CYANOPHAGES - Cyanobacteria - page 448

Atlantic Ocean slope waters (Sullivan et al ., 2009). The particle size of this phage is the largest (with
a head of 75 nm broad and 140 nm long and a 325 nm long tail of 12 nm diameter) among 17 other
similar phages that infect species of enteric bacteria, mycobacteria, Streptomyces , Methanobacterium
and Lactococcus . The genome of P-SS2 is ~108 kb long, surpassed by the T5 genome (122 kb), and
encodes 131 predicted proteins. Of these, six structural proteins have been identifi ed on the basis
of sequence comparisons while 35 proteins during infection cycle. Some of the distinctive features
of the genome of P-SS2 are that (i) it lacks the photosynthesis genes that are widely represented in
the marine cyanomyoviruses; (ii) the phage P-SS2 may be a temperate phage due to the presence of
int , bet , exo and a 53 bp attachment site; and (iii) the host attachment site is located near an insertion
sequence activity which could mobilize a gene involved in nitrogen stress response. The proteome
analysis revealed that the 35 proteins identifi ed during infection cycle corresponded to structural
proteins. The structural genes were clustered into “modules”, one of the largest such module consists
of 28 structural genes including six structural genes for tail fi bers (ORFs 067, 072, 073, 074, 077, 091).
Of these six tail fi ber genes, ORF 073 encodes a protein of 1627-amino acid residues. This has been
found to be similar to the tail fi ber protein of P-SSM2, a myovirus. The low mol% G+C of this gene
signifi es that it might have been acquired through LGT (Sullivan et al ., 2009).
i) PaV-LD : A tailless Planktothrix agardhii virus from Lake Donghu, China has been reported. The
genome of this phage is unique for carrying NblA gene (ORF 022L) that encodes non-bleaching
protein involved in phycobilisome degradation. Electron microscopy and spectroscopic analysis of
the PaV-LD infected cells revealed a signifi cant decline of phycobilin content. The expression of the
NblA gene during infection cycle has been confi rmed by Western blot analysis. Another signifi cant
feature is the presence of a linear chromosome (95,299 bp) with 142 potential genes. Protein analysis
of the phage revealed 13 major structural proteins (27 kDa to 172 kDa). Genes that encode tail proteins
are absent in the genome (Gan et al ., 2011).
Besides the individual genomes of the cyanophages presented above, comprehensive accounts
on genome comparison of Prochlorococcus and Synechococcus myoviruses, recombination studies
on Rhode Island myovirus (RIM) strains and annotation of the myoviral genomes as per the
standards laid down by the Genome Standards Consortium (GSC) are available. Sullivan et al .
(2010) reported genome analysis of marine cyanophage myoviruses (16 of them) and compared
these with the genomes of T4-like myoviruses (10) from different hosts and environments. The 16
marine cyanophage myoviruses included six of those (P-SSM2, P-SSM4, P-HM1, P-HM2, P-RSM4
and P-SSM7) isolated on Prochlorococcus and ten of those (S-PM2, Syn9, Syn19, Syn33, Syn1, S-ShM2,
S-SM2, S-SSM7, S-SSM5 and S-SM1) isolated against Synechococcus strains. Except for the sizes of the
genomes of S-SSM7 (232 kb) and P-SSM2 (252 kb), the rest of the genomes ranged in between 174 kb
to 196 kb. The mol% G+C for Prochlorococcus myoviruses was lower (37± 1%) than the mol% G+C of
Synechococcus myoviruses (40.1 + 1.0%). All the 26 genomes consisted of a T4-like core of 38 genes
that encode capsid proteins and DNA replication genes. In addition, there are 25 genes that form
cyanophage-core genes which are shared by all the 16 cyanophage myoviruses. The total number
of core genes for cyanophage myoviruses thus comes to 63 genes. The T4-like genes shared among
the non-cyanophage myoviruses are 32 genes. Of the 25 cyanophage-core genes, the cyanophage
myoviral genomes consisted of six host genes ( psbA , mazG , phoH gene that encodes a phosphate-
starvation inducible protein; hsp20 , hli03 and cobS, cobalamine synthetase gene), two virion structural
genes and 17 hypothetical genes. The Synechococcus myoviral genomes consisted of different gene
subsets such as petE , zwf , gnd , prnA and cpeT that are not found in the genomes of Prochlorococcus
myoviruses. Marston and Amrich (2009) studied recombination and microdiversity among fi ve RIM