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closely related to the marine cyanophage P-SSM4 but it is not identical to it. This cyanophage seems
to infl uence the distribution, abundance and diversity of its host identifi ed to be closer to
P. marinus
MIT9312 (Williamson
et al
., 2008).
v)
Genes related to pigment biosynthesis
:
Of the two oceanic cyanobacteria
Synechococcus
and
Prochlorococcus
, the former possesses phycobilisome rods that contain phycoerythrin (Waterbury
and Rippka, 1989) which helps in the transfer of harvested light energy to PSI and PSII. The latter
does not possess phycobilisome rods but possesses alternative antenna based on chlorophyll
(Goericke and Repeta, 1992; Lichtle
et al
., 1995; La Roche
et al
., 1996; Ting
et al
., 2002). Some strains of
Prochlorococcus
express genes that are remnants of the ancestral
Synechococcus
phycobilisomes (Hess
et al
., 1996). Genes involved in the biosynthesis of phycoerythrobilin (PEB) and phycocyanobilin
(PCB) are designated as
pebA
and
pcyA
. The sequence of reactions involved in the biosynthesis of
PEB and PCB is described here. Heme in presence of heme oxygenase (HO-1) is converted to an open
chain product by the addition of seven electrons in presence of oxygen into biliverdin 1x alpha (BV)
that constitutes the substrate for various enzymes of the FDBR family. Two sequential two electron
reductions catalysed by the enzymes PebA and PebB give rise to PEB. Phycoerythrobilin synthase
(PebS) is a product of the gene
pebS
that catalyses a unique four electron reduction of BVIx alpha
to PEB. BV is reduced to PCB in presence of the enzyme PcyA with the addition of 4 electrons. All
cyanobacteria possess the enzymes PebA and PebB. Enzymes HO-1 and PcyA are found both in
cyanobacterial cells and marine cyanophages. The PebS enzyme is found only in marine cyanophage
populations. Exceptionally, marine cyanophages P-SSP7 (a
Prochlorococcus
podovirus), Syn5 and P-60
(
Synechococcus
podoviruses) do not possess any of the bilin biosynthesis genes. Of the myoviruses,
Prochlorococcus
myovirus P-SSM2 contains
pebS
,
ho1
and
petF
genes where as P-SSM4 consists of only
one
pcyA
gene (Sullivan
et al
., 2005; Mann
et al
., 2005; Weigele
et al
., 2007).
Synechococcus
myoviruses
S-PM2 and Syn9 possess
cpeT
gene (Mann
et al
., 2005).
Synechococcus
sp. strain PCC 7002 possesses
a
cpcT
gene that is paralogous to
cpeT
gene and the
cpcT
gene encodes a lyase that attaches the
phycobilin chromophore to the D-subunit of phycocyanin (Shen
et al.,
2006). Of the cyanobacterial
strains, six of the HL-
Prochlorococcus
strains (MED4, MIT9215, MIT9301, MIT9312, MIT9515 and
AS9601) investigated possess
pebA
,
pebB
,
pcyA
,
ho1
, and
petF
genes while six of the LL-
Prochlorococcus
strains (NATL1A, NATL2A, SS120, MIT9211,, MIT9303 and MIT9313) showed in addition to these,
the gene
cpeT
.
Synechococcus
strains (CC9311, CC9605, CC9902, WH8102, JA-2-3B'a (2-13) and JA-
3-3Ab) also contained the six genes as seen in LL-
Prochlorococcus
strains. Dammeyer
et al
. (2008)
investigated the functional aspects of the genes
ho1
,
pebA
,
petF
in P-SSM2 and a
pcyA
homolog in the
phage P-SSM4. Cyanophage P-SSM2
pebA
encodes PebS via an intermediate 15, 16 dihydrobiliverdin
and in this sense
pebA
of the phage carries out a reaction in a single step that is performed in two
steps in cyanobacterial cells and these genes are expressed during the fi rst hour of infection in the
cells. On the basis of identifi cation of bilin biosynthesis genes of the host and phage genomes from
oceanic communities of marine cyanophages and the subsequent phylogenetic analysis revealed
that PebA, PebS and HO-1 are grouped into a monophyletic cluster and have been acquired from
marine cyanobacteria only once. Whereas
pcyA
phage sequences group into two clusters and may
have been obtained in two steps.
Millard
et al
. (2009) conducted a comparative genomic analysis of fi ve marine cyanomyoviruses
including cyanophage S-RSM4 and detected a hypervariable region in between genes
g15
-
g18
characterized by the presence of
ptoX
(plastoquinol terminal oxidase),
gnd
(6-phosphogluconate
dehydrogenase),
zwf
(glucose-6-phosphate dehydrogenase) and
petE
(plastocyanin) genes. The
acquisition of these host genes by the cyanomyoviruses might have taken place as independent
