CYANOPHAGES - Cyanobacteria - page 462

of Sullivan et al . (2006) who suggested the absence of psbD gene sequence from cyanophages with
restricted host range. None of the cyanophages belonging to Podoviridae and Styloviridae tested
so far contained the psbD gene. Wang and Chen (2008) investigated the prevalence of highly host-
specifi c podoviruses and siphoviruses infecting estuarine Synechococcus strains. Of these, three
podovirusres, three siphoviruses and one myovirus have been characterized with reference to
their morphology, host-range, growth and genetic features. While psbA gene sequences have not
been found in the siphoviruses, the psbA gene sequences found in the two podoviruses clustered
with some environmental psbA sequences forming a unique cluster that is distantly related to the
already known psbA clusters. Another signifi cant feature is the presence of highly conserved DNA
polymerase ( pol ) gene in these estuarine podoviruses that makes them unique to develop a host-
phage relationship different from what has been observed in waters of open Oceans.
Sharon et al . (2007) conducted a global survey of the prevalence of phage photosynthesis genes and
their relationship with cyanobacterial photosynthesis genes from diverse oceanic environments. They
concluded that the cyanophage genomes have a long term effect on the evolution of cyanobacterial
photosynthetic apparatus by shifting these genes from one host to another and the viral psbA genes
are actively expressed in the host cells in the marine environment to prolong photosynthesis albeit for
short periods of lytic cycle. Paul and Sullivan (2005) are of the opinion that psbA gene is transferred
from host to phage multiple times while psbD was transferred a single time from host to phage but
these have not gone back from phage to host. However, hlip gene acquired from host by the phage
has undergone some evolutionary changes and in some cases might have been returned to the host.
Although it is diffi cult to explain these hypotheses with the limited phylogenetic information now
available, it seems probable that the phage genomes have evolved by acquiring genes of interest
from their hosts that increase phage fi tness and potential for replication. The range of genetic
diversifi cation in psbA and psbD gene sequences discovered in cyanophages might be useful for the
evolution of the host genomes as well. Thus the cyanophage-encoded photosynthesis genes represent
one of the best examples for the occurrence of LGT in marine microbial communities and this will
no doubt continue to drive evolutionary forces to shape the biosphere of the future (Hambly and
Suttle, 2005). Summarizing the role of light in the marine cyanophage-host interactions, Clokie and
Mann (2006) suggested that it is desirable to fi nd out (i) the fi tness benefi ts of acquiring host genes
by marine cyanophages, (ii) the probable effects of diurnal cycle on the infection cycle of phages
infecting marine cyanobacteria and (iii) as to how the LGT continues to be an important component
of the arms race between the marine cyanophages and their hosts.
Photosynthesis gene psbA has been detected in the viral populations of Norwegian coastal waters
by Sandaa et al . (2008) who separated viral genomes ranging in size from 31 to 380 kb by PFGE
followed by PCR combined with cloning and sequencing. The contribution of psbA gene carried by
the phage and that of the host has been examined by modelling experiments during lytic cycle of
the phage P-SSP7 in the cells of P. marinus MED4. As phage genome replication proceeds consequent
upon the degradation of host genome and also D1 protein degradation takes place due to phage
infection, there is a shortfall in the supply of D1 protein. Since the infected cells are no longer in a
position to supply D1 protein, phage encoded psbA gene fulfi lls the role and gives advantage to the
phage for completing its life cycle (Fig. 14; Bragg and Chisholm, 2008). Lytic cycle of P-SSP7 in the
host P. marinus MED4 has been followed for genome-wide expression dynamics of both host and
phage. Four phage-encoded host metabolism genes psbA , hli , talC and ribonucleotide reductase
( nrd ) are transcribed together as a functional unit. These genes help the phage to meet its energy
requirements and in the deoxynucleotide production for phage replication in oligotropic marine
waters. The presence of such genes in marine waters leads to the formation of genome islands and