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The xylose isomerase genes (xylA) From Thermoanaerobacterium thermosulfurogenes and Streptomyces rubiginosus were introduced and expressed in three plant species (potato, tobacco and tomato) and transgenic plants were selected on xylose-containing medium. The xylose isomerase genes were transferred to explants of the target plant by Agrobacterium-mediated transformation. The xylose isomerase genes were expressed under the control of the enhanced cauliflower mosaic virus 35S promoter and the Ω′ translation enhancer sequence from tobacco mosaic virus. In potato and tomato, xylose isomerase selection was more efficient than the established kanamycin selection. The level of enzyme activity in the regenerated transgenic plants selected on xylose was 5-25-fold higher than the enzyme activity in control plants selected on kanamycin. The xylose isomerase system enables transgenic cells to utilize xylose as a carbohydrate source. In contrast to antibiotic or herbicide resistance-based system where transgenic cells survive on a selective medium but nontransgenic cells are killed, the xylose system is an example of a positive selection system where transgenic cells proliferate while non-transgenic cells are starved but still survive. The results show that a new selection method, is established. The xylose system is devoid of the disadvantages of antibiotic or herbicide selection, and depends on an enzyme which is already being widely utilized in specific food processes and that is generally recognized as safe for use in the starch industry.