Scientists studying ocean microorganisms have encountered something they have never seen before. A marine virus that cons certain photosynthetic bacteria into letting it come inside because it appears to offer a “helping hand” by bringing resources very like their own to help them acquire phosphorus, a nutrient they are desperately short of. Once inside, the virus uses the host’s cellular resources to replicate itself. About ten hours later, the host cells explode and release the viral progeny back into the ocean.
Qinglu Zeng and Sallie “Penny” W. Chisholm of Massachusetts Institute of Technology (MIT) in the US, write about their findings in the 24 January issue of Current Biology.
At first this may sound like old news: we already know that viruses invade host cells and pillage their resources in order to replicate. But what is new about this discovery is that the virus or phage, carries genetic material, co-opted from previous hosts, that tricks the new host into using its own machinery to activate the genes that sow the seeds of its destruction.
Because the bacteria live in a phosphorus-starved region of the ocean, they get stressed, phosphorous is vital to them. This puts their phosphorus-gathering machinery into high gear, and it is this heightened gear that the virus appears to sense and tempt the bacteria into accepting the offer of a “helping hand” of bacterial genes nearly identical to those of the approached host. With the newly acquired genes the host can then gather more phosphorous.
This process has never been reported before in a virus-bacteria relationship.
“This is the first demonstration of a virus of any kind — even those heavily studied in biomedical research — exploiting this kind of regulatory machinery in a host cell,” Chisholm told the press.
Chisholm, a professor of civil and environmental engineering (CEE) and biology at MIT, said the process has “evolved in response to the extreme selection pressures of phosphorus limitation in many parts of the global oceans”.
For their study, the authors used Prochlorococcus and its close relative, Synechococcus. Between them, these two bacteria produce about a sixth of the oxygen in the Earth’s atmosphere. They are abundant in seawater.
The viruses that attack them are called cyanophages: they are even more abundant.
The bacterial mechanism that the researchers focused on is the two-component regulatory system that enables the microbe to sense and respond to what is happening around it. The system prompts the organism to make extra proteins that bind to phosphorous and ferry it into the cell.
The virus or phage has acquired a gene that encodes this same protein.
Zeng said both the phage and its bacterial host have the genes that encode for the proteins that bind to and ferry the phosphorus into the cell, and in their study they found the host’s two-component regulatory system can up-regulate both of them.
“The positive side of infection for bacteria is that they will obtain more phosphorus binders from the phage and maybe more phosphorus, although the bacteria are dying and the phage is actually using the phosphorus for its own ends,” said Zeng.
The authors suggest their findings indicate the phage and the host bacteria have evolved side by side.
David Shub is a professor of biological sciences at the State University of New York at Albany, and was not involved in the study. He said the viruses have taken genes for a metabolic pathway from their host cells and that the authors have:
“… shown that these particular viral genes are regulated by the amount of phosphate in their environment, and also that they use the regulatory proteins already present in their host cells at the time of infection. The significance of this paper is the revelation of a very close evolutionary interrelationship between this particular bacterium and the viruses that seek to destroy it.”
Chisholm said their findings are further evidence of the “incredible intimacy of the relationship of phage and host”.
She and her colleagues now plan to explore the functions of other genes the phages have appropriated from host cells. This might give further insights into the selective pressures that drive these phage-host interactions in the open oceans.
“Most of what we understand about phage and bacteria has come from model microorganisms used in biomedical research. The environment of the human body is dramatically different from that of the open oceans, and these oceanic phage have much to teach us about fundamental biological processes,” said Chisholm.
Funds from the Gordon and Betty Moore Foundation, the National Science Foundation’s CMORE program and Biological Oceanography program, and the US Department of Energy helped pay for the study.
Written by Catharine Paddock PhD