Recombination,
Bacteriophages, and
Horizontal Gene Transfer
2005
Bacterial Conjugation
• transfer of DNA
by direct cell to
cell contact
• discovered 1946
by Lederberg and
Tatum
F+ x F– Mating
• F+ = donor
– contains F factor
• F– = recipient
– does not contain F factor
• F factor replicated by rolling-circle
mechanism and duplicate is transferred
• recipients usually become F+
• donor remains F+
F factor
• The F factor can exist in three different states:
• F+ refers to a factor in an autonomous,
extrachromosomal state containing only the genetic
information described above.
• The "Hfr" (which refers to "high frequency
recombination") state describes the situation when
the factor has integrated itself into the chromosome
presumably due to its various insertion sequences.
• The F' or (F prime) state refers to the factor when it
exists as an extrachromosomal element, but with the
additional requirement that it contain some section of
chromosomal DNA covalently attached to it. A strain
containing no F factor is said to be "F-".
Gene transfer and
recombination
• Genes are transferred in a linear
manner
• The F factor integrates into
chromosomes at different points and its
position determines the O site
F x F– mating
Hfr Conjugation
• Hfr strain
– donor having F factor integrated into its
chromosome
• both plasmid genes and chromosomal
genes are transferred
Hfr
• Special class of F+ strains
• This was discovered because this strain
underwent recombination 1000x more
frequently than F+ strains
• In certain Hfr strains certain stains are
more likely to recombine than others.
• The nonrandom pattern of gene
transfer was shown to vary from Hfr
strain to Hfr strain
Interrupted mating
• Wollman explained the cells that are
different between F+ and Hfr. To
facilitate the recovery, the Hfr was
sensitive to antibiotics and the F+
wasn’t.
• The cells were separated at intervals of
5 minutes is the F factor
Mating
Hfr x F– mating
Figure 13.14b
Recombination
• Exogenote
• Exogenote
Mating
• The two strains were mixed
• There were incubated.
• At intervals of 5 minutes, samples were taken
of the F- cells
• The cells were centrifuged so that they would
know which genes were transferred.
• The distance between genes was measured by
the time that it took for the genes to be
transferred.
• During the first five minutes, the strains
were mixed there was no recombination
F+ x F– mating
• In its extrachromosomal
state the factor has a
molecular weight of
approximately 62 kb and
encodes at least 20 tra
genes. It also contains
three copies of IS3, one
copy of IS2, and one
copy of a À sequence as
well as genes for
incompatibility and
replication.
F’
• In 1959 during his experiments with the Hfr
strains of E. coli Adelberg discovered that
the F factor could lose its integrated status
and revert to its F+ status.
• When this occurred, the F factor carries
along several adjacent bacterial genes.
• When you have the F factor + bacterial genes
– the condition is known as the F’
F Conjugation
• F plasmid
integrated F factor
– formed by
incorrect
excision from
chromosome
– contains  1
genes from
chromosome
chromosomal gene
• F cell can
transfer F
plasmid to
recipient
Figure 13.15a
Merozygotes
• When the F’ is then transferred to another
bacterium
• The bacterium may contain genomic copies of
a gene as well as an additional copy of the
gene in the F’.
• As a result the situation is a partial diploid
• Merozygotes have been extremely beneficial
in the study of gene regulation
Interrupted mating
Figure 13.22a
Figure 13.22b
Hfr mapping
• used to map relative location of bacterial
genes
• based on observation that chromosome
transfer occurs at constant rate
• interrupted mating experiment
– Hfr x F- mating interrupted at various intervals
– order and timing of gene transfer determined
Gene mapping
Recombinants
• Map distance can be determined by
replating the resulting colonies on agar
• For example
leu+ exconjugants by plating them on
medium containing no leucine but
containing methionine and arginine
Mapping results
Map distance
• The map distance is equal to the %
recombination
Tra Y
• Characterization of the Escherichia
coli F factor traY gene product and
its binding sites
• WC Nelson, BS Morton, EE Lahue and
SW Matson
Department of Biology, University of
North Carolina, Chapel Hill 27599.
