The nature of unstable insertion mutations and reversions in the locus cut of Drosophila melanogaster: molecular mechanism of transposition memory.

The segment of the locus cut containing the mobile genetic element mdg4 (gypsy) insertions which induce unstable ct and ct mutations has been cloned. Both mutations depend on the insertion of mdg4 into the same sequence, which coincides with that in ct allele. The ct mutation differs from ct by additional insertion of a novel mobile element jockey into mdg4. Jockey is 2.8 kb long, represented by 2-100 copies per genome, very homogeneous and lacks long terminal repeats (LTRs). The excision of mdg4 takes place in stable ct reversions. On the other hand, a complete single LTR is retained in the case of unstable ct reversions characterized by a high level of reverse directed transpositions of mdg4 into the locus cut. The LTR serves as a guide for reinsertion of mdg4 itself or mdg4 with jockey into the same site of the genome. A possible mechanism of transposition memory (homologous recombination with extrachromosomal circular DNA) is discussed.