February 28, 2012 | 1
“In biology, magnetism is a unique and virtually orthogonal physical property.”
Only a few organisms can actively sense and utilize magnetic fields. Magnetotactic bacteria contain strings of iron-dense membrane-bound organelles filled with magnetic crystals called magnetosomes, which act like microscopic compasses. Bacteria that contain magnetosomes can detect the earth’s magnetic field, telling them which direction is up and helping them find oxygen closer to the surface of the water. Migratory animals can also navigate by following the earth’s magnetic field lines, but the mechanism by which they sense geomagnetic fields remains unclear.
All other organisms contain iron, but rarely enough to be noticeably magnetic. An amazing paper published today by Keiji Nishida and Pamela Silver in PLoS Biology demonstrates how by physiologically or genetically altering the iron content inside yeast, cells can become magnetized and attracted to magnets. Almost all cells, from bacteria to humans contain the protein ferritin, which sequesters iron inside the cell (preventing iron toxicity) and releases it as needed. Yeast don’t normally contain ferritin, typically collecting iron inside organelles called the vacuole instead. Deleting genes that help the vacuole pick up iron and genetically engineering yeast to produce ferritin can increase the amount of iron that yeast cells can take up, enough to noticeably increase their magnetism.
“The cell cultures were exposed to magnets and attraction was observed.”
When you add iron the media that yeast are growing in, even wild-type, unengineered cells are a little magnetic, this “basal magnetization” being caused by the iron accumulating in the vacuole. Using a superconducting quantum interference device (SQUID), Nishida shows that the synergistic effect of deleting iron accumulation in the vacuole and expressing ferritin makes the cells 3 times more magnetic than wild-type yeast, able to be quickly attracted to magnets placed underneath the liquid culture (in cute patterns or not):
Next, they wanted to see if they could control the yeast magnetism not just by adding more iron, but by controlling genes involved in iron homeostasis or cellular redox state. Redox balance determines how many electrons are available in the cell, and when there are fewer electrons iron will be oxidized from Fe2+ to Fe3+ and precipitate out of solution into magnetic clusters. Out of 60 gene deletions screened for changes in magnetism, one gene in particular was found to be necessary for the magnetism observed in high iron media. TCO89 is a nonessential part of TORC1, a complex of many proteins involved in regulating cellular stress responses, including nutrient and redox stress. When TCO89 was deleted, the cells were not magnetic, and when it was expressed in multiple copies the cells were more strongly attracted to the magnet. Because of this genetic dose-dependence, magnetism can be induced in yeast by controlling the expression of TCO89 with gene regulatory machinery that can be activated by external conditions, such as the presence of nutrients or chemicals. This can be used as a unique biological input or output in synthetic biology, improve efforts for precipitation and bioremediation of dangerous metals, as well as impact our understanding of cellular iron and electron metabolism.
“The importance of redox state in magnetization offers insight into magnetotactic bacteria.”
Magnetotactic bacteria live exclusively in microaerobic environments, using their magnetic crystals to find the perfect oxygen concentration. Oxygen availability influences the cell’s redox state, hinting at a possible evolutionary connection between iron sequestration, redox mediation, and the evolution of bio-magnetism. Perhaps cells adapted to certain redox conditions created the ideal chemical environment for the formation of iron crystals, which evolved into magnetosomes.
Magnetism is fascinating, and the fact that biology can create magnets genetically through processes fundamental to the biochemistry of all cells can seem nothing short of magical. Check out the video with the authors below:
and the paper at PLoS Biology (open access): Nishida K and Silver PA. (2012) “Induction of Biogenic Magnetization and Redox Control by a Component of the Target of Rapamycin Complex 1 Signaling Pathway.” PLoS Biology, e1001269.