Marine Viruses 4.5 Marine Phages 39
Part A | 4.5
cyanophage genes are involved in cyanobacteria photosynthesis [4.23]. Photosynthesis genes such as high
light inducible genes (hli), psbA, and psbD are found in
cyanophages. These genes encode the photosystem II
(PSII) core reaction-center proteins D1 and D2, respectively. PSII, which catalyzes the light-dependent oxidation of water to molecular oxygen in chloroplasts, is
a large pigment–protein complex in the thylakoid membrane. The D1 and D2 proteins of PSII bind the pigments and cofactors necessary for primary photochemistry. PSII is very sensitive to photo inhibition, and the
D1 protein of the PSII reaction center is the main target
for light-induced damage among the PSII proteins. The
damaged D1 proteins are degraded and subsequently
replaced with newly synthesized polypeptides in a repair cycle. This efficient repair mechanism is crucial to
maintain PSII in a functional state. Cyanophages shut
down the most of the host’s gene expression during the
lytic cycle infection and the proton motive force must
be maintained if they are to lyse the host. Therefore, it is
necessary to prolong photosynthesis of the hosts during
the infection cycle. Thus, it is intended that the phage
generates the energy for viral production by encoding psbA and other genes involved in photosynthesis.
The phylogenetic analysis of the cyanophage psbA gene
provides evidence that the acquisition of these genes by
horizontal gene transfer from their cyanobacterial hosts
(Synechococcus and Prochlorococcus) and gene acquisition were not very recent [4.24].
Types of Cyanophages
Cyanophages have been classified into three-tailed
phage families: Myoviridae, Podoviridae, and Siphoviridae; all of them are dsDNA phages. These viruses
can be isolated from both marine and fresh water environments [4.25]. Myoviruses and siphoviruses
have broad host ranges and have frequently been
isolated from a natural marine ecosystem. However,
podoviruses have a very narrow host range with a short
noncontractile tail. Myoviruses are nonenveloped, have
a head with icosahedral symmetry, and a tail with
tubular and helical symmetry, which is separated by
a neck. The head diameter is 50110 nm, while the
tail is 1620 nm, and the capsid is made up of
152 capsomers. Furthermore, morphological evidence
supports that marine and freshwater myoviruses are
more closely related each other than other bacteriophages. Only six cyanophages (S-PM2, P-SSM2, PSSM4, P-SSP7, P60, and Syn9) infect cyanobacteria in
the marine environment and all of these cyanophage
sequences are available in GenBank. Interestingly,
all those six cyanophages have been isolated either
from Synechococcus or Prochlorococcus. S-PM2, PSSM2, and P-SSM4 are more similar in morphology
to the Myoviridae, However P-SSP7 and P60 belong
to the Podoviridae family. The genomes of podoviral
cyanophages are small and compact compared to myoviral cyanophages. For instance, the genome size of
P-SSP7 and P60 are found to be about 44 to 47 kb. Nevertheless, myoviral cyanophages have relatively large
genomes; 196, 280 bp, 178, 249 bp, and 252, 401 bp
are found in S-PM2, P-SSM4, and P-SSM2, respectively [4.26].
4.5.3 Phage Therapy
Natural bacterial viruses or bacteriophages have been
applied to control bacterial diseases. For example, instead of antibiotics, phages are used as a common
therapy for human gastrointestinal diseases such as
salmonellosis in Russia [4.27]. In 1915, the phage was
discovered by the English microbiologist F.W. Twort
and subsequently by d’Herelle, who introduced the term
bacteriophage [4.28]. Phage research was an important
field of research in the 1920s and it was desired to
treat bacterial diseases [4.28]. Despite phages having
been used as an antibacterial agent in the United States
and Europe during the 1920s and 1930s, it has been
abandoned in the western countries for various reasons, including the discovery of antibiotics. Although
the commercial production of therapeutic phages has
ceased in most of the Western world, phages continue
to be used therapeutically in Eastern Europe and in
the former Soviet Union. Moreover, several institutions
in these countries are actively involved in therapeutic
phage research and production [4.27].
Antibiotics have been widely used as an antibacterial agent. However, antibiotic drugs allow the development of mutated drug-resistant bacteria. Therefore,
an effective alternative therapy is necessary to control
the rise of anti-drug-resistant bacteria. Bacteria have
the tendency to mutate against antibiotics once in every 10
6 divisions, but the antibiotics are immutable
chemicals, which are not effective against the new
antibiotic resistant bacterium. Although bacteria are
also becoming resistant to phages, the rate of developing resistance against phages is approximately once
in every 10
7 divisions, which is approximately tenfold
lower compared to antibiotics. Furthermore, phages
are living organisms and evolve along with hosts by
mutation. Thus mutated phages can overcome the bacterial mutations within either a few days or weeks.
