Monday, January 10, 2011

Gene Transfer (part I)

Viruses play a huge role in the lives of microbes. We are used to thinking of viruses in human terms, and for humans—whether it’s a case of the sniffles caused by rhinovirus, or death from poxvirus—viruses are agents of disease. But, for the Bacteria and Archaea, viruses are much more. They are agents of evolution.

Structurally, a virus is much simpler than a cell. It is essentially an envelope of made of protein, carrying a bunch of genes on a piece of DNA or RNA. The protein envelope is supremely effective at attaching to the surface of a specific type of cell, and once it has done so, it injects the DNA or RNA it carries into that cell. Once inside the cell, the virus’ genes will instruct the cell to make lots of those envelope proteins and make lots of copies of the genes, and assemble them into new viruses. Usually, the virus genes also encode a protein that pops the cell open, killing the cell and releasing hundreds of new viruses so the cycle can start again. Most of the time—like, 99.9999% of the time—this is what happens when a virus attacks a Bacterial or Archaeal cell. However, what happens the other .00001% of the time is hugely important for the evolution of these cells.

I had an awful nightmare once—too much studying!—one of those horrible dreams where you’re in two places at once, and neither is nice. In this dream, I was a citizen of Berlin as it was overrun by the Russian army at the end of WWII...

and I was also a ribosome in a bacterial cell that was being attacked by a virus. Everything was chaos and confusion and terror. Easy enough to see in the Berliners; the second picture shows a cell exploding after viral infection. Each little pale tadpole-shaped thing is a new virus particle; remember that shape.

In such a scene it’s no surprise that mistakes are made, and when a bacterial cell dies at the microscopic hands of a virus, not everything goes according to virus’ program. Every once in a while, by random chance, a piece of the bacterial cell’s DNA ends up in a virus envelope, instead of viral DNA.

This “virus with bacterial genes” can go and attach itself to another bacterial cell and inject its DNA—but instead of that cell getting sick, it will get some perfectly good (if second-hand) bacterial genes. These genes could potentially increase the fitness of the cell, and so they could change the course of evolution.

This process is called “transduction,” and it is a major way in which Bacteria and Archaea can exchange genes. For humans (and all eukaryotes), sex is a medium for genetic exchange, and it can affect the course of evolution by putting genes into new and potentially useful combinations. The Bacteria and Archaea don’t have sex, but they do have transduction to shuffle the genetic cards (even though the DNA donor dies in the process). As one of my professors joked, in humans, sex brings viruses; but in the Bacteria and Archaea, viruses bring sex.

Transduction is well known and described. It has even been “domesticated” by humans so it is a commonly used tool in microbial genetics research. If I wished to move a gene from one bacterium to another, I’d grow up a trillion of my “donor” bacteria, infect them with viruses, wait a little bit, and collect the new viruses that were produced. I’d then go and take these viruses and infect a trillion of my “recipient” cells with them. It would be a massacre—all of the donor cells, and almost every single one of those recipient cells would die. But some would be lucky, infected by a transducing virus instead of a normal virus, and some small number of those would be even luckier—they would have the gene that I wanted to move.

Transduction is statistically rare—but given the numbers of microbes in the environment, transduction is also amazingly common. A milliliter of seawater contains some 1,000,000 bacteria and perhaps some 10,000,000 bacterial viruses. Plug in the number of milliliters in all the oceans and there are probably well over 1030 viruses swimming around out there, or as Ken Stedman put it, the same mass as 75 million blue whales. Given what we know about the frequency of transduction, this sort of bacterial sex happens, oh, about 20,000,000,000,000,000 times every second.

Think about that next time you dip your toes in the ocean.

(Sources--any reputable micro textbook, Ken Stedman of Portland State University, U.S. Army Museum)

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