A MODIFIED SINGLE-STRAND ANNEALING MODEL BEST EXPLAINS THE JOINING OFDNA DOUBLE-STRAND BREAKS IN MAMMALIAN-CELLS AND CELL-EXTRACTS

The joining of DNA double-strand breaks in vivo is frequently accompan ied by the loss of a few nucleotides at the junction between the inter acting partners. In vitro systems mimic this loss and, on detailed ana lysis, have suggested two models for the mechanism of end-joining. One invokes the use of extensive homologous side-by-side alignment of She partners prior to joining, while the other proposes the use of small regions of homology located at or near the terminus of the interacting molecules. To discriminate between these two models, assays were cond ucted both in vitro and in vivo with specially designed substrates. In vitro, molecules with limited terminal homology were capable of joini ng, but analysis of the junctions suggested that the mechanism employe d the limited homology available. In vivo, substrates with no extensiv e homology end-joined with equal efficiency to those with extensive ho mology in two different topological arrangements. Taken together, thes e results suggest that extensive homology is not a prerequisite for ef ficient end-joining, but that small homologies close to the terminus a re used preferentially, as predicted by the modified single-strand ann ealing model.