Abstract
DNA repair mechanisms in mitotically proliferating cells avoid generating crossovers, which can contribute to genome instability. Most models for the production of crossovers involve an intermediate with one or more four-stranded Holliday junctions (HJs), which are resolved into duplex molecules through cleavage by specialized endonucleases. In vitro studies have implicated three nuclear enzymes in HJ resolution: MUS81–EME1/Mms4, GEN1/Yen1, and SLX4–SLX1. The Bloom syndrome helicase, BLM, plays key roles in preventing mitotic crossover, either by blocking the formation of HJ intermediates or by removing HJs without cleavage. Saccharomyces cerevisiae mutants that lack Sgs1 (the BLM ortholog) and either Mus81–Mms4 or Slx4–Slx1 are inviable, but mutants that lack Sgs1 and Yen1 are viable. The current view is that Yen1 serves primarily as a backup to Mus81–Mms4. Previous studies with Drosophila melanogaster showed that, as in yeast, loss of both DmBLM and MUS81 or MUS312 (the ortholog of SLX4) is lethal. We have now recovered and analyzed mutations in Drosophila Gen. As in yeast, there is some redundancy between Gen and mus81; however, in contrast to the case in yeast, GEN plays a more predominant role in responding to DNA damage than MUS81–MMS4. Furthermore, loss of DmBLM and GEN leads to lethality early in development. We present a comparison of phenotypes occurring in double mutants that lack DmBLM and either MUS81, GEN, or MUS312, including chromosome instability and deficiencies in cell proliferation. Our studies of synthetic lethality provide insights into the multiple functions of DmBLM and how various endonucleases may function when DmBLM is absent.
The maintenance of a stable genome is crucial to organismal survival. Genome stability is perpetually threatened by spontaneous DNA damage, and DNA repair proteins are required to accurately and efficiently repair DNA damage in ways that minimize genome alterations. Some repair pathways are linked to increased risk of genome changes. One example is repair associated with the production of crossovers between homologous chromosomes. The DNA helicase BLM suppresses genome changes by promoting non-crossover forms of repair; without BLM, spontaneous crossovers, deletions, and genome rearrangements increase. Using Drosophila as a model organism, our studies reveal the complex interactions between BLM and three structure-selective endonucleases with overlapping substrate specificities and partial functional redundancy. Loss of BLM and any one of the nucleases results in severe genome instability, reduced cell proliferation, and, ultimately, death of the animal. Our work suggests that these nucleases differentially rescue the loss of functions of BLM associated with problems that arise during DNA replication, illuminating the complexity of repair mechanisms required to maintain genome stability during replication. Further, our work advances models of replication-associated repair by suggesting specific roles for BLM and structure-selective endonucleases.
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