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Morphology of selected bacteriophages
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Single-stranded DNA bacteriophages
ϕX174
The phage fX174 is an icosahedral phage that contains a circular singlestranded
DNA molecule of 5386 nucleotides. It codes for 11 proteins, each of
which has been identified. Adding together the size of all those proteins
comes to 2330 amino acids, which1 would require 6990 nucleotides (3
2330) – substantially more than the total length of the genome
Firstly the genes are very tightly packed – there is very little non-coding
sequence in the genome. In most cases, the end of one gene is directly
adjacent to (or slightly overlaps with) the start of the next.
Secondly, one of the proteins (A*) corresponds to the C-terminal region
of protein A
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Single-stranded DNA
bacteriophages
ϕX174
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Replication of single-strand bacteriophages
After the single-stranded ϕX174 DNA enters
the cell, it is converted into a doublestranded molecule (replicative form, RF) by
synthesis of the complementary (or ‘minus’)
strand (step A).
The minus strand provides a template for
the production of further copies of the plus
strand (step B).
These are in turn converted to the doublestranded replicative form by synthesis of the
complementary (minus) strand (step C).
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Single-stranded DNA bacteriophages
M13
M13, represent another type of single-stranded phage. These
are ‘male-specific’ phages, since they infect the cell by
attaching to the tips of the pili specified by the F plasmid
As the replication cycle proceeds, one of the phage proteins
that is produced is able to bind to the single-stranded DNA and
divert it into the production of phage particles by targeting it to
the cell membrane
where it is extruded from the cell, with phage coat proteins
being polymerized around it during its passage through the
membrane
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Single-stranded DNA bacteriophages
M13
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RNA-containing phages
MS2(3600 N)
Another type of male-specific
phage is represented by MS2
This is an icosahedral RNAcontaining
phage
which
attaches to the sides of the Fpili, rather than to the tip as
M13 does.
It is an extremely simple
phage, containing some 3600
nucleotides ,coding for just
three genes: a coat protein, a
maturation protein and a
replicase
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Double-stranded DNA phages
Bacteriophage T4(165Kb)
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Terminal redundancy of bacteriophage T4. Multiple length linear DNA is the substrate for
packaging into phage particles. The amount of DNA packaged in longer than the genome size,
leading to terminal redundancy (a sequence of about 1600 base pairs at one end of the
molecule is repeated in the same orientation at the other end)
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Control of phage
development
B. subtilis
phage SPO1
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Cutting λ DNA at the cos sites
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Lysogeny
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Control of the lytic/lysogenic switch in λ. CII stimulates transcription of the cI repressor
gene from PE; CIII stabilizes CII. CI represses PL and PR and stimulates PM allowing
further synthesis of repressor. Cro represses PL, PR and PM
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