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Wednesday, 1 March, 2000, 09:33 GMT
Plants produce human protein
Tobacco is commonly used in plant research
Scientists have developed a new way of getting plants to make a human protein.

Researchers at the biotech company Monsanto engineered tobacco to express human somatotropin (hST), a hormone most often used to treat hypopituitary dwarfism in children.

The gene coding for the protein was introduced into the chloroplasts, the small compartments in leaf cells that generate energy from sunlight. This is a first for genetic modification (GM).

Until now, most GM work has focused on introducing foreign genes into nuclear DNA, not the small amount of DNA that is located outside the nucleus in the chloroplasts.

Gene containment

The ease of growing plants means the new route promises a high-yield, relatively low-cost alternative to the manufacture of pharmaceuticals using traditional GM technology such as in modified bacteria.

Interestingly, chloroplast DNA is not transferred through pollen. This means that crossing the modified plants with wild-type tobacco would result in offspring that contained no foreign genes within their nuclear or chloroplast DNA.

This may be a way of preventing genetic modifications from "leaking" into the environment, a major concern of environmental groups.

However, the Monsanto team claim that this issue, although worthy of note, was not the focus of the research. Their main aim was simply to produce as much of the protein as possible in the tobacco leaves, and the chloroplast technique resulted in a 300-fold increase in yield over that achieved using nuclear modifications.

Critical difference

"We tried to produce the protein in the nuclear compartment but we were only able to produce very low levels," Dr Jeffrey Staub, leader of the Monsanto research team, told BBC News Online. "The chloroplast genetic system was able to produce a high level of the protein.

"Truthfully, we don't know what the critical difference is at this time. This is not to say that chloroplasts would be better for every protein - it is just to say that in this case, they were better for the human somatotropin."

Monsanto plan to extend the research which is published in the journal Nature Biotechnology. It was a small scale study and far more work will be needed to fully exploit the potential of chloroplast genetics.

"By introducing genes into the chloroplasts, which are the sites of many metabolic pathways, we may be able to produce larger amounts of metabolic end products - such as vitamins, which may be used to enhance the nutritional value of foods," Dr Staub said.

"Another possibility might be to have better seed-specific accumulation of other important nutritional components like proteins and oils. And of course, there is the possibility of over-producing pharmaceutical proteins."

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