DNA Secrets of a Salty Survivor

Reading the "book of life"

To understand how these cells of Halobacterium managed to survive in their experiments, DiRuggiero's team sent the "victims" of their tests to the Institute for Systems Biology in Seattle. There, scientists used a modern genetics tool called a "DNA microarray" to see a complete picture of Halobacterium's response to being damaged: the full set of molecular tools that spring into action in the wake of a UV dose or exposure to space-like vacuum.

These "molecular repair tools" belong to a category of proteins called enzymes. Enzymes are the workhorses of all living cells: they catalyze the thousands of chemical reactions necessary for life, such as breaking down food or repairing flaws in DNA. Halobacterium always keeps a certain amount of repair enzymes on hand, so when a radiation dose occurs, this stash of enzymes can quickly administer "first aid" to the DNA. But then it must also ramp up production of other repair enzymes to continue the repair, activating the genes that produce those enzymes. It's that boost in gene activity that the microarray tests can detect, thus showing which enzymes are important for Halobacterium's remarkable DNA-repair abilities.

Image credit: James Smiley A DNA microarray, as seen through a microscope. Each tiny dot corresponds to one of the organism's thousands of genes, and the color of the dot indicates the activity level of that gene.

From those microarrays, DiRuggiero's team has learned that when it comes to DNA repair, Halobacterium is something of a "Renaissance bug." It dabbles in a bit of everything. Its genome of only 2,400 genes contains several distinct sets of DNA-repair mechanisms. Some of these sets of tools are like the DNA-repair tools found in plants and animals, other sets are more like those of bacteria, and still others are characteristic of a lesser-known group of life called "Archaea" (the group that Halobacterium belongs to). Halobacterium has them all. Beyond even that, Halobacterium has a few novel DNA-repair mechanisms that no one has ever seen before!

Learning how all these repair mechanisms work could teach scientists a lot about how DNA repair occurs in humans, and perhaps point to ways to enhance people's natural ability to cope with damage to their DNA - a possible boon to astronauts.

"Many of the repair proteins in the Archaea are very similar to that of Eukarya - [the group of life that includes] you and me - and therefore Archaea can be used as a simple model system to study the more complex processes that occur in eukaryotes," DiRuggiero explains.

Some of these novel molecular tools could also prove to be useful for industry and biotechnology, DiRuggiero suspects. After all, it was in studying a cousin of Halobacterium - a heat-loving microbe - that scientists found the DNA-copying protein that made it possible to sequence entire genomes. The Human Genome Project would have never happened without it.

Not bad for a humble microbe.