Tra Genes
• Tra Y gene codes for the protein binds
to the Ori T
• Initiates the transfer of plasmid across
the bridge between the two cells
• Tra I Gene is a helicase responsible for
the conjugation
• strand-specific transesterification
(relaxase)
Conjugative Proteins
• Key players are the proteins that
initiate the physical transfer of ssDNA,
the conjugative initiator proteins
• They nick the DNA and open it to begin
the transfer
• Working in conjunction with the
helicases they facilitate the transfer of
ss RNA to the F- cell
DNA Transformation
• Uptake of naked DNA molecule from
the environment and incorporation into
recipient in a heritable form
• Competent cell
– capable of taking up DNA
• May be important route of genetic
exchange in nature
Streptococcus pneumoniae
DNA binding
protein
competence-specific
protein
nuclease – nicks and degrades one
strand
Artificial transformation
• Transformation done in laboratory with
species that are not normally competent
(E. coli)
• Variety of techniques used to make cells
temporarily competent
– calcium chloride treatment
• makes cells more permeable to DNA
Cloning vectors
pAmp
Transformation mapping
• used to establish gene linkage
• expressed as frequency of
cotransformation
• if two genes close together, greater
likelihood will be transferred on single
DNA fragment
Microbial Genetics
Bacteriophages
Diversification of Escherichia coli genomes: are
bacteriophages the major contributors? Makoto
Ohnishi – Trends in Microbiology
• E. coli is a diverse species
• 4.5 – 5.5 MB
• E. coli strains are commensals of higher
vertebrates, but some are pathogenic
• There are 5subtypes of the diarrheagneic
strainsd
• The pathogenicity of the strains has been
traced to a subtype that retains a large
segment of virulence factors or pathogenicity
islands
E. Coli O 157
• Sixteen sections of this pathogenic strain
differ from the lab strain
• These are subtype specific
• Within sections of the DNA and these large
segments
• The G-C content varies from the lab strain
• The 4.1 kb common backbone sequence mainly
represent the DNA that RE. coli possesses
from a common ancestor.
E. coli
• There are 98 copies of IS elements
within this section as well as genes
enxoding hemolysins, proteases, and
other virulence factors.
• More interesting O 157 also contains 18
remnants of prophages
Horizontal gene transfer
• Clearly this plays a central role in the
diversity of E. coli
• Among the 18 prophage remnants on
O157 – 12 resemble lambda pahge
• They all contain a variety of deletions
and or insertions
• Some of the phages are so similar that
they contain a 20 kb segment tat is
identical.
Recombinant phages
• It is believed that the phages have
undergone recombination and
diversification
• Recombination could occur with in a
single cell
• It could occur as the result of
recombination
Virulence and Strptococcus
pyogenes
• Streptococcal pyrogenic exotoxins(SPE)
contribute to the diverse symptoms of a
streptococcal infection.
• These antigens compare to Staphylococcal
antigens of the same type.
• The A + C genes coding for these toxins were
horizontally transferred from strain to strain
by a lysogenic bacteriophage.
• In addition the genes contributed by the
phages produce hyaluronidase, mitogenic
factor, and leukocyte( WBC) toxins
Streptococcus pyogenes
• There are 15 prophages that have been
identified in E. coli
• These prophages belong to the group
Siphoridae
• All but one of these produce a toxin
• In both strep and staph – the prophage
is found at the site of recombination
Bacteriophages
Bacteriophages
•
•
•
•
Bacterial viruses
Obligate intracellular parasites
Inject themselves into a host bacterial cell
Take over the host machinery and utilize it for
protein synthesis and replication
T- 4 Bacteriophage
• Ds DNA virus
• 168, 800 base pairs
• Phage life cycles studied by Luria and
Delbruck
Bacteriophage structure
Bacteriophage structure(con)
• Most bacteriophages have tails
• The size of the tail varies.
• It is a tube through which the nucleic acid is
injected as a result of attachment of the
bacteriophage to the host bacterium
• In the more complex phages the tail is
surrounded by a contractile sheath for
injection of the nucleic acids
Bacteriophage structure
• Many bacteriophages have a base plate
and tail fibers
• Some have icosahedral capsids
• M13 has a helical capsid
Bacteriophage structure(con)
• Most bacteriophages have tails
• The size of the tail varies.
• It is a tube through which the nucleic acid is
injected as a result of attachment of the
bacteriophage to the host bacterium
• In the more complex phages the tail is
surrounded by a contractile sheath for
injection of the nucleic acids
Bacteriophage structure
• Many bacteriophages have a base plate
and tail fibers
• Some have icosahedral capsids
• M13 has a helical capsid
PhiX 174
• The spherical phage (PhiX174, G4, S13) are
broadly similar to the filamentous phage.
