New Worlds: Host-virus relations can be a win

Researchers have for the first time recorded the entire genomic expression of both a host bacterium and an infecting virus over the eight-hour course of infection.

October 13, 2007 21:06
New Worlds: Host-virus relations can be a win

dna 88. (photo credit: )


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A genetic discovery at the Massachusetts Institute of Technology and the Technion Institute in Haifa is likely to encourage a rethinking of host-virus systems believed to play a key evolutionary role by facilitating the transfer of genes between species. The researchers have for the first time recorded the entire genomic expression of both a host bacterium and an infecting virus over the eight-hour course of infection. Prof. Debbie Lindell of the Technion and Prof. Sallie Chisholm of MIT reported in Nature that their study of a system involving the marine bacteria Prochlorococcus led them to speculate that viral infection may play a role in shaping the genetic repertoire of families of bacteria, even though individual infected bacteria die. This could indicate that the meeting between a marine bacterial host and its virus may not be just a battle between two individuals, but an evolutionarily significant exchange that helps both species become more fit for life. "The current status of host-virus relations has been influenced by a rich history of interactions," said Lindell. "While we can't pin down the sequence of past co-evolutionary events, our findings suggest a novel means through which the exchange of beneficial genes between host and virus have been triggered." And, because the pattern of genomic expression in this host-virus system differed from that in the more commonly studied system of intestinal bacteria such as E. coli and a virus called T7, the research will likely lead to increased appreciation for the need to study diverse types of marine bacteria. "We hope this work will encourage scientists to explore a wide range of host-pathogen systems, and thus lead to a significant broadening of our understanding of the diversity of the host-pathogen interactions existing in nature," said Chisholm, one of the discoverers of Prochlorococcus in 1985. "More importantly, these studies will help us understand the role these interactions play in shaping microbial ecosystems." Researchers have only in the past few decades begun studying ecologically relevant ocean bacteria such as Prochlorococcus, which play a very important role in our lives. These single-celled photosynthetic bacteria use light to produce oxygen and organic carbon - supplying a significant portion of the oxygen we breathe, and forming the basis for the ocean food chain. In previously studied host-virus systems, a virus hijacks the bacterial host cell and shuts down genome expression immediately, preventing the bacterium from conducting its own metabolic processes. The attacking virus redirects expression to its own genome and activates the genes beneficial for its activity, which is to replicate itself quickly at the cost of the host. But uncharacteristically, in the system of Prochlorococcus and virus P-SSP7, an unprecedented 41 of the bacteria's 1,717 genes were upregulated: That is, the researchers detected increased quantities of the messenger RNA encoded by these genes in the cell during the infection process. The upregulation of so many host genes during infection is unseen before in the world of bacteriology. Lindell and Chisholm believe the most plausible scenario to explain the gene upregulation and gene trading is that the bacterium activates certain genes in response to infection as a means of self-protection. The virus has "learned" to use those genes to its own advantage and so incorporates them into its own genome. Later, when infecting another bacterium, the virus upregulates those genes itself to facilitate its own reproduction within the host bacterium. When a bacterium survives an infection, those viral modified genes are incorporated back into the bacterial DNA in genome islands, making that bacterium and its descendants more likely to survive in the ocean. "These viral parasites cooperate with their hosts during infection, providing proteins that probably function within host metabolic pathways to squeeze every bit of energy out them before killing them off," said Lindell. "Yet on evolutionary scales, such host-pathogen interactions are influencing the evolution of gene content in both host and virus, which in turn is likely impacting their ability to colonize new niches." Next steps in the research are to see if host-like genes in the virus really do confer a fitness advantage to the virus and then to the host bacterium when transferred back. DIFFERENT, NOT DETESTED You don't have to hate other groups to love your own, according to a Ohio State University psychologist. While it may appear that conflict is an inevitable part of interaction between groups, Prof. Marilynn Brewer's research actually suggests that fighting, hating and contempt between groups is not a necessary part of human nature. This should offer a glimmer of hope in the Middle East. "There's still this belief that a group's cohesion depends on conflict with other groups, but the evidence doesn't support that," said Brewer. "Despite evidence to the contrary, you still see this theory in the research literature and in many textbooks." Brewer has spent much of her career studying "in-groups" - the groups we belong to - and their relations with "out-groups" - those to which others belong. She discussed the nature of these intergroup relations recently in San Francisco at the annual meeting of the American Psychological Association, which gave Brewer its 2007 Distinguished Scientific Contribution Award. In her address, Brewer said recent evidence suggests that people's attachment to their own groups has nothing to do with relations to other groups. Instead, people join groups to find a place of trust and security. "Simply put, we prefer people of our kind, people we know we can rely on. That doesn't mean you have to hate anyone else. But you will be more likely to trust people from your own group," Brewer said. In one recent study, for example, Brewer found that people tended to put more trust in total strangers when they learned this stranger attended the same university they did. The evolutionary history of humans suggests there is no need to require intergroup conflict. Early humans didn't live under dense population conditions. Given the costs of fighting, and the lack of need to compete, groups would have been more likely to flee from each other. That doesn't mean in-group bias is benign, Brewer said. The favoring of people in your group over those in another is the basis for discrimination. "You don't have to hate people from other groups in order to deny them the opportunities you have," she said. "That's a real downside to in-group bias."

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