Genetic control of the error-prone repair of a chromosomal double-strand break with 5' overhangs in yeast.

A targeted double-strand break introduced into the genome of Saccharomyces cerevisiae is repaired by the relatively error-prone nonhomologous end joining (NHEJ) pathway when homologous recombination is not an option. A zinc finger nuclease cleavage site was inserted out-of-frame into the LYS2 locus of a haploid yeast strain to study the genetic control of NHEJ when the ends contain 5' overhangs. Repair events that destroyed the cleavage site were identified either as Lys+ colonies on selective medium or as surviving colonies on rich medium. Junction sequences in Lys+ events solely reflected NHEJ and were influenced by the nuclease activity of Mre11 as well as by the presence/absence of the NHEJ-specific polymerase Pol4 and the translesion-synthesis DNA polymerases Pol ζ and Pol η. Although most NHEJ events were dependent on Pol4, a 29-bp deletion with endpoints in 3-bp repeats was an exception. The Pol4-independent deletion required translesion synthesis polymerases as well as the exonuclease activity of the replicative Pol δ DNA polymerase. Survivors were equally split between NHEJ events and 1.2 or 11.7 kb deletions that reflected microhomology-mediated end joining (MMEJ). MMEJ events required the processive resection activity of Exo1/Sgs1, but there unexpectedly was no dependence on the Rad1-Rad10 endonuclease for the removal of presumptive 3' tails. Finally, NHEJ was more efficient in nongrowing than in growing cells and was most efficient in G0 cells. These studies provide novel insights into the flexibility and complexity of error-prone DSB repair in yeast.