Gene Returned to Ancient Home

Reunited? Researchers have swapped genes between the nucleus and chloroplast.

A long time ago, the ancestors of plants acquired the ability to make their own food, perhaps by engulfing photosynthetic bacteria. Over time, these bacteria evolved into chloroplasts, tiny organelles with their own small genomes; while that small genome remains, many of its original genes slipped into a plant's nucleus. Now, for the first time, researchers have reversed this ancient migration in a tobacco plant. By transferring a gene from the nucleus to its chloroplasts, they may be able to help plants create proteins in greater abundance.

In earlier experiments, researchers transferred genes found in chloroplasts and mitochondria, which also have their own genes, to the plant's nucleus. It's hard to put genes into chloroplasts, but recently researchers have done so with a number of species, including potato, tomato, and tobacco.

Inspired by this work, plant geneticists Archie Portis Jr. and Jack Widholm at the University of Illinois, Urbana-Champaign, transplanted a nuclear tobacco gene back to where it was originally found, in the chloroplast. Portis, Widholm, and their colleagues report in the September issue of Plant Physiology that they worked with the gene for a subunit of anthanilate synthesis (AS), a key enzyme in the manufacture of an amino acid called tryptophan. The researchers used a mutant form of the gene that overexpresses itself. So when the AS subunit was produced in 10 times the normal concentration, they knew they'd successfully transplanted the gene. The bumper crop was also maternally heritable, again showing that the gene had been successfully transferred.

The researchers have performed a "cute trick," says Jeffrey Palmer, a geneticist at the University of Indiana in Bloomington. Pal Maliga, a geneticist at Rutgers University in New Brunswick, New Jersey, geneticist says that organelle gene transplants will help scientists engineer plants that crank out desirable natural products because it liberates their production from the nucleus' regulatory controls.

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