A COMPLEX ARRANGEMENT OF GENES AT A STARCH BRANCHING ENZYME-I LOCUS IN THE D-GENOME DONOR OF WHEAT

Genomic DNA fragments from Triticum tauschii (D-genome donor to wheat) carrying starch branching enzyme I (SEE I) type genes have been chara cterized. One fragment contains one complete gene and two partial gene s in 16 kb of DNA. One of the partial genes is oriented in the opposit e strand to the other two. The gene that is complete was sequenced. It s structure corresponds closely to that of rice in that exons 3-8 are retained at similar sizes and spacings. A cDNA closely corresponding t o the complete gene was isolated and characterized; it codes for a put ative protein that represents a novel type of SEE I, as it is shorter at the 3' end than the forms reported so far in other plants. A second genomic fragment contains a different SEE I gene. There appear to be approximately 10 copies of SEE I type genes in wheat (approximately 5 in T. tauschii) and most of them have been assigned to group 7 chromos omes. In situ hybridization indicates that a major locus for the genes is located at the distal end of the short arm of chromosome 7D.