The 30 kb insertion mentioned in the preceding report is larger than most known transposable elements in other organisms. We considered the possibility that the 30 kb insertion is a transposon-like structure with two transposable elements flanking some other DNA. In order to investigate this hypothesis, we sequenced 4.3 kb of DNA at the junction between the insertion and the 3'-end of the sucrose synthase gene. The structure of this 4 kb segment was known to contain duplicated sequences from previous hybridization studies. The DNA sequence confirmed this: A 2040 bp sequence is present twice at the junction. One copy is inserted into the middle of the second, and the two copies are inverted relative to each other. The 2040 bp sequences share properties with known transposable elements: They are terminated by 11 bp inverted repeats, as is common for transposons. The 8 bp sequences flanking the central copy of the 2040 bp structure are identical. This same 8 bp sequence is present within the central copy only once. This is the only difference between the outer and the inner copy of the 2040 bp structure, and is explained by assuming that the inner sequence is an insertion into the outer one, creating a short duplication of 8 bp at the site of insertion. This is a second property common to transposable elements.
The sequence similarities between the 2040 bp structure and other known transposons do not prove that it, and not the large 30 kb structure present in the sh-m5933 sucrose synthase gene, is a Ds element. To evaluate what constitutes a Ds sequence, we have examined other Ds-induced mutations for similarities with the 2040 bp structure. We have shown that the 2040 bp sequence hybridizes to a 1.3 kb insert found in mutant Adh1-2F11, a Ds-induced mutation (see a subsequent report). This sequence also hybridizes to the ends, but not the middle, of an Ac element we
have isolated from the wx locus (see a subsequent report). This strongly supports our contention that the 2040 bp structure is Ds and is derived from an internal deletion of an active Ac element. This work is in press in Nature.
H. P. Doring, E. Tillmann and P. Starlinger
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