Jumping genes into soybeans?

Pieces of DNA that move from one position to another within chromosomes, turning off or on the more sedentary genes they invade, have been well studied in maize, but few have been described in other plants. Now Lila O. Vodkin, Patsy R. Rhodes and their colleagues at ARS report the first evidence for such a mobile element, often called a transposon, in soybean plants. They believe transposons will be useful for identifying and isolating plant genes of agricultural interest, and transposons may eventually serve as a genetic engineering tool for carrying genes from plant to plant.

The ARS scientists discovered the soybean transposon during their genetic analysis of lectin, a major protein of the soy seed. A few varieties of soybean produce no lectin at all. Vodkin and her colleagues found that the lectin gene in a lectin-less variety is interrupted near the middle by a large piece of DNA, which prevents the gene's function. They next observed junctions between the lectin gene and the insert that are characteristic of mobile elements. Vodkin has named the soybean element Tgm1--indicating it is the first transposon of soybean, which has the species name Glycine max.

The newly discovered soybean transposon has surprising similarities to elements of other plant species, Vodkin says. It resembles transposons recently identified in snapdragons and in corn. These soybean, snapdragon and corn transposons all have unusual structures containing many repeats of short DNA sequences that shape the molecule into a series of linked "hairpin" structures.

So far Vodkin has no direct evidence that Tgm1, identified as a transposon on the basis of its structure, moves in and out of the soybean gene. She is searching for the Tgm1 in other soybean genes, especially those that act like corn genes known to harbor transposons and create a variegated pattern--for instance, dappled green and yellow coloration in the levels.