makes cAMP) (Figure 19.12).
a.In E. coli, the phosphorylated form of III
Glc
enzyme activates
adenylate cyclase.
b.Glucose transport into the cell triggers events including
dephosphorylation of III
Glc
.
c.With III
Glc
protein dephosphorylated, adenylate cyclase is
inactivated, and no new cAMP is produced.
4.Catabolic genes for other sugars are also regulated by
catabolite repression. In all cases, a CAP site in their
promoters is bound by a CAP-cAMP complex, increasing
RNA polymerase binding.

29. 台大農藝系 遺傳學601 20000Chapter 19 slide 29
Molecular Details of lac Operon Regulation
1. The sequences of significantlac regulatory regions are known. DNase
protection by regulatory molecules (e.g., repressor) is useful in these studies.
2. The lacI DNA sequence shows the expected transcription and translation
signals, except that the start codon is GUG (not AUG). The single base-pair
mutation of IacI
Q
is also characterized (Figure 19.13).
3. Operon controlling sites (Figure 19.14) were derived from several types of data:
a. Amino acid sequences of the repressor protein and (β-galactosidase were known,
allowing coding regions for lacI and lacZ
+
to be identified.
b. Protection assays identified binding sites for:
i. CAP-cAMP complex.
ii. RNA polymerase.
iii. Repressor protein.

30. 台大農藝系 遺傳學601 20000Chapter 19 slide 30
Fig. 19.13 Base pair sequences of thelacoperon lacI
+
gene promoter (P
lac+) and of the
5' end of the repressor mRNA

31. 台大農藝系 遺傳學601 20000Chapter 19 slide 31
Fig. 19.14 Base pair sequence of the controlling sites, promoter and operator, for the
lactose operon of E. coli

32. 台大農藝系 遺傳學601 20000Chapter 19 slide 32
4. The lac operon promoter region begins at -84, immediately next to the lacI gene
stop codon, and ends at -8, just upstream from the transcription start site.
Features of the promoter region include:
a. The consensus sequence for CAP-cAMP binding is in two regions: -54 to -58, and -
65 to -69.
b. The RNA polymerase binding site (including a Pribnow box) spans DNA from -47
to -8, with consensus sequence matches at -10 and -35.
5. The operator is immediately next to the promoter, with repressor protein
protecting DNA from -3 to +21. With repressor bound to the operator, RNA
polymerase can bind but cannot transcribe.
6. The operon transcript begins at +1, which is within the operator region bound by
repressor. (Part of the operator is transcribed.)
a. The β-ga1actosidase gene has a leader region before the start codon.
b. Start codon for 3-galactosidase (AUG) is at + 39 to +41.
c. Several lacO
C
mutations have been characterized. All are single base pair
substitutions.
7. The lac operon was the first molecular model for gene regulation. Operons are
common in bacteria and phages, but unknown in eukaryotes.

33. 台大農藝系 遺傳學601 20000Chapter 19 slide 33
The trpOperon of E. coli
1.If amino acids are available in the medium, E.
coliwill import them rather than make them, and
the genes for amino acid biosynthesis are
repressed. When amino acids are absent, the
genes are expressed and biosynthesis occurs.
2.Unlike the inducible lacoperon, the trpoperon is
repressible. Generally, anabolic pathways are
repressed when the end product is available.

34. 台大農藝系 遺傳學601 20000Chapter 19 slide 34
Gene Organization of the Tryptophan
Biosynthesis Genes
1.Yanofsky and colleagues characterized the controlling
sites and genes of the trp operon (Figure 19.15).
a.There are 5 structural genes, trpAthrough E.
b.The promoter and operator are upstream from the trpEgene.
c.Between trpEand the promoter-operator is trpL, the leader
region. Within trpLis the attenuator region (att).
d.The trp operon spans about 7 kb. The operon produces a
polygenic transcript with five structural genes for tryptophan
biosynthesis.

