© 2005 Sullivan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The definitive version was published in PLoS Biology 3 (2005): e144, doi:10.1371/journal.pbio.0030144.
The oceanic cyanobacteria Prochlorococcus are globally important, ecologically diverse primary producers. It is thought
that their viruses (phages) mediate population sizes and affect the evolutionary trajectories of their hosts. Here we
present an analysis of genomes from three Prochlorococcus phages: a podovirus and two myoviruses. The morphology,
overall genome features, and gene content of these phages suggest that they are quite similar to T7-like (P-SSP7) and
T4-like (P-SSM2 and P-SSM4) phages. Using the existing phage taxonomic framework as a guideline, we examined
genome sequences to establish ‘‘core’’ genes for each phage group. We found the podovirus contained 15 of 26 core
T7-like genes and the two myoviruses contained 43 and 42 of 75 core T4-like genes. In addition to these core genes,
each genome contains a significant number of ‘‘cyanobacterial’’ genes, i.e., genes with significant best BLAST hits to
genes found in cyanobacteria. Some of these, we speculate, represent ‘‘signature’’ cyanophage genes. For example, all
three phage genomes contain photosynthetic genes (psbA, hliP) that are thought to help maintain host photosynthetic
activity during infection, as well as an aldolase family gene (talC) that could facilitate alternative routes of carbon
metabolism during infection. The podovirus genome also contains an integrase gene (int) and other features that
suggest it is capable of integrating into its host. If indeed it is, this would be unprecedented among cultured T7-like
phages or marine cyanophages and would have significant evolutionary and ecological implications for phage and
host. Further, both myoviruses contain phosphate-inducible genes (phoH and pstS) that are likely to be important for
phage and host responses to phosphate stress, a commonly limiting nutrient in marine systems. Thus, these marine
cyanophages appear to be variations of two well-known phages—T7 and T4—but contain genes that, if functional,
reflect adaptations for infection of photosynthetic hosts in low-nutrient oceanic environments.
was supported by the US DOE under grant numbers DEFG02–
99ER62814 and DE-FG02–02ER63445, and the National Science
Foundation under grant number OCE-9820035 (to SWC).
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