DE-NOVO SYNTHESIS OF AN INTRON BY THE MAIZE TRANSPOSABLE ELEMENT DISSOCIATION

The mechanisms by which introns are gained or lost in the evolution of eukaryotic genes remain poorly understood. The discovery that transpo sable elements sometimes alter RNA splicing to allow partial or imperf ect removal of the element from the primary transcripts suggests that transposons are a potential and continuing source of new introns. To d ate, splicing events that precisely restore the wild-type RNA sequence at the site of insertion have not been detected. Here we describe alt ernative RNA splicing patterns that result in precise removal of a Dis sociation (Ds) insertion and one copy of its eight-nucleotide host sit e duplication from an exon sequence of the maize shrunken2-mutable1 (s h2-m1) mutant. In one case, perfect splicing of Ds was associated with aberrant splicing of an intron located 32 bp upstream of the insertio n site. The second transcript type was indistinguishable from wild-typ e mRNA, indicating that Ds was spliced like a normal intron in about 2 % of the sh2-m1 transcripts. Our results suggest that the transpositio n of Ds into sh2 in 1968, in effect, marked the creation of a new intr on in a modern eukaryotic gene. The possibility of precise intron form ation by a transposable element demonstrated here may be a general phe nomenon of intron formation, since consensus intron splice sites can b e explained by insertions that duplicate host sequences upon integrati on. A model is presented.