Viruses con bacteria into working for them

January 26, 2012

By Denise Brehm
Civil & Environmental Engineering

MIT researchers have discovered that certain photosynthetic ocean bacteria need to beware of viruses bearing gifts: These viruses are really con artists carrying genetic material taken from their previous bacterial hosts that tricks the new host into using its own machinery to activate the genes, a process never before documented in any virus-bacteria relationship.

The con occurs when a grifter virus injects its DNA into a bacterium living in a phosphorus-starved region of the ocean. Such bacteria, stressed by the lack of phosphorus (which they use as a nutrient), have their phosphorus-gathering machinery in high gear. The virus senses the host’s stress and offers what seems like a helping hand: bacterial genes nearly identical to the host’s own that enable the host to gather more phosphorus. The host uses those genes,— but the additional phosphorus goes primarily toward supporting the virus’ replication of its own DNA.

Once that process is complete (about 10 hours after infection), the virus explodes its host, releasing progeny viruses back into the ocean where they can invade other bacteria and repeat this process. The additional phosphorus-gathering genes provided by the virus keep its reproduction cycle on schedule.

In essence, the virus (or phage) is co-opting a very sophisticated component of the host’s regulatory machinery to enhance its own reproduction — something never before documented 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, and it has evolved in response to the extreme selection pressures of phosphorus limitation in many parts of the global oceans,” says Sallie (Penny) W. Chisholm, a professor of civil and environmental engineering (CEE) and biology at MIT, who is principal investigator of the research and co-author of a paper published in the Jan. 24 issue of Current Biology. “The phage have evolved the capability to sense the degree of phosphorus stress in the host they’re infecting and have captured, over evolutionary time, some components of the bacteria’s machinery to overcome the limitation.”

Chisholm and co-author Qinglu Zeng, a CEE postdoc, performed this research using the bacterium Prochlorococcus and its close relative, Synechococcus, which together produce about a sixth of the oxygen in Earth’s atmosphere. Prochlorococcus is about one micron in diameter and can reach densities of up to 100 million per liter of seawater; Synechococcus is only slightly larger and a bit less abundant. The viruses that attack both bacteria, called cyanophages, are even more populous.

The bacterial mechanism in play is called a two-component regulatory system, which refers to the microbe’s ability to sense and respond to external environmental conditions. This system prompts the bacteria to produce extra proteins that bind to phosphorus and bring it into the cell. The gene carried by the virus encodes this