• The capsid is icosahedral not helical and is not
enveloped (these phage lyse the host cell).
• Their genome consists of a circular ssDNA
molecule. A well-known examples is PhiX174,
which was the first genome to be sequenced by Fred Sanger's group in 1976.
• Its genome of 5386 bp coded for 11 genes,
including several examples of overlapping
genes.
PhiX174 – economy and
overlapping genes
• The coding frames for 7
proteins overlap: A* is a
truncated form of A;
• B is coded within A in a
different reading frame;
• K is encoded in a third
reading frame at the end of
A which extends into and
overlaps with that of C; E is
coded within D in a different
reading frame. These were
the first examples of
overlapping genes.
• Other relatives of PhiX174
are G4 and S13.
PhiX174
• Gene A – RF replication: viral strand
synthesis
• A* Turning off host DNA synthesis
• B Formation of capsid
• E Lysis of bacterium
• F major coat protein
• G Major spike protein
Genetics
• Conversion of a parental single stranded
DNA molecule to the viral + strand to a
covalently closed double stranded
molecule
• This is called the Replicative form( RFI)
• Synthesis of many copies of RFI. The –
strand is transcribed. – Gene A product
is made and the process continuew
Synthesis of + strands for
encapsidation
• There is not switch – It just occurs
• During the period that the phage
capsids( heads ) are being synthesized
Ss RNA viruses
• ssRNA phages
• Tailess icosahedral
• Single stranded linear molecule, having a
great deal of intramolecular hydrogen
bonding
• Consists of 3600 nucleotides
• Genes for attachment, coat protein and
an RNA polymerase
Replication
• The RNA molecule serves a both a
replication template and the mRNA
Filamentous phages
•
•
•
•
Fd
Filamentous
Circular ss DNA
Lies in the middle of
the filment
• Infects through the
pilus
• Create a symbiotic
relationship with the
host
M 13
•
•
•
•
•
•
•
•
•
•
•
•
•
Sequential steps
- M13
Cloning vector – Joachim Messier
phage particles bind to F pilus
– only infects F+, Hfr, F' cells
single-stranded DNA genome enters cell
designated as “+” strand
“+” strand repaired
– double-stranded replicative form (RF)
RF contains “+” and “–” strands
“–” strand is template – for mRNA synthesis
– for production of new “+” strands
– by rolling circle replication
“+” strands are packaged in phage coat protein
– exit cell as phage particle
Important points for cloning vectors
M13 and cloning
• M13 occurs in both single and double stranded
forms
• RF can be digested with restriction
endonucleases
• inserts can be cloned in – like plasmid
• “+” strands from phage particles
• – convenient source of single-stranded DNA
•
M13
• – used for sequencing and site-directed
mutagenesis
• different sized DNA molecules packaged as
phage particle
• – (within reason)
• – phage with inserts > 2 kb replicated slower
• different sized DNA molecules
• – produce different size phage particles
M13 Phage
General Steps
T even phages
Luria and Delbruck
• Four distinct periods in the release of phages from
host cells
• Latent period- follows the addition of phage( no
release of virions)
• Eclipse period – virions were detectable before
infection and are now hidden or eclipsed
• Rise or burst period – Host cells rapidly burst and
release viruses
• The total number of phages released can be
determined by the burst size – the number of viruses
produced per
infected cell
Steps in the life cycle
• Adsorption of the virus to the host
• This is mediated by tail fibers or some analagous
structure
• When the tail fibers make contact, the base plate
settles to the surface
• This connection which is maintianed by electrostatic
attraction and the ions Mg++ and Ca++
Attachment
• There is host specificity in the attachment
and adsorption of the bacteriophage
• There are receptors for the attachment.