Part A | 4.5
cyanophage genes are involved in cyanobacteria photosynthesis [4.23]. Photosynthesis genes such as high
light inducible genes (hli), psbA, and psbD are found in
cyanophages. These genes encode the photosystem II
(PSII) core reaction-center proteins D1 and D2, respectively. PSII, which catalyzes the light-dependent oxidation of water to molecular oxygen in chloroplasts, is
a large pigment–protein complex in the thylakoid membrane. The D1 and D2 proteins of PSII bind the pigments and cofactors necessary for primary photochemistry. PSII is very sensitive to photo inhibition, and the
D1 protein of the PSII reaction center is the main target
for light-induced damage among the PSII proteins. The
damaged D1 proteins are degraded and subsequently
replaced with newly synthesized polypeptides in a repair cycle. This efficient repair mechanism is crucial to
maintain PSII in a functional state. Cyanophages shut
down the most of the host’s gene expression during the
lytic cycle infection and the proton motive force must
be maintained if they are to lyse the host. Therefore, it is
necessary to prolong photosynthesis of the hosts during
the infection cycle. Thus, it is intended that the phage
generates the energy for viral production by encoding psbA and other genes involved in photosynthesis.
The phylogenetic analysis of the cyanophage psbA gene
provides evidence that the acquisition of these genes by
horizontal gene transfer from their cyanobacterial hosts
(Synechococcus and Prochlorococcus) and gene acquisition were not very recent [4.24].
Types of Cyanophages
Cyanophages have been classified into three-tailed
phage families: Myoviridae, Podoviridae, and Siphoviridae; all of them are dsDNA phages. These viruses
can be isolated from both marine and fresh water environments [4.25]. Myoviruses and siphoviruses
have broad host ranges and have frequently been
isolated from a natural marine ecosystem. However,
podoviruses have a very narrow host range with a short
noncontractile tail. Myoviruses are nonenveloped, have
a head with icosahedral symmetry, and a tail with
tubular and helical symmetry, which is separated by
a neck. The head diameter is 50110 nm, while the
tail is 1620 nm, and the capsid is made up of
152 capsomers. Furthermore, morphological evidence
supports that marine and freshwater myoviruses are
more closely related each other than other bacteriophages. Only six cyanophages (S-PM2, P-SSM2, PSSM4, P-SSP7, P60, and Syn9) infect cyanobacteria in
the marine environment and all of these cyanophage
sequences are available in GenBank. Interestingly,
all those six cyanophages have been isolated either
from Synechococcus or Prochlorococcus. S-PM2, PSSM2, and P-SSM4 are more similar in morphology
to the Myoviridae, However P-SSP7 and P60 belong
to the Podoviridae family. The genomes of podoviral
cyanophages are small and compact compared to myoviral cyanophages. For instance, the genome size of
P-SSP7 and P60 are found to be about 44 to 47 kb. Nevertheless, myoviral cyanophages have relatively large
genomes; 196, 280 bp, 178, 249 bp, and 252, 401 bp
are found in S-PM2, P-SSM4, and P-SSM2, respectively [4.26].
4.5.3 Phage Therapy
Natural bacterial viruses or bacteriophages have been
applied to control bacterial diseases. For example, instead of antibiotics, phages are used as a common
therapy for human gastrointestinal diseases such as
salmonellosis in Russia [4.27]. In 1915, the phage was
discovered by the English microbiologist F.W. Twort
and subsequently by d’Herelle, who introduced the term
bacteriophage [4.28]. Phage research was an important
field of research in the 1920s and it was desired to
treat bacterial diseases [4.28]. Despite phages having
been used as an antibacterial agent in the United States
and Europe during the 1920s and 1930s, it has been
abandoned in the western countries for various reasons, including the discovery of antibiotics. Although
the commercial production of therapeutic phages has
ceased in most of the Western world, phages continue
to be used therapeutically in Eastern Europe and in
the former Soviet Union. Moreover, several institutions
in these countries are actively involved in therapeutic
phage research and production [4.27].
Antibiotics have been widely used as an antibacterial agent. However, antibiotic drugs allow the development of mutated drug-resistant bacteria. Therefore,
an effective alternative therapy is necessary to control
the rise of anti-drug-resistant bacteria. Bacteria have
the tendency to mutate against antibiotics once in every 10
6 divisions, but the antibiotics are immutable
chemicals, which are not effective against the new
antibiotic resistant bacterium. Although bacteria are
also becoming resistant to phages, the rate of developing resistance against phages is approximately once
in every 10
7 divisions, which is approximately tenfold
lower compared to antibiotics. Furthermore, phages
are living organisms and evolve along with hosts by
mutation. Thus mutated phages can overcome the bacterial mutations within either a few days or weeks.