How can a short single strand of DNA help yeast repair a broken chromosome?

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Repairing chromosomal double-strand breaks (DSBs) is essential for normal cell growth, and homologous recombination is a universal process for repairing these breaks. The researchers experimentally explored DSB repair in the yeast Saccharomyces cerevisiae using single-strand oligonucleotides—short DNA strands—with homology to DNA on both sides of a break.

Oligonucleotide-directed repair occurred exclusively through Rad52- and Rad59-mediated single-strand annealing (SSA), in which matching single strands pair up. Even the SSA domain of human Rad52 partially complemented a null rad52 mutation. Repair did not involve Rad51-driven strand invasion; suppressing strand invasion actually increased repair using oligonucleotides.

A DSB also activated targeting by oligonucleotides homologous to only one side of the break at sites at least 20 kb from the break. This targeting was strand-biased, suggesting extensive DNA resection in the 5′-to-3′ direction, followed by restoration of the resected DNA to a double-stranded state.

The researchers concluded that a single-strand DNA oligonucleotide repairs long, resected chromosomal DSB ends through two rounds of annealing. Repair by single-strand DNA can be conservative and may allow accurate restoration of chromosomal DNA with closely spaced DSBs.