Adh1-2F11, an unstable allele of the Adh1 gene on chromosome 1, is due to a Ds insertion into the fourth exon of the gene. This mutation reverts only in the presence of Ac. Thus, the mutation is physiologically caused by a Ds element, and we have reported last year that indeed an insertion is found in the Adh gene.

This insertion has now been sequenced. It is 1319 bp long. 600 bp on the left and 300 bp on the right are about 95% homologous to Ac at its related Ds elements on chromosome 9. The remaining sequence of 400 bp in the center distinguishes this element from other Ds elements. This central sequence can be divided into two segments. One segment of 150 bp at one terminus is clearly related to Ac. In this region, several short DNA sequences of between 10 and 17 bp can be found that are also present in a limited region of 250 bp extension in Ac. The order of these fragments, however, is not colinear with Ac and they are interrupted with sequences not identifiable in Ac. This DNA segment may have arisen by a yet unknown sequence-shuffling mechanism. The remaining 250 bp of the central segment of Ds-2F11 has no detectable homology to Ac. Neither do they have a detectable similarity to Ds1, the "aberrant" Ds element described at the Adh1 locus by Sachs et al. (1984).

The structure of Ds-2F11 is thus intermediate between the Ds elements that are internal deletions of Ac, and Ds1, which shares with Ac only 13 and 19 bp respectively at either terminus and is otherwise unrelated to Ac.

A. Merckelbach, P. Starlinger

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