Download Bio-261-Chapter-13-Phages

Viruses of Bacteria
Bio 261 Microbiology
Medgar Evers College
Prof. Santos
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Recall that bacteria are prokaryotes
– With cells much smaller and more simply
organized than those of eukaryotes! can
someone give me some differences between
eukaryotes and prokaryotes!
• Viruses
Virus
– Are smaller and simpler still!
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of Viruses
• Viruses
– Are very small infectious particles consisting of
nucleic acid enclosed in a protein coat and, in
some cases, a membranous envelope.
– A virus has a genome but can reproduce only
within a host cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Viral Genomes
• Viral genomes may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Capsids and Envelopes
• A capsid
– Is the protein shell that encloses the viral genome
– Can have various structures
Capsomere
of capsid
RNA
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18  250 mm
20 nm
50 nm
(a) Tobacco mosaic virus (b) Adenoviruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Capsids are formed from identical protein
subunits called capsomeres.
• Capsid + nucleic acid = nucleocapsid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Some viruses have envelopes
– Which are membranous coverings derived
from the membrane of the host cell
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
50 nm
(c) Influenza viruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Spike proteins present on the capsid allow for
attachment to host.
• In the enveloped viruses, just inside the lipid
envelope there is often matrix proteins, which
are found only on enveloped viruses.
• The attachment spikes project from the
envelope.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A naked virus is one that does not have an
envelope. Virtually all phages are naked.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Bacteriophages, also called phages
– Have the most complex capsids found among
viruses
Head
Tail
sheath
DNA
Tail
fiber
80  225 nm
50 nm
(d) Bacteriophage T4
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure
• In general, there are three main morphological
virus types:
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1- helical
• These viruses are composed of a single type of capsomere
stacked around a central axis to form a helical structure,
which may have a central cavity, or hollow tube.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 2-Icosahedral
• An icosahedron is a convex polyhedron with
twenty faces
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 3- Complex- usually consists of a head piece
and a sheath or tail.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
General Features of Viral Reproductive Cycles
• Viruses are obligate intracellular parasites
– They can reproduce only within a host cell
• Each virus has a host range
– A limited number of host cells that it can infect
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Viruses use enzymes, ribosomes, and small
molecules of host cells
– To synthesize progeny viruses
Entry into cell and
uncoating of DNA
DNA
Capsid
VIRUS
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new
virus particles and
their exit from cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Reproductive Cycles of Phages
• Phages
– Are the best understood of all viruses
– Go through two alternative reproductive
mechanisms: the lytic cycle and the lysogenic
cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Lytic Cycle
• The lytic cycle (productive) or virulent
– Is a phage reproductive cycle that culminates
in the death of the host
– Produces new phages and digests the host’s
cell wall, releasing the progeny viruses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Lysogenic Cycle
• The lysogenic cycle
– Replicates the phage genome without
destroying the host
• Temperate phages
– Are capable of using both the lytic and
lysogenic cycles of reproduction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Temperate phages "live in harmony" with their
host bacteria in lysogeny.
When the phage enters the host cell
Some of the bacteria will be lysogenized
(lysogenic cycle)
Others will be lysed (lytic pathway).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lytic/lysogenic cycle of a ds DNA phage
• The lytic and lysogenic cycles of phage , a
temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle
is induced
or
New phage DNA and
proteins are synthesized
and assembled into phages.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lysogenic cycle
is entered
Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
What decides whether lytic or lysogeny take place?
factors!
1- amount of nutrients
low nutrients favors lysogeny, If host cell
doesn’t have enough resources to become a
“phage factory”, wait for better times.
2-If host cell is damaged, better to enter lytic
mode before the cell dies.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Review of lytic and lysogeny
• http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapt
er17/animation_quiz_2.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lytic cycle of a ds DNA virus
• The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Head
Tails
Tail fibers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
Stages involved in T4 replication (ds DNA)
1- Attachment
2- penetration
3- transcription and translation
4- replication of phage DNA and synthesis of
proteins
5-assembly
6-release
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lytic single stranded RNA phages
Some important things to consider;
1- common ones are MS2 and QB which infect
E.coli.
2- they only infect the F+ strains of E.coli
because they attach to the F pilus.
3- large burst size and replication rate
4- Need a special enzyme, RNA dependent RNA
polymerase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Filamentous Phages
• Filamentous phages are viruses that appear as
long thin fibers.
• These viruses infect bacteria, replicate their
DNA and make proteins but do not lyse the
host.
• Examples are the M13 virus and fd virus.
• The host continues to multiply but do so slowly.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Filamentous phages adsorb to the tip of F+
pilus of E.coli.
• The ss DNA enters the bacteria and replicates.
• The phage protein capsomeres are made but
they are stored in the cell membrane where
they are not detected.
• As the DNA leaves through the membrane it
gains its protein coat.
• Carrier cells carry the viral components.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lytic infection by ss DNA phages
• The phi X174 virus is capable of lytic infection.
• It contains a + DNA strand.
• Not filamentous in shape
• Can infect non F+ cells.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transduction
• In the process known as transduction
– Phages carry bacterial genes from one host
cell to another
Phage DNA
A+ B+
1 Phage infects bacterial cell that has alleles A+ and B+
A+ B+
2 Host DNA (brown) is fragmented, and phage DNA
and proteins are made. This is the donor cell.
Donor
cell
3 A bacterial DNA fragment (in this case a fragment with
the A+ allele) may be packaged in a phage capsid.
A+
4 Phage with the A+ allele from the donor cell infects
a recipient A–B– cell, and crossing over (recombination)
between donor DNA (brown) and recipient DNA
(green) occurs at two places (dotted lines).
Crossing
over
A+
A– B–
Recipient
cell
5 The genotype of the resulting recombinant cell (A+B–)
differs from the genotypes of both the donor (A+B+) and
the recipient (A–B–).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A+ B–
Recombinant cell