They vary from bacteria to bacteria
• The receptors are on the bacteria for other
purposes: the bacteriophages evolved to
utilize them for their invasion
T even phages
• The phage sheath shortens from 24
rings to 12 rings
• The sheath becomes shorter and wider
• This causes the central tube to push
through the bacterial cell wall
Gp5
• The baseplate
contains the protein
gp5 with lysozyme
activity which made
aid in the
penetration of the
host
Penetration and other Phages
• Penetration by other phages may differ
• PRD1 phage attaches to a surface
receptor by a spike on one of its capsid
vertices
Conformational Changes and
PRD1
• As a result of the binding a tubular
structure is formed that allows the
virus to penetrate the
• Penetration of the membrane tube is
made by the membrane enzyme P7
• These phages have a major effect on
the bacteriaceae
Early Genes
• E. coli RNA polymerase starts
transcribing genes( phage genes) within
minutes of entering the bacterial cell
• The early m RNA direct the synthesis
of proteins and enzymes that are
needed for hostile tack over
• Some early virus specific enzymes
degrade host DNA to nucleotides wo
that virus DNA synthesis can commence
Hydroxymethylcytosine
• HMC is needed for synthesis instead of
cytosine
• HMC must be glucosylated by the
addition of glucose to protect from
restriction enzymes
T4 and terminal redundancy
• The end has terminal redundancy
• When multiple coies have been made
enzymes join the copies by therse ends
• When several untis are linked together
this forms concatamers
Late mRNA
• Phage structural structural proteins
• Proteins that help with pahge assembly
• Proteins involved in cell lysis and release
Capsid
• The base plate requires 12 protein
products
• The head or capsid requires 10 genes
• The capside requires scaffolding
proteins for assembly
• DNA packaging a mysterious process
• Many phages lyse their host cells at the
end of the intracellular phase
Release
• Interference with the synthesis of the
bacterial cell wall
• PhiX 174 produces a lytic enzyme that
interfers with the urein precursos
Irreversible attachment
• The attachment of the tail ribers to the bacterium is
a weak attachment
• The attachment of the bacterophage is also
accompanied by a stronger interaction usually by the
base plate
Sheath contraction
• The irreversible binding results in the
sheath contraction
Injection
• When the irreversible attachment has
been made and the sheath contracts,
the nucleic acid passes through the tail
and enters the cytoplasm
Phage Multiplication Cycle –
Lytic phages
• Lytic phages or virulent phages enter the bacterial
cell, complete protein synthesis, nucleic acid
replication, and then cause lysis of the bacterial cell
when the assembly of the particles has been
completed.
Eclipse Period
• The bacteriophages may be seen inside or outside of
the bacterial cells
• The phages take over the cell’s machinery and phage
specific mRNA’s are made
• Early mRNA’s are generally needed for DNA
replication
• Later mRNA’s are required for the synthesis of phage
proteins
Intracellular accumulation phase
• The bacteriophage sub units accumulate
in the cytoplasm of the bacterial cell
and are assembled
Lysis or Release Phase
• A lysis protein is released
• The bacterial cell breaks open
• The viruses escape to invade other
bacterial cells
Plaque assay
• Phage infection and lysis can easily be
detected in bacterial cultures grown on agar
plates
• Typically bacterial cells are cultured in high
concentrations on the surface of an agar
plate
• This produces a “ bacterial lawn”
• Phage infection and lysis can be seen as a
clear area on the plate. As phage are
released they invade neighboring cells and
produce a clear area
Plaque assay
Lambda and Plaques
• The plaque produced by Lambda had a
different appearance on the Petri Dish.
• It is considered to be turbid rather
than clear
• The turbidiy is the result of the growth
of phage immune lysogens in the plaque
• The agar surface contains a ratio of
about a phage /107 bacteria
MOI
• Average number of phages /bacterium
• After several lytic cycles the MOI gets
higher due to the release of phage
particles
Transduction
• Transfer of bacterial genes by viruses
• Virulent bacteriophages
– reproduce using lytic life cycle
• Temperate bacteriophages
– reproduce using lysogenic life cycle
Generalized transduction
• http://www.cat.cc.md.us/courses/bio141
/lecguide/unit4/genetics/recombination
/transduction/gentran.html
• http://www.cat.cc.md.us/courses/bio141
/lecguide/unit1/control/genrec/u4fg21a
.html
Generalized transduction
• E. coli phage P21 or P22.
• As a part of the lytic cycle, the phage cuts the bacterial DNA
into fragments
• This fragmentation prevents the expression of bacterial genes
• Nucleotides can be used to make phage DNA
• Occasionally these DNA fragments are about the same size as
phage DNA
• They become mistakenly packaged into phage capsids in place of
phage DNA
Types of Lysogenic Cycle
• The most common type is the classic model of the
Lambda phage
• The DNA molecule is injected into a bacterium
• In a short period of time, after a brief period of
transcription, an integration factor and a repressor
are synthesized
• A phage DNA molecule typically a replica of the
injected molecules is inserted into the DNA
• As the bacterium continue to grow and multiply and
the phage genes replicate as part of the bacterial
chromosome
The P1 temperate phage
• There is not integration into the host
• The phage becomes a plasmid
• It exists as an independently replicating
entity in the bacterial cell in the same
way a plasmid exists.
