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of marine cyanophages from the brackish Baltic Sea infecting
No
.
spumigena
. The encoded protein
has been shown to be identical to the
No
.
spumigena
-specifi c cyanophages described above (Jenkins
and Hayes, 2006). Jia
et al
. (2006) reported molecular characterization of T4-type bacteriophages
from rice straw and a surface soil in a Japanese rice fi eld by the amplifi cation of
g23
gene sequence.
They detected great diversity in T4-type cyanophages distinctive from those found in the marine
environments. But interestingly, the phylogenetic analysis showed that two novel sub-groups of
the T4-type phages were more closely related to the marine cyanophage isolates of exo-T-evens
described ealier by Hambly
et al.
(2001).
iii)
phoH
: The
pho
regulon regulates phosphate uptake and metabolism under low phosphate
conditions. A bioinformatics study of the fully sequenced marine phage genomes available with
GenBank revealed the presence of
pho
regulon genes in nearly 40% of them. Of the genes of the
pho
regulon,
phoH
from marine viruses appeared as a distinct cluster, different from the
phoH
sequences
of the bacterial hosts. With the help of the PCR amplifi cation of the
phoH
sequences, Goldsmith
et al
.
(2011) determined the diversty of the
phoH
sequences from different depth profi les of Sargasso Sea
and six geographically distinct locations. Due to the presence of unique
phoH
clusters at different
depths and geographic location,
phoH
gene sequence has been projected as a molecular marker to
assess marine phage diversity.
iv)
“Photosynthetic” phages
:
Photosynthesis genes
psbA
and
psbD
encode PSII core reaction centre
proteins D1 and D2, respectively. The
hli
gene encodes the high-light inducible proteins (HLIPs).
HLIPs are suggested to protect photosynthetic apparatus from excess excitation energy during
stressful conditions in cyanobacteria (He
et al
., 2001). Two other related genes
petE
and
petF
encode
plastocyanin and ferredoxin, respectively. Mann
et al
. (2003) reported for the fi rst time the existence
of
psbD
and an interrupted
psbA
gene sequences in the genome of cyanophage S-PM2 that infects
Synechococcus
sp. strain WH7803. Lindell
et al
. (2004) investigated the different aspects of transfer of
photosynthesis genes to and from
Prochlorococcus
viruses (Table 11). Upon a close examination of the
photosynthetic gene sequences of selected cyanophages P-SSP7, P-SSM2 and P-SSM4 and those of
their hosts, it is concluded that the phage photosynthesis genes are (i) highly conserved sequences,
(ii) show high degree of homology to the host gene sequences (85% and 95% in nucleotide and
amino acid identities, respectively), (iii) arranged together suggesting that they might be expressed
simultaneously during infection (Miller
et al
., 2003), (iv) the presence of a 7-amino acid sequence at
the C-terminal portion of phage D1 protein similar to the cyanobacterial D1 proteins as well as in
non-green algal plastids, (v) the presence of a 7-amino acid sequence in the centre of the D2 protein
Table 11:
Photosynthetic genes in certain cyanophages (based on the data from Lindell
et al.
, 2004).
Cyanophage
Characteristics
Host strain
Photosynthetic Genes
P-SSP7
T7-like Podovirus
Capsid ~50 nm;
Non-contractile tail; genome 45 kb
High-light adapted
Prochlorococcus
Strain MED4
D1 and one HLIP
P-SSM2
T4-like Myovirus
Capsid~85 nm; long contractile
tails
Low-light adapted
Strains of
Prochlorococcus
NATL1A, NATL2A and MIT9211
D1, six HLIPs, ferredoxin
and plastocyanin
P-SSM4
T4-like Myovirus
Capsid ~80 nm
Long contractile tails
Two low-light adapted strains
(NATL1A, NATL2A)
and two high-light adapted strains
(MED4 and MIT9215) of
Prochlorococcus
D1, D2 and four HLIPs