MOBILIZATION OF A MINOS TRANSPOSON IN DROSOPHILA-MELANOGASTER CHROMOSOMES AND CHROMATID REPAIR BY HETERODUPLEX FORMATION

Transposase-mediated mobilization of the element Minos has been studie d in the Drosophila melanogaster genome. Excision and transposition of a nonautonomous Mines transposon in the presence of a Minos transposa se gene was detected with a dominant eye color marker carried by the t ransposon. Frequencies of excision in somatic tissues and in the germ line were higher in flies heterozygous for the transposon than in homo zygotes or hemizygotes. Transposition of a X chromosome-linked inserti on of Mines into new autosomal sites occurred in 1-12% of males expres sing transposase, suggesting that this system is usable for gene taggi ng and enhancer trapping in Drosophila. Sequence analysis of PCR-ampli fied donor sites after excision showed precise restoration of the orig inal target sequence in similar to 75% of events in heterozygotes and the presence of footprints or partially deleted elements in the remain ing events. Most footprints consisted of the four terminal bases of th e transposon, flanked by the TA target duplication. Sequencing of a ch romosomal donor site that was directly cloned after excision showed a characteristic two-base mismatch heteroduplex in the center of the 6-b p footprint. Circular extrachromosomal forms of the transposon, presum ably representing excised Mines elements, could be detected only in th e presence of transposase. A model for chromatid repair after Minos ex cision is discussed in which staggered cuts are first produced at the ends of the inverted repeats, the broken chromatid ends are joined, an d the resulting heteroduplex is subsequently repaired. The model also suggests a simple mechanism for the production of the target site dupl ication and for regeneration of the transposon ends during reintegrati on.