Temperate
• A bacteriophage that can exist as a
lytic or lysogenic phage is referred to
as a temperate phage
• A bacterium containing a full set of
phage genes is a lysogen
• The process of infecting a bacterial
culture with a temperate phage is called
lysogenization
Immunization
• A bacterial cell or lysogen cannot be
reinfected by a phage of the same type
• This is resistance to superinfection is
called immunity
• More than 90% of the bacteriophages
are temperate
• These are unable to produce bursts
such as T4 and T7
Lysogenic Phage
Lambda Phage
• Temperate phage
• Alternate life cycle
• Ds DNA – linear then circularizes when it enters the
host
• 48,502 base pairs
• Molecular biology workhorse – because of its life
cycle
Genes
Lambda genes
• 46 genes have been identified
• 14 are non esswential to the lytic cycle
• Only 7 are nonessential to both the lytic
and lysogenic cycles
Lambda Gene Map
•
•
•
•
•
•
•
Genes are clustered according to function
There are four clusters
Head
Tail
Replication
Recombination genes
Regulatory genes act at specific site on the
DNA
Restriction sites
Life cycle of λ Phage
Latency
• Lysogenic conversion can lead to
virulence
• Botulism, cholera,and diptheria toxins
are encoded by prophages that convert
their host into a pathogenic bacterium
Control of lysogeny and lytic
cycle
• Genes needed to
establish lysogeny
• cI yes
• cII yes
• cIII yes
• Genes needed for
maintenance of
lysogeny
• cI yes
• cII no
• cIII no
Lambda
• In order for the lambda prophage to
exist in a host E. coli cell, it must
integrate into the host chromosome
which it does by means of a sitespecific recombination reaction.
Preferred site of integration
• It is inserted into the E. coli
chromosome between the gal operon and
the biotin operon.
• The site of attachment is specific just
for the Lambda phage ( att)
Lambda Phage Genes
•
•
–
–
–
–
repression of all lytic functions
cI: the lambda repressor, when present and active, will
repress lytic functions. The counterpart of cI is cro, a
repressor of cI. The initial "decision" that lambda makes is
based on the outcome of a battle over cI synthesis
lambda DNA is injected and circularized
initially, cII is made
cII is a positive regulator of cI synthesis. If cII is around long
enough, then cI will be made, and cI will repress cro and other
lytic functions
if cII is degraded quickly, cro will build up, and cI will not be
synthesized
Lambda Phage Genes
•
whether cII is degraded or not depends on the health of the host. A
healthy cell will degrade cII quickly, and in effect signal to lambda that
a lytic cycle would be good. An unhealthy cell will not degrade cII,
which is like telling lambda that the cell is too sick to make viral
progeny, so lysogeny is the better idea
Terminology
• LEGEND
• att: an E.coli seqence for the "attachment" or integration of
lambda's circular chromosome.
• oriC: E.coli's origin of Chromosome replication (given here for
orientation only)
• gal: E.coli's gene for galactose utilization
• pe:prophage ends (site of integration)
• cos: joined sticky ends of vegetative DNA; sometimes called ve
("vegetative ends")
• int: gene for the enzyme integrase
• c: gene for lambda repressor to maintain lysogeny
• Q: another gene concerned with lysogeny
• h: the last of the many capsomer genes.
Bacteriophages
Specialized transduction
Attachment site
• The E. coli chromosome contains one site at which
lambda integrates. The site, located between the gal
and bio operons, is called the attachment site and is
designated attB since it is the attachment site on the
bacterial chromosome.
• The site is only 30 bp in size and contains a conserved
central 15 bp region where the recombination
reaction will take place.
• The structure of the recombination site was
determined originally by genetic analyses and is
usually represented as BOB', where B and B'
represent the bacterial DNA on either side of the
conserved central element
Recombination site
• The bacteriophage recombination site - attP is more complex. It contains the identical
central 15 bp region as attB.
• The overall structure can be represented as
POP'. However, the flanking sequences on
either side of attP are very important since
they contain the binding sites for a number of
other proteins which are required for the
recombination reaction. The P arm is 150 bp in
length and the P' arm is 90 bp in length.
