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They found the presence of a homing endonuclease (HE;
FCphl
) gene adjacent to
psbA
gene sequence
in the genome of S-PM2. Furthermore, the intergenic region between
psbA
and
FCphl
is occupied
by antisense RNA gene (asRNA). Millard
et al
. (2010) are of the opinion that asRNA is co-regulated
with
psbA
and
F-Cphl
genes and is involved with their expression.This appears to be the fi rst report
on the presence of asRNA in the genomes of bacteriophages, a property that is widely represented
in eukaryotes and to certain extent in prokaryotes.
Besides the core PSII genes in marine cyanophages, Sharon
et al
. (2009) demonstrated the
presence of core PSI genes (
PsaA
,
B
,
C
,
D
,
E
,
K
and a unique
J
and
F
fusion gene) in the genomes of
marine cyanophage sequences collected from metagenomic data of the GOS expedition. The gene
products of these seven genes have been suggested to be suffi cient to form an intact monomeric
PSI complex and thus the marine cyanophages are able to funnel reducing power from respiratory
and other electron transfer chains to the PSI complex. Furthermore, the presence of gene sequences
of electron transfer protein genes like NADP(H) dehydrogenase (NDH-1) in the marine cyanophage
genomes signifi es that the marine cyaophages tinker with the photosynthetic electron fl ow of their
hosts and can derive benefi ts of ATP and NADPH from PSII or by the operation of PSI obtaining
only ATP, depending on the metabolic state of the host cell (Alperovitch-Lavy
et al
., 2011; Philosof
et al
., 2011).
Other host-related genes that play an important role in the multiplication of marine cyanophages
relate to the biosynthesis of polyamines. Polyamines infl uence the structure and oxygen evolution
rate of the PSII reaction centre (Bograh
et al
., 1997). These are suggested to stabilize
psbA
-2 mRNA
(Mulo
et al
., 1998). Sequencing of marine cyanophage genomes revealed the presence of
speD
gene
in S-PM2 (Mann
et al
., 2005) and P-SSM4 (Sullivan
et al
., 2005) that encodes S-adenosylmethionine
a key enzyme in the biosynthesis of polyamines spermine and spermidine. During infection these
phages may derive either of the functions mentioned above to prolong the functional integrity of
photosystem.
Metagenomic analysis of Chesapeake Bay waters led to the identifi cation of two myoviruses
P-SSM2 and P-SSM4 represented by 878 and 723 homologous sequences, respectively. The
podoviruses have been represented by P-SSP7 and P60 with homologous sequences of 543 and 368,
respectively. Thus the cyanomyoviruses and cyanopodoviruses are equally well represented in the
Chesapeake Bay metagenome. Cyanophage P60 appears to be the most commonly detected phage
based on the second version of phage proteomic tree (Rohwer and Edwards, 2002; Edwards and
Rohwer, 2005). The cyanophage homologous sequences accounted for 14% of the total metagenomic
sequences specifi c for phages. A total of 17,124 bp of
psbA
homologous sequences have been noted
that constitute 5% of the 341 kb of cyanophage homologues identifi ed in the library. On the basis of
sequence length and percentage of total genome,
psbA
gene sequences have been found in the majority
of cyanophages. Thus it has been concluded that cyanomyoviruses with larger genomes carried both
psbA
and
psbD
genes while cyanopodoviruses and cyanosiphoviruses that exhibited narrow host
range contained only
psbA
gene. Another interesting feature is the existence of sequences homologous
to ORFs of the genome of cyanomyovirus P-SSM2 in the metagenomic library of Chesapeake Bay
(Bench
et al
., 2007). Metagenomic analysis of the sequences from the GOS expedition (conducted
at fi ve sampling sites GS19, GS20, GS26, GS34 and GS51) revealed a high concentration of viral
sequences pertaining to 1,54, 662 viral genes encoding various metabolic and cellular functions.
The acquisition of host genes that are environmentally important for the viral population seems to
be a more wide-spread phenomenon than previously envisaged. The most predominant sequences
relate to tailed bacteriophage sequences. Of the 27 complete aquatic viral genomes analysed only
one reference bacteriophage genome has been abundantly represented. This sequence seems to be