Producing silk proteins in plant tissues

RESEARCHERS HAVE inserted spider genes into potatoes and tobacco plants so that they produce large quantities of silk proteins in their tissues. If these proteins could be spun into thread, they might be used to make not only strong fibres, but also non-toxic and biodegradable biomedical fabrics.

Udo Conrad from the Institute Pflanzgenetik und Kulturpflanzenforschung in Gatersleben, Germany, and co-workers made artificial versions of the silk genes from the golden orb- weaving spider (Nephila clavipes), and spliced them into the genomes of several plants. They found that more than 2 per cent of the total protein mass of some of these plants consisted of the silk protein. The findings were reported in Nature Biotechnology.

Spider silk genes have already been transferred into bacteria. The bacteria are then cultured in fermentation vats, where they make silk protein, but they have to be fed with the relatively expensive components of the silk protein, the amino acids glycine and alanine. Silk genes have also been inserted into the DNA of goats, which express the protein in their milk.

The researchers estimate that producing silk in transgenic plants should cost only a tenth to a half as much as using genetically engineered bacteria. In contrast to bacteria, plants can make their own amino acids from cruder raw materials, and the researchers say that silk genes in plants are less prone to getting reshuffled.

Silkworms and spiders have a special gland for making silk proteins, which they spin into fine fibres that are stronger than steel. For sheer resilience, silk is rivalled by only a few man- made materials such as Kevlar fibres, made by DuPont, which are used in bullet-proof clothing, sports equipment, aircraft components and tethers for oil- rigs.

But Kevlar is stiff, whereas silk is highly elastic. This means that silk absorbs a lot of energy before it snaps - which is why the spider uses it to capture the fast-moving fly.

Engineers and materials scientists would dearly like to use silk for technological purposes. But collecting it from natural sources is veryexpensive. So researchers are looking for ways to mass-produce artificialsilk with the same composition and properties as real silk.

As silk is a protein, its chemical composition is encoded in the genes of the organisms that make it. Researchers have unravelled this composition, but it is too complicated to put silk together 'by hand' using industrial methods. A better approach is to turn organisms into living silk factories.

It is not easy to make strong fibres from the water-soluble silk protein produced by the plants; no one yet knows how to do it as well as a spider. But the researchers think that the availability of large quantities of raw protein will aid the development of spinning techniques.

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Section  : Science & Tech
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