Integration
• Integration of bacteriophage lambda requires
one phage-encoded protein - Int, which is the
integrase - and one bacterial protein - IHF,
which is Integration Host Factor.
• Both of these proteins bind to sites on the P
and P' arms of attP to form a complex in
which the central conserved 15 bp elements
of attP and attB are properly aligned.
• The integrase enzyme carries out all of the
steps of the recombination reaction, which
includes a short 7 bp branch migration.
Enzymes and Recombination
• There are two major groups of enzymes that
carry out site-specific recombination
reactions; one group - known as the tyrosine
recombinase family - consists of over 140
proteins.
• These proteins are 300-400 amino acids in
size, they contain two conserved structural
domains, and they carry out recombination
reactions using a common mechanism involving
a the formation of a covalent bond with an
active site tyrosine residue.
Enzymes and Recombination
• The strand exchange reaction involves
staggered cuts that are 6 to 8 bp apart
within the recognition sequence.
• All of the strand cleavage and re-joining
reactions proceed through a series of
transesterification reactions like those
mediated by type I topoisomerases.
Excision of bacteriophages
• Excision of bacteriophage lambda requires two phageencoded proteins:
• Int (again!) and Xis, which is an excisionase. It also
requires several bacterial proteins.
• In addition to IHF, a protein called Fis is required.
• All of these proteins bind to sites on the P and P'
arms of attL and attR forming a complex in which the
central conserved 15 bp elements of attL and attR
are properly aligned to promote excision of the
prophage.
Normal Excision
Excision and lysis
•
•
•
•
•
•
The reverse of integration happens upon induction
UV light is a good inducer
induction actually involves RecA. RecA is activated by ssDNA.
Activated RecA interacts with cI, and causes cI to proteolyze
itself. Without cI, lytic functions are derepressed, and the
lytic cycle begins.
induction also results in the synthesis of both Int and Xis
only int is required for integration, but both are required for
excision
normal excision (which usually occurs) produces the cicular
viral genome, and lysis continues
Generalized Transduction
• Any part of bacterial genome can be
transferred
• Occurs during lytic cycle
• During viral assembly, fragments of
host DNA mistakenly packaged into
phage head
– generalized transducing particle
Generalized transduction
Specialized Transduction
• also called restricted transduction
• carried out only by temperate phages
that have established lysogeny
• only specific portion of bacterial
genome is transferred
• occurs when prophage is incorrectly
excised
Specialized
transduction
Figure 13.20
Figure 13.20
Generalized Transduction
Mapping
• used to establish gene linkage
• expressed as frequency of
cotransduction
• if two genes close together, greater
likelihood will be carried on single DNA
fragment in transducing particle
Recombination and Genome
Mapping in Viruses
• viral genomes can also undergo recombination
events
• viral genomes can be mapped by determining
recombination frequencies
• physical maps of viral genomes can also be
constructed using other techniques
Specialized transduction
mapping
• provides distance of genes from viral
genome integration sites
• viral genome integration sites must first
be mapped by conjugation mapping
techniques
Recombination mapping
• recombination
frequency
determined
when cells
infected
simultaneously
with two
different
viruses
Figure 13.24
Physical maps
• heteroduplex maps
– genomes of two different viruses
denatured, mixed and allowed to anneal
• regions that are not identical, do not reanneal
– allows for localization of mutant alleles
Physical maps…
• restriction endonuclease mapping
– compare DNA fragments from two
different viral strains in terms of
electrophoretic mobility
• sequence mapping
– determine nucleotide sequence of viral
genome
– identify coding regions, mutations, etc.
Lambda Phage Genes
•
•
–
–
–
–
repression of all lytic functions
cI: the lambda repressor, when present and active, will
repress lytic functions. The counterpart of cI is cro, a
repressor of cI. The initial "decision" that lambda makes is
based on the outcome of a battle over cI synthesis
lambda DNA is injected and circularized
initially, cII is made
cII is a positive regulator of cI synthesis. If cII is around long
enough, then cI will be made, and cI will repress cro and other
lytic functions
if cII is degraded quickly, cro will build up, and cI will not be
synthesized
Lamda Phage Genes
•
whether cII is degraded or not depends on the health of the host. A
healthy cell will degrade cII quickly, and in effect signal to lambda that
a lytic cycle would be good. An unhealthy cell will not degrade cII,
which is like telling lambda that the cell is too sick to make viral
progeny, so lysogeny is the better idea