Chromosomal rearrangements caused by the aberrant transposition of double Ds elements are formed by Ds and adjacent non-Ds sequences

Summary

The Ds-induced unstable sh-m6258 allele is caused by the insertion of at least 45 kb of non-sh DNA. Genomic clones spanning the two junctions of sh sequences with insert sequences were isolated and analysed. The long insert in the sh-m6258 allele is bordered by Ds sequences on either side. A 3 kb double Ds structure which consists of one complete Ds element of approximately 2 kb and one half Ds element of approximately 1 kb was found in the sh-m6258 allele at the junction between the 3′ region of the sh locus and the insert. At the junction between the 5′ region of the sh locus and the insert a half Ds element is present. This truncated Ds element was probably left behind after the excision of a 2 kb Ds element from a 3 kb double Ds structure similar to that found at the 3′ junction. Sequence analysis of the insert sequences beyond the Ds elements demonstrated that the long inserts in both the sh-m6258 and the sh-m5933 alleles originated from the same contiguous chromosomal segment in their common progenitor strain. One revertant derivative of the sh-m6258 allele was investigated. In the revertant strain, which displays a normal non-shrunken phenotype, a 2 kb Ds element is present at the site of the 45 kb insert in the mutant allele. This 2 kb insertion, which is located in the last but one intervening sequence of the Sh gene, does not inhibit expression of the gene. The 2 kb Ds element in the revertant allele is identical to the 2 kb Ds element which forms part of the 3 kb double Ds structure on the 3′ side of the insert of the sh-m6258 allele. We propose a hypothetical insert structure for the progenitor allele of the revertant. An intramolecular recombination or an unequal crossing-over event in this insert structure would account for the presence of a 2 kb Ds element in the revertant allele.