Saccharomyces cerevisiae Sae2- and tell-dependent single-strand DNA formation at DNA break promotes microhomology-mediated end joining

Microhomology-mediated end joining (MMEJ) joins DNA ends via short stretches [5-20 nucleotides (nt)] of direct repeat sequences, yielding deletions of intervening sequences. Non-homologous end 'joining (NHEJ) and single-strand annealing (SSA) are other error prone processes that anneal single-stranded DNA (ssDNA.) via a few bases (<5 nt) or extensive direct repeat homologies (>20 nt). Although the genetic components involved in MMEJ are largely unknown, those in NHEJ and SSA are characterized in some detail. Here, we surveyed the role of NHEJ or SSA factors in joining of double-strand breaks (DSBs) with no complementary DNA ends that rely primarily on MMEJ repair. We found that MMEJ requires the nuclease activity of Mre11/Rad50/Xrs2, 3' flap removal by Rad1/Rad10, Nej1, and DNA synthesis by multiple polymerases including Po14, Rad30, Rev3, and Pol32. The mismatch repair proteins, Rad52 group genes, and Rad27 are dispensable for MMEJ. Sae2 and Tell promote MMEJ but inhibit NHEJ, likely by regulating Mre11-dependent ssDNA accumulation at DNA break. Our data support the role of Sae2 and Tell in MMFJ and genome integrity.