Biology Reference
In-Depth Information
Table 10:
Genome characteristics of
Prochlorococcus
cyanophages (after Sullivan
et al.
(2005)).
Cyanophages
Length (bp)
G+C content
ORFs
Functional genes
P-60
47,872 bp
80
Genes of nucleotide metabolism similar to cyanobacteria
S-PM2
196,280 bp
37.8%
239
petE
,
petF
P-SSP7
~45 kb
38.7%
54
Of the 26 core phage T7 genes 15 are present; integrase gene
present
P-SSM2
252 kb
35.5%
327
Phage T4-like genes 14% present; genes of unkown function
60%; integrase gene lacking; two large clusters of genes (24 in
total) present relating to lipopolysaccaride synthesis; phosphate
inducible genes (
phoH
and
pstS
) helpful in phosphate stress are
present
P-SSM4
178 kb
36.7%
198
Phage T4-like genes 21% present; genes of unkown function are
58%; integrase gene lacking; two large clusters of genes (24 in
total) absent; phosphate inducible genes (
phoH
and
pstS
) helpful
in phosphate stress are present
Pf-WMP4
40,938 bp
45
Syn-9
162,109 bp
184
nblA
gene that degrades phycobilisomes; integrase (recombinase)
present, 44 ORFs with known function and the rest of unknown
function
DNA, DNA replication, DNA packaging and virion formation are important among others. Some of
the T7 functions absent in P-SSP7 relate to ligation of DNA fragments, inhibition of host RNAP, lysis
events and interactions specifi c to host cell envelope during virion formation are important. There
seems to be a functional
int
gene in P-SSP7 genome that has a 42-bp matching sequence downstream
of tRNA
Leu
gene of the host
P. marinus
MED4 suggesting the probability of P-SSP7 undergoing a
lysogenic event (Sullivan
et al
., 2005).
c)
Phage S-PM2
: The genome of S-PM2 is a dsDNA molecule that is circularly permuted and consists
of 196,280 bp with mol% G+C of 37.8. The total ORFs represented are 239 and possess sequences
for 25 tRNAs. Of these, 40 ORFs have been organized into discrete clusters that are associated with
virus morphogenesis, nucleotide metabolism, gene regulation and DNA replication and repair.
ORFs of unkown function suspected to be involved in the adaptation of the metabolism of host
cell to the requirements of phage replication are present. Twenty ORFs have been shown to be
identical to the cyanobacterial hosts (Mann
et al
., 2005). It may be mentioned that S-PM2 is the fi rst
“photosynthetic” marine cyanophage that has been reported to carry PSII reaction center genes,
psbD
and an interrupted
psbA
gene sequences in its genome (Mann
et al
., 2003). Two photosynthesis
electron transport genes encoding plastocyanin (
petE
) and ferredoxin (
petF
) that are present in the
Prochlorococcus
phage P-SSM2 are not represented in S-PM2 genome (Mann
et al
., 2005). Though the
adsorption of S-PM2 to the cells of
Synechococcus
sp. strain WH7803 was light-dependent, neither
photosynthesis is required for adsorption nor does it make any difference of the presence or absence
of PSII reaction center genes in its genome (Jia
et al
., 2010).
d)
Phage Syn5
: This phage, isolated by Waterbury and Valois (1993) from Sargasso Sea, infects
Synechococcus
sp. strain WH8109 belonging to MC-A, Calde II. It exhibited a restricted host range as
it did not infect 19 other strains of
Synechococcus
and
Prochlorococcus
. Pope
et al
. (2007) investigated
the structural properties, capsid organization and genome sequence of this cyanophage. Cryo-
electronmicroscopy revealed that the virus particles have an isometric icosahedral head of ~60
nm in diameter with a short tail of 25 nm length. One of the most distinguishing features of this
