Genetics, Vol. 167, 699-705, June 2004, Copyright © 2004
doi:10.1534/genetics.103.025411

Evidence for Multiple Cycles of Strand Invasion During Repair of Double-Strand Gaps in Drosophila

Mitch McVey^*,1, Melissa Adams^,1, Eric Staeva-Vieira^,2 and Jeff J. Sekelsky^,,3

Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599
^* SPIRE Program, University of North Carolina, Chapel Hill, North Carolina 27599
Program in Molecular Biology and Biotechnology, University of North Carolina, Chapel Hill, North Carolina 27599
Developmental Genetics Program, Skirball Institute of Biomolecular Medicine, New York University School of Medicine, New York, New York 10016

³ Corresponding author: Department of Biology, CB 3280, 303 Fordham Hall, University of North Carolina, Chapel Hill, NC 27599.
E-mail: sekelsky{at}unc.edu

DNA double-strand breaks (DSBs), a major source of genome instability, are often repaired through homologous recombination pathways. Models for these pathways have been proposed, but the precise mechanisms and the rules governing their use remain unclear. In Drosophila, the synthesis-dependent strand annealing (SDSA) model can explain most DSB repair. To investigate SDSA, we induced DSBs by excision of a P element from the male X chromosome, which produces a 14-kb gap relative to the sister chromatid. In wild-type males, repair synthesis tracts are usually long, resulting in frequent restoration of the P element. However, repair synthesis is often incomplete, resulting in internally deleted P elements. We examined the effects of mutations in spn-A, which encodes the Drosophila Rad51 ortholog. As expected, there is little or no repair synthesis in homozygous spn-A mutants after P excision. However, heterozygosity for spn-A mutations also resulted in dramatic reductions in the lengths of repair synthesis tracts. These findings support a model in which repair DNA synthesis is not highly processive. We discuss a model wherein repair of a double-strand gap requires multiple cycles of strand invasion, synthesis, and dissociation of the nascent strand. After dissociation, the nascent strand may anneal to a complementary single strand, reinvade a template to be extended by additional synthesis, or undergo end joining. This model can explain aborted SDSA repair events and the prevalence of internally deleted transposable elements in genomes.

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