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BACTERIOPHAGES: STRUCTURE AND PROPERTIES OF BACTERIAL VIRUSES
Bacteriophage (phage) are obligate intracellular parasites that multiply inside bacteria by
making use of some or all of the host biosynthetic machinery (pathmicro
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sc
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The
term is commonly used in its shortened form, phage
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Phages are estimated to be the most widely distributed
and diverse entities in the biosphere
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One of the densest
natural sources for phages and other viruses is sea water
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phages
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Typical phages have hollow heads (where the
phage DNA or RNA is stored) and tunnel tails, the tips of
which have the ability to bind to specific molecules on
the surface of their target bacteria
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These "young" phages burst from the
host cell (killing it) and infect more bacteria
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Depending upon the phage, the nucleic acid can be either DNA or RNA but not
both and it can exist in various forms
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These modified
bases protect phage nucleic acid from nucleases that break down host nucleic acids during
phage infection
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The simplest
phages only have enough nucleic acid to code for 3-5 average size gene products while the
more complex phages may code for over 100 gene products
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The simplest phages have many copies of
only one or two different proteins while more complex phages may have many different kinds
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Phages are also commonly employed in gene cloning, especially those exhibiting
lytic and lysogenic cycles (http://www
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com)

Structure of bacteriophages
Bacteriophage comes in many different sizes and shapes
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Size - T4 is among the largest phages; it is approximately 200 nm long and 80-100 nm wide
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Most phages range in size from 24-200 nm in length
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Head or Capsid - All phages contain a head structure which can vary in size and shape
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The head or capsid is composed of
many copies of one or more different proteins
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The
head acts as the protective covering for the nucleic acid
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Tail - Many but not all phages have tails attached to the phage head
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The size of the tail can vary and some
phages do not even have a tail structure
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At the end
of the tail the more complex phages like T4 have a base plate and one or more tail fibers
attached to it
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Not all phages have base plates and tail fibers
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Infection of Host Cells (http:// biology
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com/od/virology)

A
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This step is mediated by the tail fibers or by some analogous structure on those phages that
lack tail fibers and it is reversible
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e
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The nature of the bacterial receptor varies
for different bacteria
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These receptors are on the bacteria for other purposes and phages have
evolved to use these receptors for infection
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Irreversible attachment
The attachment of the phage to the bacterium via the tail fibers is a weak one and is
reversible
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Phages lacking base plates have other ways of becoming tightly
bound to the bacterial cell
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Sheath Contraction
The irreversible binding of the phage to the bacterium results in the contraction of the
sheath (for those phages which have a sheath) and the hollow tail fiber is pushed through the
bacterial envelope
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Some phages have enzymes that digest various
components of the bacterial envelope
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Nucleic Acid Injection
When the phage has gotten through the bacterial envelope the nucleic acid from the
head passes through the hollow tail and enters the bacterial cell
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The remainder of the phage remains
on the outside of the bacterium
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This is different from
animal cell viruses in which most of the virus particle usually gets into the cell
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LYTIC AND LYSOGENIC CYCLES - PHAGE MULTIPLICATION CYCLE
A
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Lytic or Virulent Phages
a
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The phage nucleic acid takes over the host biosynthetic
machinery and phage specified m-RNA's and proteins are made
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Early
m-RNA's code for early proteins which are needed for phage DNA synthesis and for shutting off
host DNA, RNA and protein biosynthesis
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After phage DNA is made late m-RNA's and late proteins are made
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b
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c
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The number of
particles released per infected bacteria may be as high as 1000
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Plaque assay - Lytic phage are enumerated by a plaque assay
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Each plaque arises from a single infectious phage
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B
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Definition - Lysogenic or temperate phages are those that can either multiply via the lytic
cycle or enter a quiescent state in the cell
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The phage DNA in this
repressed state is called a prophage because it is not a phage but it has the potential to
produce phage
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The cell
harboring a prophage is not adversely affected by the presence of the prophage and the
lysogenic state may persist indefinitely
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2
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Circularization of the phage chromosome - Lambda DNA is a double stranded linear
molecule with small single stranded regions at the 5' ends
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In
the cell the free ends of the circle can be ligated to form a covalently closed circle as illustrated
in Figure 5
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Site-specific recombination - A recombination event, catalyzed by a phage coded enzyme,
occurs between a particular site on the circularized phage DNA and a particular site on the host
chromosome
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c
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The result is a stable
repressed phage genome which is integrated into the host
chromosome
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3
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This
process is called induction
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Adverse conditions lead to the production of
proteases (rec A protein) which destroy the repressor
protein
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4
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The cro protein turns off the synthesis of the repressor and thus prevents the establishment
of lysogeny
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5
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Model for animal virus transformation - Lysogeny is a model system for virus
transformation of animal cells

b
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These genes can change the properties of the
bacterial cell
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This can be of significance
clinically
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g
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Toxin production by Corynebacterium diphtheriae is mediated by a gene carried by a
phage
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