ESSENTIAL POSTREPLICATIVE FUNCTIONS OF THE SMC5/6 COMPLEX

The structural maintenance of chromosomes (SMC) complex Smc5/6 is based on a heterodimer of two SMC subunits, Smc5 and Smc6, and six non-Smc element subunits, Nse1-6, all of which are essential for cell viability in most organisms. Smc5/6 safeguards genome integrity via different mechanisms, including stabilization of stalled replication forks, resolution of recombination intermediates, and maintenance of nucleolar integrity. However, the essential functions of Smc5/6 remain elusive. The aim of the present work was to understand when in the cell cycle the crucial functions of Smc5/6 are manifested and to identify them. Through the use of cell cycle regulated alleles, which enabled the restriction of various Smc5/6 subunits expression to either S or G2/M phases of the cell cycle, we uncovered that the essential roles are executed postreplicatively in G2/M. By further genetic screens, molecular approaches and genome-wide studies, we identified three chromosome topology and recombination-related processes that are crucially sensitive to low amounts of Smc5/6 specifically in G2/M. First, Smc5/6 plays a topological role affecting the formation and/or the resolution of Rad5-Mms2-Ubc13 chromatin structures that are later engaged by Sgs1-Top3-Rmi1. Second, Smc5/6 facilitates an epigenetic pathway that ensures silencing of specific loci, such as repetitive DNA regions, thereby preventing unrestrained recombination. Third, Smc5/6 has an anti-fragility function, facilitating replication through natural pausing elements and site-specific replication fork barriers and preventing their breakage in mitosis during chromosome segregation.