EFFICIENT COPYING OF NONHOMOLOGOUS SEQUENCES FROM ECTOPIC SITES VIA P-ELEMENT-INDUCED GAP REPAIR

P-element-induced gap repair was used to copy nonhomologous DNA into t he Drosophila white locus. We found that nearly 8,000 bp of nonhomolog ous sequence could be copied from an ectopic template at essentially t he same rate as a single-base substitution at the same location. An in vitro-constructed deletion was also copied into,white al high frequen cies. This procedure can be applied to the study of gene expression in Drosophila melanogaster, especially for genes too large to be manipul ated in other ways. We also observed several types of more complex eve nts in which the copied template sequences were rearranged such that t he breakpoints occurred at direct duplications. Most of these can be e xplained by a model of double strand break repair in which each termin us of the break invades a template independently and serves as a prime r for DNA synthesis from it, yielding two overlapping single-stranded sequences. These single strands then pair, and synthesis is completed by each using the other as a template. This synthesis-dependent strand annealing (SDSA) model as a possible general mechanism in complex org anisms is discussed.