The Smc5–Smc6 complex and SUMO modification of Rad52 regulates recombinational repair at the ribosomal gene locus

Abstract

Homologous recombination (HR) is crucial for maintaining genome integrity by repairing DNA double-strand breaks (DSBs) and rescuing collapsed replication forks. In contrast, uncontrolled HR can lead to chromosome translocations, loss of heterozygosity, and deletion of repetitive sequences. Controlled HR is particularly important for the preservation of repetitive sequences of the ribosomal gene (rDNA) cluster. Here we show that recombinational repair of a DSB in rDNA in Saccharomyces cerevisiae involves the transient relocalization of the lesion to associate with the recombination machinery at an extranucleolar site. The nucleolar exclusion of Rad52 recombination foci entails Mre11 and Smc5-Smc6 complexes and depends on Rad52 SUMO (small ubiquitin-related modifier) modification. Remarkably, mutations that abrogate these activities result in the formation of Rad52 foci within the nucleolus and cause rDNA hyperrecombination and the excision of extrachromosomal rDNA circles. Our study also suggests a key role of sumoylation for nucleolar dynamics, perhaps in the compartmentalization of nuclear activities.

In vivo modeling of polysumoylation uncovers targeting of Topoisomerase II to the nucleolus via optimal level of SUMO modification

“…However, we may have uncovered a more general role of heavy sumoylation in nucleolar maintenance, by showing that the bulk of SUMO is concentrated in the nucleolus in the smt4Δ mutant. While some reports showed that increase in sumoylation correlates with proteins leaving the nucleolus (Mo et al 2002;D'Amours et al 2004;Torres-Rosell et al 2007), in neither case, the localization of the sumoylated pool was tracked.…”

Smc5–Smc6 complex suppresses gross chromosomal rearrangements mediated by break-induced replications

“…Temperature-sensitive mutants of Smc6 have been shown to affect the stability of repetitive sequences in budding yeast by influencing their repair [35,36]. These observations have been recently extended to human cells, where the Smc5-Smc6 complex has been shown to play a role in alternative lengthening of telomeres ALT pathways, which are mediated by homologous recombination events [37].…”

“…For example, the cohesion defect impairs sister chromatid recombination [31,66], increasing intra-chromatid recombination in the tandem array of ribosomal repeats [36]. Recent work even suggested that Smc5-Smc6 would be required for the efficient loading of cohesin to DSB sites [67,68].…”

“…Some of these defects due to incorrect repair during the G2/M phases likely cause an increase in spontaneous double strand breaks (DSBs) [36]. To further gain an insight into the mechanisms of GCR formation when the Smc5-Smc6 complex is impaired, we measured GCR rates in strains defective in different subunits of Smc5-6 complex.…”

Perinuclear tethers license telomeric DSBs for a broad kinesin- and NPC-dependent DNA repair process

“…In yeast, DSBs within the highly repetitive ribosomal RNA genes (rDNA) can lead to unequal sister chromatid exchange events, compromise chromosome stability and shorten cellular lifespan [4][5][6][7] . DSBs within rDNA repeats were found to transiently exit the Rad52/recombination-repressive environment of the nucleolus in which they typically reside to undergo repair without compromising genome integrity 8,9 . In flies, heterochromatic DSBs exit their Rad51/recombinationrepressive subnuclear domain to undergo repair 10 .…”