Figure 8.—

Models for DmBlm function in DSB and gap repair. (A) Hypothesized functions for DmBlm in DSBR. (Left) The steps involved in generating a crossover, according to a modified version of the DSBR model of Szostak et al. (1983). (i) Initial processing of the DSB involves resection of the 5′-ends, which generates 3′-ended, single-stranded overhangs. (ii) One overhang invades a homologous duplex, generating a D-loop, and the D-loop is enlarged when the invading strand is extended by repair DNA synthesis. (iii) The displaced strand anneals to the other 3′ overhang. (iv) Additional synthesis extends this end of the break, using the displaced strand as a template, and ligation at both ends leads to a DHJ. (v) The DHJ is resolved by nicking of two strands at each junction. In the example shown here, the two inner (crossing) strands are cut at the left junction, and the two outer (noncrossing) strands are cut at the right junction, giving rise to crossover chromatids. Resolution can also give rise to noncrossover chromatids. In the disruptase model, DmBlm removes the invading strand during or after synthesis, as in the SDSA model. The nascent sequence anneals to the other resected end, resulting a noncrossover repair product. In the dissolvase model, DmBlm migrates the two Holliday junctions toward one another. Decatenation by a type I topoisomerase generates a noncrossover repair product. (B) Hypothesis for the formation of deletions during gap repair. The first diagram illustrates a chromatid from which a transposable element (solid lines) has excised and the ends have been resected. (i) As in DSBR, a resected end invades a homologous template, such as the sister chromatid, and primes new synthesis. Displacement of the D-loop does not extend far enough to allow capture of the second end of the break. (ii) If DmBlm cannot dissociate the invading strand, this strand is cut (open arrowhead), resulting in an enlarged gap. (iii) If the two ends of the enlarged gap are repaired through end joining, as is common during gap repair in Drosophila, the product will lack the transposable element and will be deleted for sequences adjacent to the insertion site. If the left end of the break has invaded the sister chromatid and primed repair synthesis, it may contain some sequences from the left end of the transposable element or may also have a deletion. The fate of the template chromatid is not shown. Cutting of the invading strand may allow another helicase to remove the annealed strand, or it may be removed at the next S phase. A more complete illustration of this model for gap repair is given in supplemental Figure S1 at http://www.genetics.org/supplemental/.