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that is also found characteristically in the
Prochlorococcus
strains MED4 and SS120 (but not in other
cyanobacterial or eukaryotic D2 proteins) and (vi) the absence of additional amino acid sequences in
the D2 proteins of
Synechococcus
sp. strain WH8102 as well as the
Synechococcus
phage S-PM2. When
cyanophage S-PM2 infects
Synechococcus
cells it encodes D1 and D2 proteins that enable the host cells
to overcome photoinhibition. It means the phage is able to get energy supply for its replication in
an otherwise inhospitable host environment (Bailey
et al
., 2004). Phage-mediated genes synthesized
hemoxygenase (
ho1
), 15,16-dihydrobiliverdin:ferredoxin oxidoreductase (
pebA
) in case of P-SSM2
and phycobilin:ferredoxin oxidoreductase (
pcyA
) gene in case of P-SSM4 during their replication
in
Prochlorococcus
thus enhancing their fi tness in the host that is otherwise inherently incompetent
to synthesize phycobiliproteins (Dammeyer
et al
., 2008).
Phylogenetic analysis of phage
psbA
gene sequences revealed that the
psbA
genes fall into a clade
of
Synechococcus
or
Prochlorococcus
hosts. The cyanophages used in this study are S-PM2, S-WHM1
and S-BM4, S-RSM2 (isolated by Wilson
et al
., 1993) and S-RSM28 and S-RSM88 isolated from the
Gulf of Aquaba by Millard
et al
. (2004). By the use of degenerate PCR primers and Southern blotting,
cyanomyovirus populations screened from Red Sea showed the presence of a copy of the
psbA
gene
among 37 of the 68 isolates. In addition to
psbA
gene, all phages showed the presence of
psbD
gene.
The DNA sequences of phages S-PM2 and S-RSM88 are identical to the extent of the presence of 212
nucleotides comprising group I intron sequences in between codons 334 and 335. However, in phages
S-RSM2, S-BM4 and S-WHM1 and the
psbA
and the
psbD
genes are adjacent to each other but differ
in the nucleotide sequences from each other (Millard
et al
., 2004). Lindell
et al
. (2005) investigated the
functional nature of the “photosynthetic” genes carried by the cyanophages infecting
Synechococcus
and
Prochlorococcus
who assayed the D1 protein levels. When cellular
psbA
gene stopped producing
D1 protein, the phage-encoded D1 protein increased gradually to cope up with the defi ciency.
These observations suggest that the phage replication cycle is dependent on photosynthesis and the
phage-encoded proteins sustained photosynthesis of the host ensuring its replication. Zeidner
et al
.
(2005) analysed environmental viral samples and prophage populations for the
psbA
and
psbD
gene
sequences and observed signifi cant differences from those of the host genes. On the basis of probable
codon usage it is suggested that photosynthesis genes carried by the cyanophages are evolving
and are under levels of purifying selection that make them indistinguishable from the sequences
of their hosts. There is every possibility of these gene sequences being exchanged and reshuffl ed
between
Synechococcus
and
Prochlorococcus
via the phage intermediates (Zeidner
et al
., 2005). Studies
on evolutionary aspects of core PSII genes (
psbA
and
psbD
) in marine cyanophages and their hosts
have been conducted by sequence analysis of 33 cultured marine cyanophages (belonging to the
three groups of tailed phages) and viral DNA samples collected from fi eld samples. Nearly 88%
of the phage genomes contained
psbA
gene sequence while 50% of the phage genomes possessed
both
psbA
and
psbD
genes. The existence of
psbA
gene in all cyanomyoviruses and
Prochlorococcus
podoviruses has been demonstrated but this gene sequence could not be amplifi ed from the
siphoviruses of
Prochlorococcus
or podoviruses infecting
Synechococcus
. Phylogenetic analysis of
psbA
and
psbD
gene sequences from cultured cyanobacteria and cyanophages segregated into distinct
clusters (Figs. 12 and 13). Furthermore,
psbA
and
psbD
genes were involved in a transfer from the
hosts to phages in at least four and two discrete steps, respectively. This was followed by horizontal
and vertical gene transfer between cyanophages (Sullivan
et al
., 2006). The existence of
psbA
gene
sequences in two of the three cyanopodoviruses (S-CBP1 and S-CBP3) and one cyanomyovirus
(S-CBM2)) infecting the estuarine
Synechococcus
strains has been demonstrated subsequently (Wang
and Chen, 2007). The sequences from the two podoviruses very much resembled ten environmental
psbA
sequences reported earlier from Chesapeake Bay (Bench
et al
., 2007), Mediterranean waters and