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Magnetic organelles point bacteria in the right direction (view PDF)
by Wendy Hansen

It's a big, scary world out there, especially if you're a bacterium just two millionths of a meter long, searching the 156 cubic kilometers of Lake Tahoe for the perfect place to call home. But what if you could use a built-in compass to simplify your search through the murky deep? Magnetic field-sensing bacteria such as Magnetospirillum magneticum do just that, using a specialized cell compartment, or organelle, called a magnetosome. UC Berkeley plant and microbial biology professor Arash Komeili is studying how these bacteria produce magnetosomes and what they have in common with more conventional organelles.

Biology textbook dogma says that bacteria don't contain internal compartments, or organelles, but that's not quite true. Though bacterial cells (prokaryotes) have far fewer membrane-encapsulated internal compartments than their protozoan and multicellular counterparts (the eukaryotes), some do exist. "Even in eukaryotic cells, the definition of organelle has changed over the years," says Komeili, but scientists generally agree that an organelle is a separate compartment in a cell with a specific enzymatic activity. Advances in imaging technology over the past few decades have enabled scientists to discover bacterial organelles despite their miniscule size. Magnetosomes, for example, run just under 50 nanometers in diameter, far too small to image clearly with conventional light microscopy.

Magneto-sensitive bacteria, however, are easy to find if you know how to look for them. "They're in Strawberry Creek and Monterey Bay, Lake Tahoe—everywhere," says Komeili. To identify them, simply hold a bar magnet near a sample and pick out the bacteria that become oriented along its magnetic field. Incidentally, this is the way Richard P. Blakemore originally discovered magneto-sensitive bacteria back in 1975.

Magnetosomes are membrane-encapsulated compartments filled with crystals of magnetite, a naturally magnetic mineral. These crystals are assembled by the bacterium in the magnetosome and act as an internal compass needle, aligning with the Earth's magnetic field. Scientists believe that bacteria use this ability to sense magnetic fields in order to find their preferred environments. Like Goldilocks in search of a comfortable bed, many bacteria need to live where the chemical environment is "just right," which they sense by a process called chemotaxis. However, searching through all three dimensions of a large body of water is taxing for these tiny life forms.

Concentrations of oxygen and other chemicals vary with water depth, so bacteria developed magnetosomes as a way to guarantee that their search will cross paths with the right layer. In much of the world, magnetic field lines are perpendicular to the surface of the water. Guided by their magnetosomes, the bacteria can just follow the magnetic field down until they reach the level that suits them. By passively determining up from down, the bacteria simplify a vast three-dimensional search to a more manageable single dimension.

Much of Komeili's work on the magnetosome was done while he was a postdoctoral researcher in Dianne Newman's lab at Caltech. Collaborating with Caltech biology professor Grant Jensen and postdoctoral researcher Zhuo Li, Komeili used a technique called electron cryotomography to visualize the tiny magnetosomes in Magnetospirillum cells. With electron cryotomography, says Komeili, "the level of detail you get is really amazing."

Their results, published in the January 13, 2006 issue of Science, showed two unexpected features of magnetosomes. First, chains of magnetosomes were associated with filaments of a protein called MamK, which seem to link the magnetosome vesicles together. Second, the membranes of individual magnetosome compartments were connected to the outer cell membrane. Komeili acknowledges that this finding is "probably still a little controversial," but says that it is "very clear that the magnetosomes are these small little blebs coming off of the cell membrane."

A faculty member at UC Berkeley since July 2005, Komeili is continuing with two aspects of the magnetosome research. In one approach, his lab is systematically deleting individual genes from a "genomic island" known to be involved in magnetosome formation. They hope to identify the role of each of the genes and perhaps tackle larger questions about how bacterial organelles form. In these efforts, the magnetosome also serves as a "model organelle" for studying the similarities between bacterial and eukaryotic organelles. The other approach focuses on the dynamics of MamK, the filamentous protein associated with magnetosome structure.

"We want to understand the process of magnetosome formation in detail so that we can eventually manipulate the magnetosomes' magnetic properties for different kinds of applications," explains Komeili. Potential uses for magnetosomes—and in a more general sense, biologically controlled mineralization—range far and wide. Possible biomedical applications include using magnetite as a contrast agent in magnetic resonance imaging (MRI). Biologically controlled magnetite is also of interest to scientists in the blooming field of bioremediation, where living organisms are used to clean up contaminated environments. Not bad for an organelle that textbooks say doesn't exist.

Wendy Hansen is a graduate student in biophysics.

Want to know more?
Check out a review of magnetosome formation: Komeili, A. (2007) Annu Rev Biochem. 2007;76:351-6

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