CYANOPHAGES - Cyanobacteria - page 463

Figure 14: Schematic diagram of model, phage genomes are made using dNTPs from two possible sources. First, dNTPs
can be made by scavenging deoxynucleotides from the host genome. This process can occur in the dark, but is bolstered by
photosynthesis. Second, dNTPs can be newly synthesized by a process that is dependent on the products of photosynthesis
(dashed lines). Photosynthesis is dependent on functional PSII subunits, which contain the D1 protein. During exposure to
light, D1 proteins can become damaged, and are excised from PSII subunits, and replaced with D1 proteins from either host or
phage encoded psbA mRNAs. With the kind permission of J. G. Bragg, Department of Civil and Environmental Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts, USA [Bragg, J. G. and Chisholm, S. W. (2008) PLoS ONE
3(10): e3550. doi:10.1371/journal.pone.0003550]. doi:10.1371/journal.pone.0003550.g001
the presence of their homologues in cyanophages suggests that phage-mediated transfer of host genes
has a major impact on microbial evolution. The ability of the phage to use up-regulated host genes
is one of the starting points for their stable incorporation into phage genomes and their subsequent
transfer back to genome islands. The activation of host genes during infection may be directing the
co-evolution of gene content in both host and phage genomes (Lindell et al ., 2007). Chénard and
Suttle (2008) conducted a phylogenetic analysis of 45 psbA gene sequences generated from a number
of environmental viral samples collected from freshwaters (Lake Contance, West Germany, and
Experimental Lakes Area of Ontario, Canada) and marine waters (collected from Arctic Ocean, Gulf
of Mexico, NE Pacifi c Ocean). The important fi ndings are that (i) the sequences from freshwaters have
an evolutionary history distinct from those of marine waters, and (ii) psbA sequences from the same
geographical area clustered into different clades suggesting that the area supported the presence of
different types of phages. However, other fi ndings such as the differences in psbA sequences from
Prochlorococcus and Synechococcus viral genomes and as also between the sequences of myoviruses
and podoviruses agreed with the observations of other workers reported earlier. The observations of
Mann et al . (2003) on the interrupted nature of psbA gene in the genome of S-PM2 due to the presence
of a group I intron has been confi rmed by other workers but its expression during the lytic cycle of
S-PM2 has been studied by Millard et al . (2010). The group I intron is shown to be spliced during
the entire lytic cycle thus providing a continuous supply of D1 protein but due to the appearance
of both spliced and unspliced psbA transcripts they predicted a regulatory role for intron splicing.