The function of Smc5/6 in Drosophila development and at the heterochromatin-like locus HMR in budding yeast

Abstract: The structural maintenance of chromosome 5/6 (Smc5/6) complex is one out of many factors in the cell that maintain genome stability. Smc5/6 has been implicated in processes such as DNA double-strand break (DSB) repair, replication progression, and chromosome segregation. We have examined Smc5/6 in both fruit flies (Drosophila melanogaster) and budding yeast (Saccharomyces cerevisiae) during unchallenged conditions in order to further explore its function(s) in genome stability maintenance. In paper I we investigated early development in Drosophila, and found that Smc5/6 is maternally contributed. In absence of the complex, genome instability is observed during both oogenesis and embryogenesis. Oocytes lacking either Smc5 or Smc6 exhibited unrepaired DSBs, which interfered with early onset of embryogenesis. In addition, early developing embryos lacking Smc5/6 accumulated damaged nuclei. These could, however, be eliminated through a process called nuclear fallout, which allows the flies to tolerate genomic instability caused by the absence of Smc5/6 during early embryo development. In paper II, we addressed the functional relevance of Smc6 binding to the transcriptionally repressed (also known as silenced) mating-type locus HMR in budding yeast. We investigated silencing of a crippled HMR locus HMRae in an smc6-56 mutant and found that the cells contained chromosomal rearrangements in the HMR region. These were dependent on Rad52, a key player for homologous recombination. Moreover, HMRae cells lacking Topoisomerase 1 (top1) exhibited a similar recombination-dependent phenotype that was epistatic to smc6- 56. Taken together with earlier observations, this suggests that resolution of replicationinduced superhelical tension by Smc5/6 and Top1 is important for the genomic integrity of HMR. Both papers demonstrate the occurrence of genomic instability in the absence of fully functional Smc5/6 complex during unchallenged conditions, and reveal an important role of the Smc5/6 complex during development in higher eukaryotes.