35. 台大農藝系 遺傳學601 20000Chapter 19 slide 35
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.15 Organization of controlling sites and the structural genes of the E. coli trp
operon

36. 台大農藝系 遺傳學601 20000Chapter 19 slide 36
Regulation of the trpOperon
Animation: Attenuation in the trp Operon of E. coli
1.Two mechanisms regulate expression of the trpoperon:
a.Repressor/operator interaction.
b.Transcription termination.
2.When tryptophan is present, it will bind to an aporepressor protein (the trpR
gene product).
a.The active repressor (aporepressor plus tryptophan) binds the trpoperator, and
prevents transcription initiation.
b.Repression reduces transcription of the trp operon about 70-fold.
3.When tryptophan is limited, transcription is also controlled by
attenuation.
a.Attenuation produces only short (140-bp) transcripts that do not encode structural
proteins.
b.Termination occurs at the attenuator site within the trpL region.
c.The proportion of attenuated transcripts to full-length ones is related to tryptophan
levels, with more attenuated transcripts as the tryptophan concentration increases.
d.Attenuation can reduce trp operon transcription 8-to 10-fold. Together, repression
and attenuation regulate trp gene expression over a 560-to 700-fold range.

37. 台大農藝系 遺傳學601 20000Chapter 19 slide 37
4.The molecular model for attenuation:
a. Translation of the trpL gene produces a short polypeptide. Near
the stop codon are two tryptophan codons.
b.Within the leader mRNA are four regions that can form
secondary structures by complementary base-pairing (Figure
19.16).
i. Pairing of sequences 1 and 2 creates a transcription pause
signal.
ii. Pairing of sequences 3 and 4 is a transcription termination
signal.
iii. Pairing of 2 and 3 is an antitermination signal, and so
transcription will continue.
c.Tight coupling of transcription and translation in prokaryotes
makes control by attenuation possible.
i. RNA polymerase pauses when regions 1 and 2 base pair just
after they are synthesized (Figure 19.16).

38. 台大農藝系 遺傳學601 20000Chapter 19 slide 38
ii. During the pause, a ribosome loads onto the mRNA and begins
translation of the leader peptide. Ribosome position is key to
attenuation:
(1)When tryptophan (Trp) is scarce(Figure 19.17):
(a)Trp-tRNAs are unavailable, and the ribosome stalls at the Trp codons in the leader
sequence, covering attenuator region 1.
(b)When the ribosome is stalled in attenuator region 1, it cannot base pair with region
2. Instead, region 2 pairs with region 3 when it is synthesized.
(c)If region 3 is paired with region 2, it is unable to pair with region 4 when it is
synthesized. Without the region 3-4 terminator, transcription continues through the
structural genes.
(2)When Trp is abundant:
(a)The ribosome continues translating the leader peptide, ending in region 2. This
prevents region 2 from pairing with region 3, leaving 3 available to pair with
region 4.
(b)Pairing of regions 3 and 4 creates a rho-independent terminator known as the
attenuator. Transcription ends before the structural genes are reached.
5. Genetic evidence for attenuation includes:
a.Mutations in the leader sequence within regions 3 or 4 disrupt base
pairing and decrease the efficiency of termination (Figure 19.18)
b.Changes in the Trp codons of the leader peptide sequence, so that they
encode a different amino acid, cause attenuation controlled by levels
of the amino acid specified by the mutant codon, not by Trp

39. 台大農藝系 遺傳學601 20000Chapter 19 slide 39
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.16 Four regions of the trpoperon leader mRNA and the alternate secondary
structures they can form by complementary base-pairing

40. 台大農藝系 遺傳學601 20000Chapter 19 slide 40
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.17a Models for attenuation in the trpoperon of E.coli

41. 台大農藝系 遺傳學601 20000Chapter 19 slide 41
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.17b Models for attenuation in the trpoperon of E.coli

42. 台大農藝系 遺傳學601 20000Chapter 19 slide 42
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.18 In the trpLregion, mutation sites that show less efficient transcription at
the attenuator site

43. 台大農藝系 遺傳學601 20000Chapter 19 slide 43
Regulation of Other Amino Acid Biosynthesis
Operons
1.Attenuation is involved in regulating many
operons(Figure 19.19).
a.When the operon is for amino acid biosynthesis,
the leader sequence always includes codons for
that amino acid.
b.Other operons regulated by attenuation include
rRNA (rrn) and E. coliampicillin resistance
(ampC).

