Requirement for the SRS2 DNA helicase gene in non-homologous end joining in yeast

Mitotic cells experience double-strand breaks (DSBs) from both exogenous and endogenous sources. Since unrepaired DSBs can result in genome rearrangements or cell death, cells mobilize multiple pathways to repair the DNA damage. In the yeast Saccharomyces cerevisiae, mitotic cells preferentially use a homologous recombination repair pathway. However, when no significant homology to the DSB ends is available, cells utilize a repair process called non-homologous end joining (NHEJ), which can join ends with no homology through resection to uncover microhomologies of a few nucleotides. Although components of the homologous recombination repair system are also involved in NHEJ, the rejoining does not involve all of the homologous recombination repair genes. The SRS2 DNA helicase has been shown to be required for DSB repair when the homologous single-stranded regions are short. Here it is shown that SRS2 is also required for NHEJ, regardless of the cell mating type. Efficient NHEJ of sticky ends requires the Ku70 and Ku80 proteins and the silencing genes SIR2, SIR3 and SIR4. However, NHEJ of blunt ends, while very inefficient, is not further reduced by mutations in YKU70, SIR2, SIR3, SIR4 or SRS2, suggesting that this rejoining process occurs by a different mechanism.