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Yeast Gene Accumulates Cadmium

No one really knows exactly how plants gobble up and store metals.

In trying to solve this puzzle, researchers at the Plant Gene Expression Center in Albany, California, discovered a gene that may hold a key to bioengineering plants that can clean metal-contaminated soil.

They uncovered the gene and dubbed it hmt1, for heavy metal tolerance, while working with one of nature's simplest organisms—the yeast Schizosaccharomyces pombe.

"As with plants," says geneticist David W. Ow, "S. pombe produces small molecules called phytochelatins that bind to metals such as cadmium.

"Simply put, the hmt1 gene cues the yeast to manufacture a protein. The protein, in turn, pumps phytochelatinbound cadmium through cell membranes and into cell compartments known as vacuoles."

Yeasts and plants seem to use vacuoles either for storing things they need or as tiny trash bags for dumping things they don't. When shuttled into vacuoles, the phytochelatin-bound cadmium apparently stays put—yet is harmless to the plant.

The researchers' next step: duplicate the yeast's metal-works in plants that might be used as metal scavengers.

Ow and colleagues succeeded in slipping hmt1 into tobacco—a potential candidate for bioaccumulation chores. But they haven't been able to get the transferred hmt1 to make the protein yet. Says Ow, "Tobacco apparently reads some signals within hmt1 as stop signs."

To sidestep the problem, the scientists are streamlining the gene. If that tactic succeeds, high-tech plants with the improved hmt1—and perhaps other metal-transporting genes, as well—may be less than a decade away.

In the meantime, Ow's studies have won him a new, multiyear grant for environmental cleanup research from the U.S. Department of Energy. -- By Marcia Wood, ARS.

David W. Ow is with the USDA-ARS Plant Gene Expression Center, Albany, CA; phone (510) 559-5909.

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