( p>We can now start changing how phytoc...)Scientists map one of biology's critical light-sensing structures

According to Vierstra, there are many kinds of phytochromes found in every plant, and they exist in virtually all cells. They occur in greater concentrations in cells that respond directly to light, such as in root tips and new shoots.

The phytochrome revealed by the Wisconsin team was derived from a microbe known as Deinococcus radiodurans, a bacterium renowned for its tolerance to ionizing radiation. It was only within the last eight years that scientists from Vierstra's and other labs discovered that, like plants, some bacteria harbor phytochromes. That finding opened the way for the Wisconsin team to define the structure of a phytochrome, as bacteria are easy to grow in the lab and their proteins are easier to purify and manipulate than plant proteins.

Once isolated, the phytochrome was crystallized and its molecular structure was mapped using a beam of X-rays to develop a three-dimensional picture of the protein. That three-dimensional portrait, which reveals the atom-by-atom configuration of the molecule, is the key to understanding the "nuts and bolts" of how the photoreceptor senses light and triggers a series of downstream events that control growth and development, according to Katrina Forest, the other senior member of the team and a UW-Madison professor of bacteriology. "There were some surprises," says Forest of the ribbon-like protein. "This protein has a knot."

That is a startling feature, she notes, observed in only a handful of proteins out of tens of thousands whose structures are known. The group speculates the knot may help stabilize the protein so it can do its job of capturing light and triggering the cascade of downstream events under its control. "We think
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Source:University of Wisconsin-Madison