44. 台大農藝系 遺傳學601 20000Chapter 19 slide 44
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.19 Predicted amino acid sequences of the leader peptides of a number of
attenuator-controlled bacterial operons

45. 台大農藝系 遺傳學601 20000Chapter 19 slide 45
Regulation of Gene Expression in Phage Lambda
1.Phage use many bacterial components for
replication, and control their use with phage gene
products.
2.Bacteriophage λhas two possible pathways when it
enters its E. colihost:
a.The lytic cycle, in which the phage takes over the cell and
produces progeny phage.
b.The lysogenic cycle, where phage chromosome is inserted
into the E. colichromosome, and replicates with the
bacterial genome.

46. 台大農藝系 遺傳學601 20000Chapter 19 slide 46
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.20 A map of phage , showing the major genes

47. 台大農藝系 遺傳學601 20000Chapter 19 slide 47
Early Transcription Events
1.The λchromosome is linear, with “sticky” ends used to circularize
it in the host cell. The regulatory system for choosing between
the lytic and lysogenic pathways is contained in the λ
chromosome (Figure 19.20).
a.First, transcription begins at promoters P
L(leftward transcription)
and P
R(rightward) (Figure 19.21).
i. The first gene transcribed from P
Ris cro(control of repressor
and other). The Cro protein is involved in the genetic switch
to the lytic pathway.
ii. The first protein transcribed from P
Lis N, which is a
transcription antiterminator that allows RNA synthesis to go
through termination regions into the early genes.
(1)Nprotein allows expression of the cIIprotein, which in
turn activates:
(a)cI(λrepressor)
(b)Oand P(DNA replication proteins).
(c)Q(activation of late genes for lysis and phage particle proteins, only when Q
protein accumulates to certain levels).

48. 台大農藝系 遺傳學601 20000Chapter 19 slide 48
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.21 Expression of genes after infecting E. coli and the transcriptional events
that occur when either the lysogenic or lytic pathways are followed

49. 台大農藝系 遺傳學601 20000Chapter 19 slide 49
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Fig. 19.21 Expression of genes after infecting E. coli and the transcriptional events
that occur when either the lysogenic or lytic pathways are followed (cont.)

50. 台大農藝系 遺傳學601 20000Chapter 19 slide 50
The Lysogenic Pathway
1.Early transcription events determine whether the lytic or lysogenic pathway
occurs.
2.The lysogenic pathway results when cIIand cIIIare expressed.
a.The action of cIIand cIIIproteins activates the P
REpromoter, causing transcription
of the cI(λrepressor) gene.
b.λrepressor binds to 2 operator regions, O
Land O
R, whose sequences overlap P
Land
P
R, respectively. This prevents transcription from P
Land P
R.
c.Transcription of Nand crogenes is blocked, and concentrations of their proteins
drop rapidly.
d.In addition, repressor bound to O
Rcauses more repressor mRNA to be made from
another promoter, P
RM.
e.High levels of repressor cause lysogeny by binding operators O
Land O
R.
f.Integrase protein is used to integrate λDNA into the E. colichromosome. Integrase
transcript is initiated at the P
Ipromoter, which is controlled by the cIIprotein.
g.As cIIconcentration drops, the P
Ipromoter is shut down, leaving P
RMas the only
active promoter.
3.Thus, the lysogenic pathway occurs when enough λrepressor is
made to turn off early promoters. Lytic genes, including Q, are not expressed.
Without the Qprotein, phage coat and lysis proteins are not produced.

51. 台大農藝系 遺傳學601 20000Chapter 19 slide 51
The Lytic Pathway
1.An example of induction of the lytic pathway is exposure to UV light.
a.UV causes the bacterial RecAprotein (normally used in DNA repair) to
stimulate the λrepressor proteins to autocleave and become
inactivated.
b.Absence of repressor at O
Rallows transcription of the crogene.
c.Cro protein decreases RNA synthesis from P
Land P
R, reducing
synthesis of cII, therefore blocking synthesis of λrepressor.
d.Transcription from P
Ris also decreased, but Qprotein levels are
sufficient to allow transcription of the late genes needed for the lytic
pathway.
2.Thus, λuses complex regulatory systems to control entry into the lytic
or lysogenic pathway. The decision depends on competition between
the repressor and the Cro protein.
a.If repressor dominates, lysogeny takes place.
b.If the Cro protein dominates, the lytic pathway occurs.