Structure and origin of a tandem duplication of a Drosophila metallothionein gene

A strain of cadmium-resistant Drosophila was isolated that contained a chromosomal duplication of the metallothionein gene, Mtn. This duplication was a direct, tandem repeat of 2.2 kilobases of DNA: 228 bases of 5' flanking DNA, the entire transcription unit, and 1.4 kilobases of 3' flanking DNA. The entire duplication was cloned and DNA sequences of the regions relevant to the duplication process were determined. Comparison of the sequences of the 5' and 3' boundaries revealed no extensive regions of similarity, thus indicating that this duplication was formed by nonhomologous breakage and reunion. Recently, results of similar analyses by other investigators have suggested that this process was involved in the origin of three other eukaryotic duplications. The authors have observed a chi-like sequence near one of the boundaries of each duplication, and therefore suggest that this sequence may be important in generating one of the breaks required for duplication formation.